"'" '

• _ • -•_,_; .-. - c. ±

, .^. • _ -,-_ -^-:_ ^. .-.:.- -

STUDIES OF HEREDITY
IN RABBITS, RATS, AND MICE

BY
W. E. CASTLE

RESEARCH ASSOCIATE OF THE CARNEGIE INSTITUTION OF WASHINGTON

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

WASHINGTON, 1919

CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 288

[From the Laboratory of Genetics of the Bussey Institution]

Copies of this Book
were first issued

JUL 24 *i

PRESS OF J. B. LIPPINCOTT COMPANY
PHILADELPHIA

CONTENTS.

PART I. FURTHER EXPERIMENTS UPON THE MODIFIABILITY OF THE HOODED

CHARACTER IN RATS.

PAGE
Selected races crossed with wild 2

PART II. THE INHERITANCE OF WHITE-SPOTTING IN RABBITS, WITH SPECIAL
REFERENCE TO GAMETIC CONTAMINATION.

Dutch 5

English 19

Relation of Dutch to English 25

PART III. OBSERVATIONS ON THE OCCURRENCE OF LINKAGE IN RATS AND MICE.

Tables 37

Bibliography 56

STUDIES OF HEREDITY IN RABBITS, RATS, AND MICE.

BY W. E. CASTLE.

I. FURTHER EXPERIMENTS UPON THE MODIFIABILITY
OF THE HOODED CHARACTER OF RATS.

In publications Nos. 195 and 241 of the Carnegie Institution of
Washington reports have been made on the results of a series of experi-
ments designed to show to what extent a mendelizing character, the
hooded pattern of piebald rats, may be altered by selection or by
crossing. At the last report (Castle and Wright, 1916) the plus-
selection series had been carried without out-crossing through 16
successive generations, in the course of which the mean grade of the
offspring had advanced from +2.05 to +4.13, in terms of the arbi-
trary grading scale depicted in both previous publications. Since
then the plus-selection series has been carried through four additional
generations of selection (17 to 20) and the mean of the selected race
has been raised to 4.61. In some respects this part of the series is
less satisfactory than that previously reported on, because smaller
numbers of animals were available from which to select and the selec-
tion has therefore been less rigorous. The race has unmistakably
fallen off in vigor and fecundity in later generations. It is uncertain
whether this should be ascribed to inbreeding alone, uncorrected by
selection for vigor (as in Miss King's experiments), or to increase in
the prevalence of disease, or to both causes. Certain it is, however,
that notwithstanding increasing care in regard to feeding and sanita-
tion a very large proportion of our breeding-pens in the case of the
selected races produce no young at all.

Tables 1 to 4 show in detail the grade distribution of the young
produced by plus-selected parents of generations 17 to 20. The
numbers of young produced in each of these generations are respec-
tively 351, 420, 280, and 92. The mean grade of the young advances
from 4.13 (in generation 16) to 4.48 (in generation 17), remains prac-
tically stationary in generations 18 and 19 at 4.46 and 4.49 respectively,
and then advances again (in generation 20) to 4.61.

The minus-selection series — which at the last report had been car-
ried through 17 generations, with an advance in mean grade of the
young from grade minus 1.00 to grade minus 2.70 — has now been

l

2 HEREDITY IN RABBITS, RATS, AND MICE.

carried through 4 additional generations of selection. (See tables
5 to 8.) But this race has shown even poorer vitality than the plus-
selection race, so that in order to keep it alive practically no young
could be rejected as parents and consequently no further progress
has been made. The numbers of young recorded for the four addi-
tional generations have been 330, 130, 79, and 35 respectively, and
their mean grades -2.84, -2.89, -2.78, and -2.74. The race is
now practically stationary in grade, but seems likely soon to become
extinct despite our strongest efforts to keep it alive. Notwithstanding
the fact that the race is verging on extinction after long-continued
close breeding, the variability of the hooded character is still as great
as ever. The standard deviation ranges from 0.25 to 0.45 within
about the same limits as in the previous 17 generations from minus-
selected parents. In the plus-selection series the standard deviation
was also fully as high in the last generations as it had been in the
previous 10 generations. Only the initial 7 generations had shown
an appreciably higher variability.

SELECTED RACES CROSSED WITH WILD.

We may now inquire what happens to the races modified by selec-
tion in opposite directions, when they are crossed with an unselected,
non-hooded race, the wild race. This question was considered in
some detail, so far as the plus-selected race is concerned, in a previous
publication (1916), where it was shown that a cross of the plus-selected
race reduced the grade of the hooded character, undoing in a measure
the work of selection. Selected animals which mated with their like
should have produced young of about mean grade 3.75, actually pro-
duced hooded grandchildren, extracted in F2 from a wild cross, of
mean grade 3.17, a falling off in grade of over 0.50. A second cross
with wild showed no further falling off, but instead a movement in
the reverse direction to 3.34 (a movement probably not significant,
in the light of further experiments). A third cross with wild has
been made on a small scale; 19 hooded grandchildren extracted in F2
from this third cross have a mean grade of 3.04. (See table 9.) It
seems probable, therefore, in the case of the plus-selected hooded char-
acter, that the maximum effect exerted by the residual heredity of the
wild race is to reduce the hooded character in grade by about three-
fourths of a grade. Selection in 10 previous generations had elevated
the grade of the hooded character by about If grades. A cross with
wild eliminated less than half of this change. The remaining change
must be ascribed to changes effected in the course of selection.

The minus-selected hooded race has also been crossed with this
same wild race. Originally of grade -1, it had been altered by 15
generations of selection to the extent of about 1^ grades, to mean
grade —2.54. Females of generations 15 to 16 (table 10) and of

MODIFIABILITY OF HOODED CHARACTER 6

grade —2.75 were crossed with wild males of the same race used in
the crosses of the plus-selected race. The extracted hooded F2 young
were highly variable, ranging in grade from -2.25 to +3.00, mean
— 0.38, a remarkable change in the plus direction of over 2 grades.

A second cross with the wild race (table 11) brought about a further
movement of the mean in a plus direction but by a somewhat smaller
amount, 1.39 grades, the mean of the twice-extracted hooded young
being +1.01. A third cross with the wild race (table 12) brought
still further contamination of the hooded character, which now ceased
to vary below grade +1.00, and had a mean of +2.55 in the case of
over 100 thrice-extracted hooded young, this being a change in the plus
direction of 1^- grades. It will be observed that the hooded grand-
children of 0^2068, table 12, the most plus in character of the hooded
grandparents, are very similar in grade to the hooded young resulting
from three crosses of the plus-selected hooded character with the
same wild race (table 9). In other words, the same wild race, when
its residual heredity is made fully effective by repeated crosses, brings
both the plus-selected and the minus-selected hooded lines to a pheno-
type of common grade. This shows, contrary to my earlier opinion,
that what has really happened in the case of the selected races was
more largely due to residual heredity than to any change in the gene
for the hooded character itself. My critics have been wrong when
they insisted that selection could not change racial characters that
mendelize and change them permanently, and when on this ground
they denied to gradual change through selection an important part
in the evolution of characters and thus of races. But my critics have
been right when they insisted that evidence is wanting that change
in single genes occurs other than spontaneously, uninfluenced by sys-
tematic selection.

II. THE INHERITANCE OF WHITE-SPOTTING IN RABBITS,

WITH SPECIAL REFERENCE TO GAMETIC

CONTAMINATION.'

One of the commonest color variations of mammals is white-
spotting — the occurrence of wholly unpigmented areas in the skin and
the hair arising from it. Small unpigmented areas are frequently
found in the coats of wild mammals, as, for example, in the fur of wild
mice, rats, the common North American rabbit (Lepus sylvaticus),
and cavies (e.g., Cavia cutleri). The white-spotting found in these
wild forms is usually not extensive. It consists of a white "star"
in the forehead or a spot on the chest, or at the end of the tail, or on
a foot. Such locations of the white-spotting suggest a deficiency of
pigment in the skin, either where it closes together in the median line
during development of the embryo or at the extreme limits of its
peripheral extension during development. At places where the skin
regenerates after injury, even in self-colored animals, a white spot
frequency develops. This is especially noticeable on the backs and
shoulders of horses where the harness has "galled" them. That such
slight congenital deficiencies of pigment as occur in wild mammals
are hereditary has been shown by Little in the case of the house-
mouse, and by Phillips and myself (unpublished observations) in the
case of the field-mouse, Peromyscus. We observed in a colony of
Peromyscus reared from animals taken in Massachusetts the occur-
rence of individuals having tails partly or wholly white. This con-
dition was found to be a Mendelian recessive character in crosses.
After one or two selections of white-tailed individuals, we noted exten-
sion of the white area on to the belly.

In some wild mammals the white-spotting is more extensive, taking
the form of a definite pattern, as in skunks, the harp-seal, and the
Malay tapir. The color pattern of skunks, while characteristic, is
known to vary slightly, the value of a pelt increasing with the amount
of black which it contains, a fact which the incipient industry of
skunk-farming in the United States notes with interest. Selective
breeding is being directed toward the establishment of all-black
strains and no doubt it will ultimately be successful.

White-spotting is so common in the domestic animals as to need
no comment. White-spotting in more or less definite patterns char-
acterizes the majority of our breeds of cattle, horses, dogs, and swine.
Often the pattern is so definite and so strongly inherited as to con-
stitute a sort of trade-mark of breed purity, as in Hereford (white-
faced) cattle, Dutch belted cattle, and Dalmatian coach-dogs. Much

1 Valuable assistance in the conduct of this investigation was given by my former pupil, Prof.
H. D. Fish.
4

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 5

interest attaches to the inheritance of these patterns in crosses, a
subject which has been studied for some years at the Bussey Institu-
tion. The present paper will deal with the subject of white-spotting
in domestic rabbits.

Patterns of white-spotting mendelize without known exception
but with some irregularity as regards dominance. In some cases
white-spotting is not expressed in the heterozygote, but if expressed
at all in the heterozygote its expression is always stronger yet in the
homozygote. When the character is nearly or quite suppressed in
the heterozygote, we may call it recessive; when the character is
strongly expressed in the heterozygote, we may call it dominant. But
neither term is applicable without qualification in the way that
recessive is applicable to complete albinism in rodents.

With this qualification of terms, it may be said that there occur
among domestic rabbits two forms of white-spotting, probably of
independent origin and certainly of quite different genetic behavior,
since one is recessive and the other dominant in crosses with the
same race of unspotted rabbits. The dominant form of white-spotting
is found in the so-called English rabbit and its inheritance has been
discussed by Castle and Hadley (1915). The recessive form of white-
spotting is found in Dutch rabbits, as observed independently by
Hurst and by Castle. Punnett assents to this conclusion with the
qualification that the inheritance is possibly not that of a simple
(one-factor) sort.

In September 1910 three standard-bred Dutch rabbits were obtained
from a fancier who had bred and exhibited prize-winning animals
derived from stock imported from England. They resembled grades
7, 8, and 9 respectively (plate 1). The female proved to be sterile
and was ultimately discarded. One of the males (c?3037, grade 7)
was mated with two heterozygous English does, which produced
self-colored blacks as recessives when mated with English bucks of
their own race. For the present we shall consider only the non-
English young produced by these matings. Such young would be of
the same character as those produced by self-colored ani-
mals mated with Dutch, since they would arise from a self
gamete furnished by the mother and this would be fer-
tilized by a Dutch gamete furnished by the father. Six
young of this character were produced of the accom-
panying grades.

This same male was mated also with 3 Himalayan albino
does of a race entirely free from spotting but which
lacked the color factor. Potentially these does were self-
colored. This cross produced 18 young of the accom-

panying grades. (See table 13)!

1 Grade 0 signifies a self animal, i.e., one without white spotting.

6 HEREDITY IN RABBITS, RATS, AND MICE.

Two does derived from the first-mentioned cross and one derived
from the second were employed in various matings presently to be
described. The results observed in the case of all three were so similar
that they may conveniently be described together. It will be borne
in mind that all are F: hybrids between Dutch and self.

When crossed back with the other original pure Dutch buck ( cf 3036,
grade 9), these three does produced 20 young of the grades shown in
table 14. We get here indications of segregation into two groups, one
like the FI mothers in grade, the other like the Dutch father, but no
sharp line of division separates the two.

The same three F! females were also mated successively with an
FI male from each of the two crosses already described, with the
results shown in table 15. The results are similar in both cases, but it
will be noticed that the lower grade FI male (5029, derived from the
Himalayan cross) produced F2 young of slightly lower grade. The
F2 range extends from 0 to grade 5 inclusive, average 1.80. The back-
cross range was from 1 to 7 inclusive, average 4.60.

Certain of the F2 young and the back-cross young of grade 4 or
higher, wThich presumably would be homozygous for the Dutch char-
acter, if it mendelizes, were employed in building up a race of Dutch
rabbits for further study. This was done by back-crossing the selected
does a second time with the pure buck, cf3036, grade 9, with the
results shown in tables 16 and 17. Young were obtained which
ranged from grade 1 to grade 17, but which grouped themselves round
two modes situated at about grade 6 and grade 15 respectively. We
shall presently consider the distribution further.

These same does were also mated with a male similar in origin to
themselves, viz, d7|5167 (table 18), a typical and evenly marked
Dutch buck of grade 7, produced by the original back-cross (table 14) .
He bred in all respects like his father (cT'3036, grade 9) when mated
with the same does, producing a bimodal group of Dutch young of
only slightly lower mean grade than the young which his father sired,
as might be expected from the fact that his grade was less than his
father's grade. (See table 18.)

It was now evident that we had secured a race of Dutch rabbits
which produced only Dutch young and which derived their Dutch
character exclusively from the two bucks 3036 and 3037, and yet
which fluctuated in grade around two different modes. In fact, it
was soon discovered that the two original Dutch bucks were them-
selves heterozygotes of two different types of Dutch pattern which
corresponded with the two modal conditions found among their
descendants. Our next task was to isolate these two types in
homozygous form. This was easy in the case of the higher grade
(whiter) type, which proved to be recessive. A male of this "white"
type, 6175, grade 17, when mated with females of the same sort

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 7

produced only one type of Dutch young varying around grade 16.
(See table 19.)

By studying the results of various matings of our Dutch does it
was found possible to classify them in three categories: (1) The
"white" type of grade 15 to 17, which produced only the "white"
type when mated with bucks of the same sort, as already described.
(See plate 2, fig. 19.) (2) A "dark" type of grade 1 to 7, which when
mated with bucks of the white type produced no "white" offspring,
but only those of the dark type. These mothers were evidently homozy-
gous "dark," the other pure type (see plate 2, fig. 20). (3) The third
type of doe is scarcely distinguishable from the pure dark type except
by breeding test. It consists of heterozygotes between the two types,
a little whiter on the average than the dark type, but not conspicu-
ously so. When they are mated with "white" bucks, young of two
types are produced in about equal numbers, viz, heterozygous darks
and pure whites. The original Dutch bucks from which the entire race
was derived were both of this heterozygous dark type.

Neither the dark type nor the white type isolated from this race of
rabbits conforms closely with the ideal Dutch type of the fancier (our
grade 8). The one is usually too dark and the other too white. It
seems probable that the fanciers in breeding "prize- winners" have
consciously or unconsciously been producing heterozygotes, very much
as in the case of the Andalusian fowl. Certain it is that all the rabbits
which we have produced from this stock, which would have any
chance of winning a prize at an exhibition, have been heterozygotes
between these two types.

While the experiments with standard-bred fanciers' Dutch rabbits
were in progress, and after the white and the dark types of Dutch had
been isolated, a third type of Dutch was discovered which kept crop-
ping out hi a stock of black-and-tan rabbits under observation for
another purpose. This stock was derived from a single pure-bred
black-and-tan buck which had been crossed with various other stocks
of rabbits then in the laboratory. The Dutch pattern had been
introduced as a recessive character in a certain yellow rabbit of
unknown pedigree obtained by purchase. When the descendants of
this yellow rabbit were bred with each other, certain of the young
produced were Dutch marked. This type of Dutch resembled the
fanciers' type of Dutch (grade 8, plate 1) so far as the head markings
were concerned, but the belt was very narrow and placed far forward
over the shoulders. (See plate 2, fig. 21.) Because of its origin within
the black-and-tan stock we have adopted the name "tan" Dutch to
distinguish it from the other two types.

In describing its variations we have used the same set of grades
(shown in plate 1) which were used in classifying the variations of
the other two types, but it must be understood that rabbits of the

8 HEREDITY IN RABBITS, RATS, AND MICE.

dark and of the tan type which are given the same grade are not
exactly alike in pattern. The grade is assigned to express roughly
the total amount of white-spotting on the animal. A rabbit of grade 3
in the tan series will usually have a whiter head with a broader white
spot on the nose but with a narrower collar than one of the same
grade in the dark series. With this explanation it may be stated that
tan Dutch rabbits bred with each other have produced 40 Dutch
young with the grade distribution shown in table 21.

The variation is close about grades 3 and 4, the race being very
uniform in character for a white-spotted race. In origin it is derived
from a single gamete which introduced the character as a recessive
into the original yellow ancestor.

Having now secured 3 distinct strains of Dutch rabbits, it was our
next task to determine what were their genetic relationships to each
other, whether they were allelomorphs or due to wholly independent
factors; whether due to single or to multiple genetic factors, and
whether these factors were constant or variable. Before these ques-
tions can be intelligently discussed the variability of each type by
itself must first be known. That of the tan race has just been referred
to. It is shown in table 21. It will be observed that the mean grade
of the young has a tendency to rise with the grade of the parents.

The variation of the uncrossed "dark" race is shown in table 20.
The same homozygous buck (6701, plate 2, fig. 20) was mated with ten
different homozygous dark does ranging in grade from 2 to 5. They
produced 172 young ranging in grade from 1 to 7, mean 3.30. The
variation is shown graphically in text-figure 1, D. The higher-grade
mothers, it will be observed, produce higher-grade young, although
the differences are not striking.

The variation of the uncrossed "white" race is shown in table 19
and text-figure 1, w. Two bucks of grade 17 were mated with 8 does
of grade 15, 16, or 17; they produced 59 young with the same range
of variation as the mothers and of the mean grade 16.25. Again we
observe a tendency for the higher-grade mothers to produce the
higher-grade young.

The same homozygous white buck (6175) which was used in matings
recorded in table 19 was mated also with 5 homozygous does of the dark
race, with the results shown in table 22 and text-figure 1, Fi,DxW.
These matings produced 28 young of mean grade 7.28. All the young,
from their parentage, should be heterozygotes between white and dark
Dutch, like the original animals from which these two races were
isolated. In reality they agree closely with the foundation stock in
grade.

Table 23 shows the results of matings of homozygous white bucks
(one of which was the same individual, 6175, as sired the young of
tables 19 and 22) with dark does which were heterozygous for white

INHERITANCE OF WHITE-SPOTTING IN RABBITS.

9

and so were like the foundation stock. The young thus produced
numbered 130 and fall into two distinct groups, each varying about a
different mode. The two groups apparently do not overlap. Taking
the minimal class, grade 12, as on the line between them, there are
65 individuals below this class and 64 above it. The former should be

No.
50-

40-

30-

20-

10-

0
10-

10-

20-

W

F,, DxW

F,,DxW

•

Grade, 0 I

4 5

7 8 9 10 II 12 13 14 IS 16 17

TEXT-FIGURE 1.

heterozygotes like the young recorded in table 22. Their mean grade
is very similar, 7.04 as against 7.28. The latter are evidently homozy-
gous white like the young of table 19. They have a similar but slightly
lower mean grade, viz, 15.56 as against 16.25. This result indicates a
1 : 1 segregation of white and dark Dutch and that accordingly white
and dark are allelomorphs. If so, an F2 generation should show
a 3 : 1 segregation. To test this point we may summarize from

10 HEREDITY IN RABBITS, RATS, AND MICE.

tables 16 to 18 all mat ings between two heterozygous parents (those
which produce both dark and white young). In this way we get the
totals shown in table 28, "white X dark." Two groups of young are
here shown, one dark (grade 10 or lower), the other white (grade 12
or higher) . The numbers of individuals in these groups are 56 and 25
respectively (a rather poor 3 : 1 ratio); their mean grades are 5.82
and 14.40. (See text-figure 1, F2, DXW.) The dark group evidently
includes homozygotes (mode on 5) and heterozygotes (mode on 8)
which overlap in the intervening region. Accordingly all facts thus
far noted indicate that white and dark are allelomorphic forms of
Dutch marking.

One other test of this hypothesis is possible. FI may be back-
crossed with the dark race. The result of such a test is shown in
table 29, "Fi (white X dark) X dark," and text-figure 1, FI X D.
The expectation here is the formation of two groups of equal size,
homozygous and heterozygous dark, with modes on 3 and 7 respec-
tively. In reality the intervening grades are the modal ones. This
does not disprove segregation. The flat-topped variation curve ob-
served is exactly what we might expect from the combination of two
simple variation curves which overlap. Compare F2, SxW, text-figure 2.

The several facts developed as regards the relation of "dark" to
"white" Dutch pattern are presented graphically in text-figure 1.
The variability of the uncrossed races is shown at the top; the races
are distinct, monomodal, and do not overlap in range. Immediately
below is shown the character of FI. It also is monomodal and it is
intermediate. Next lower is shown the variability of F2. The extracted
white race is here seen to have a lower mode than the uncrossed white
race and the extracted dark race has a higher chief mode than the
uncrossed dark race. In other words, the extracted races reappear
in F2 in character mutually modified, although their extreme range is
the same. The mode of the heterozygotes lies between the modes of
the homozygotes, as in FI just above.

The results of back-crosses with each of the parental races are
shown in the lower part of text-figure 1. In the back-cross with the
white race (FiXW), the 1 : 1 segregation is unmistakable; in the
back-cross with the dark race (FiXD), segregation is obscured by the
closeness to each other of the modes for heterozygous and homozygous
dark Dutch, which results in producing a composite fiat-topped curve.
Everything indicates that dark and white are simple allelomorphs, but
are quantitatively fluctuating and mutually modify each other in
crosses. If they were not allelomorphs, individuals which contained
neither form of Dutch should appear in F2. None such is produced.

We may next consider whether white Dutch is a simple allelomorph
of self or is composite. The pertinent facts are recorded in tables 27
to 29 and are shown graphically in text-figure 2. The same white

INHERITANCE OF WHITE-SPOTTING IN RABBITS.

11

male (6175) that was crossed with dark females was also crossed with
self-colored (unspotted) females. (See table 27.) There were pro-
duced 33 young, all showing a small amount of white-spotting, ranging

Ko.
20-

10-

^

0

20-

10-

40-

30-

20-

10-

20-

10-

60-

50-
40-
30-
20-

F,,SxW

F2,SxW

F, xW

F,.xS

v/

Grade 0123456789 10

TEXT-FIGURE 2.

12 13 14 15 16 17

in grade from 1 to 3, mean 1.51 (see text figure 2, Fi, SxW). F2 is
somewhat puzzling in character. (See table 28 and text-figure 2.)
Nearly 25 per cent of the 191 young recorded are extracted selfs, but

12

HEREDITY IN RABBITS, RATS, AND MICE.

if we take the whitest 48 individuals to be extracted " whites" it is
evident that they are considerably modified from the condition of the
uncrossed race, since their mode lies at 12 to 14, not at 16 to 17, and
the highest grade (17) of the uncrossed white race does not reappear
at all. The back-cross of FI with white must be investigated before
one can interpret the F2 result with confidence. But the back-cross
(table 29 and text-figure 2, Fx X W) makes it very clear that segrega-
tion occurs on a 1 : 1 basis. The 116 young thus produced fall into
two groups which do not overlap and each of which is monomodal.
Each contains 58 individuals. But the ex-
tracted groups show mutual modification. The
mode of the lower group is not at grade 1, as
in FI, but at grades 4 to 6, while the mode of
the upper group is not at 16 to 17, as in the
uncrossed white race, but at 15.

The back-cross of FI with self (table 29 and
text-figure 2, FI X S) gives a variation nearly
covering the combined range of self and FI, as
expected, but the two expected groups (if they
are distinct) lie so close together that it is
impossible to separate them.

The several facts developed in crosses of
white with self indicate that white is a simple
allelomorph of self. If so, and if dark is also
an allelomorph of white, as indicated by text-
figure 1, then self and dark should be allelo-
morphs of each other. Such is probably the
case, but crosses of self with dark are incon-
clusive because the two conditions are so
close to each other on the grading scale that
it is difficult to demonstrate segregation. See
tables 27 and 29 and text-figure 3. This figure
shows (at the top) the variation of uncrossed dark in relation to self.
Below is shown the variation of FI, self being usually dominant. A
back-cross with dark (Fi X D) produces less than the expected propor-
tion of self (nearly 50 per cent as indicated by FI) and produces extracted
darks of lower mean grade than the uncrossed darks. This is evidence
of mutual modification of self and dark in the heterozygote, so that
they emerge in the gametes modified. So far, then, the evidence indi-
cates that self, dark, and white are all allelomorphs, but that they
fluctuate quantitatively and mutually modify each other when asso-
ciated in heterozygotes.

We may now consider the relation of these three to tan Dutch. If
tan is an allelomorph of either of the other forms of Dutch, it should
be an allelomorph of all three conditions, in fact a, fourth allelomorph.

Grade

0123
TEXT-FIGURE 3.

4-567

INHERITANCE OF WHITE-SPOTTING IN RABBITS.

13

To test this matter, crosses of tan have been made with each of the
other three, white, dark, and self. Tan was found to be allelomorphic
with self in the black-and-tan race where it appeared (table 12), but
self being a purely negative term (meaning as here used unspotted)
one could not be sure in advance that self, as the allelomorph of
different kinds of Dutch, would be one and the same thing in all
cases. This would require demonstration. It is necessary, then, to
ascertain first whether tan Dutch is an allelomorph of white and of
dark.

Crosses of tan with white have given the results shown in tables 26,
28, and 29, and also graphically in text-figure 4. FI is intermediate.
F2 is likewise interme-
diate, but varies to or
into the range of the
uncrossed races with
indications of segre-
gation of modified tan
and modified white
individuals. The ab-
sence of selfs in F2
shows white Dutch
and tan Dutch to be
allelomorphs. The
back-cross with white
(FiXW) produces in-
dividuals varying (as
expected) all the way
from the Fx to the un-
crossed white race,
but with an apparent
tendency to form&rada
modes on 10 and 15.
These, it seems, represent the mutually modified modal conditions of
the FI and the white race. The extracted whites have their mode low-
ered to 15, instead of on 16 or 17, as in the uncrossed white race; and
the extracted FI'S have their mode advanced from grade 7 (in the orig-
inal FI'S) to grade 10.

The races dark and tan are similar to each other in grade (see text-
figure 5), but differ in the location of their unpigmented areas, as
already explained. Dark has a wider collar and a darker head; tan
has a narrower collar and a whiter head. FI is nearly or completely
self-colored, since dark tends to make the head pigmented and tan to
make the collar pigmented. Consequently there is little space left
unpigmented. F2 is quite variable, some individuals being darker
than either uncrossed race, while others are whiter. The range is from

13 14- 15 IS 17

TEXT-FIGURE 4.

14 HEREDITY IN RABBITS, RATS, AND MICE.

grade 0 to grade 11. (Text-fig. 5, F2, DxT.) Indications of segregation
are plainly seen in F2, but it is impossible to be sure of the number of
factors involved because dark and tan are so similar in grade. But we
know that dark is an allelomorph of white. If it is also an allelomorph
of tan, then Fi individuals should produce dark gametes and tan game-
tes in equal numbers. These, in a cross of FI with white, uniting with
white should produce two kinds of heterozygotes : (a) white-dark hete-
rozygotes like those of text-figure 1, Fi, and (6) white-tan heterozygotes
like those of text-figure 4. The former range from grade 5 to grade 11,
the latter from grade 6 to grade 9. The ranges are similar and the
mode of each group is on grade 7. Mutual modification of dark and
tan would tend to extend the range of segregates in both directions,
as observed in F2 (text-figure 5).

The observed back-cross generation (table 29 and text-figure 5,
FI X W) is distinctly bimodal, as the hypothesis just formulated
(that dark and tan are allelomorphs) would demand. Indeed, the
evidence of 1 : 1 segregation are clearer than we should expect, but
the modes of the expected groups are farther apart than we should have
expected. (See text-figure 5.) The lower mode is at 4, not at 7 as in
the FI produced by dark crossed with white; and the upper mode is
at 12 to 14, not at 7, as in the FI of tan crossed with white. It appears,
therefore, that while dark segregates from tan in the gametes formed
by FI individuals, each segregates in an altered form, the dark having
become darker and the tan lighter as a result of their association in
the heterozygous FI. This is indicated both by the bimodal condition
and increased range shown by the cross of FI with white, as just described,
and also by the wide range of the F2 from the cross of dark with tan.

An alternative hypothesis, which has been given careful considera-
tion and which in fact the cross of FI with white was especially designed
to test, is this : that dark and tan are due to independent factors and
that the two together constitute white. With this hypothesis the
following facts harmonize : Dark crossed with tan produces in FI and
Fo individuals which are self-colored (i.e., which have no white-spotting)
as well as others which are whiter than either the dark or the tan
race. The former may be interpreted as those which lack (or are
heterozygous for) both dark and tan; the latter as those which have
both dark and tan. But it should be observed that none of the 275 F2
young which have been produced extend into the range of the uncrossed
white race, where about 25 per cent of them should lie if the hypothesis
is correct. Compare text-figures 1 and 5.

Another way of testing the hypothesis that dark and tan are due
to independent Mendelian factors is the cross of FI with white. If the
hypothesis is correct, FI individuals should produce four kinds of
gametes, viz, those which will transmit (a) both dark and tan, (6)
dark alone, (c) tan alone, and (d) neither dark nor tan. By the hypoth-

INHERITANCE OF WHITE-SPOTTING IN RABBITS.

15

esis white is dark plus tan. Therefore, white crossed with FI should
produce four kinds of zygotes: (a) dark and tan united with white
(dark and tan), equivalent to homozygous white; (6) dark united

No
50-

40

30-

20-

20-

10-

0
90-

80-
70-
60-
50-
40-
30-
20-
10-

•

F,,DxT

2. DxT

50-

30-

20-

10-

20
10

10-

20-

20-

10-

L

Fi,DxT

F, xT

F, xD

F, xW

Grade 0 I 2 3 4 5 6 7 8 9 10 II

0 I 2 3 4 5 6 7 8 9 10
TEXT-FIGURE 5.

12 13 14- 15 16 17

with white; (c) tan united with white; and (d) self (neither dark nor
tan) united with white. The observed modal values of these four
kinds of combinations are respectively, in terms of grades: (a) 16

16 HEREDITY IN RABBITS, RATS, AND MICE.

(text-figure 1, W); (6) 7 (text-figure 1,^); (c) 7 (text-figure 4, Fi);
and (d) 1 or 2 (text-figure 2, FI). That is, this cross should, under the
hypothesis considered, produce a trimodal figure, with a mode for
25 per cent of the individuals at either end of the grading scale and
with a still larger mode (for 50 per cent of all individuals) at an inter-
mediate point. But what is actually observed is very different. The
figure (text-figure 5, FI X W) is not trimodal but bimodal, and neither
of the modes is where the hypothesis demands that modes should be.
This is conclusive evidence against the correctness of the hypothesis
in question, but is entirely in harmony with the alternative one, that
dark and tan are allelomorphs but segregate in modified form, one
on the whole darker, the other on the whole lighter than before they
were crossed with each other.

This matter of modification on crossing is one deserving further
consideration. It is in evidence in all the crosses made. It is clearest
where the races crossed differ most in grade, and it seems to consist
in a partial obliteration of those differences. Thus the uncrossed
white race has its mode at 17, the self race at 0. (See text-figure 2.)
In F2 the extracted whites contain no individual as high in grade as
17, and the highest mode lies at 12 to 14, facts which indicate that
white has been lowered in grade by its association with self in the FI
zygotes. The back-cross of FI with white also shows modification but
intermediate in amount, as might be expected from the fact that, in
the case of each zygote formed, only one of the two conjugating
gametes had been subjected previously to modifying influences, viz,
that one which was furnished by the FI parent. The mode in this
case lies at 15, instead of at 17, as in the uncrossed race, or at 12 to 14,
as in the F2 extracted whites.

That the extracted white has less influence in whitening an FI zygote
than the uncrossed white is shown further by a comparison of FI self X
white (text-figure 2) with the back-cross of FI with self. FI had its
mode at grade 1 and ranged upward to grade 3, but contained no self
individuals. If extracted white were identical with uncrossed white in
its whitening influence, then in the back-cross with self half the zygotes
should be of grade 1 or higher. But in the observed back-cross less
than one-third of the zygotes show any white, viz, 29 out of 94, and these
are lower in grade than the original FI'S, viz, 1.10 as compared with 1.51.
The self character also emerges modified after the cross with white.
For the FI zygotes, consisting of pure self united with pure white
(text-figure 2, FI, S X W) were all close to self in grade, with a mode
on grade 1 and ranging upward only to grade 3. But the zygotes
formed by extracted self united with pure white produced in the back-
cross of FI with white (text-figure 2, F: X W) range in grade from 1 to
9 with a broad low mode at 4 to 7. Evidently they have been much
increased in grade in the direction of white.

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 17

Again, in the crosses of dark with white (text-figure 1), we see the
mutual modification of the contrasted conditions taking place. Un-
crossed dark has its mode on grade 3, and uncrossed white on grade 17.
Extracted white as seen in F2 has its mode on grade 15, but in the back-
cross with pure white it has its mode on the intermediate grade, 16.

As regards the modification of dark, it will be observed that the
original Fi individuals are of lower average grade than the lower group
of individuals produced by the back-cross with white. The mode of
the former also is on grade 7, that of the latter is on grade 8. The one
consists of pure dark united with pure white, the other of extracted
dark united with pure white. The extracted dark has evidently been
whitened, exactly as extracted white has been darkened.

Uncrossed tan and uncrossed white have their modes on 3 and 17,
respectively (text-figure 4). The mode of FI is on grade 7, but the
mode of extracted tan united with white is on grade 10, as seen in the
back-cross of FI with white. This shows that extracted tan has been
whitened as compared with uncrossed tan. That white has had its
grade lowered by the cross with tan is also shown in the back-cross
of FI with white. Its mode lies at 15, not at 17 as in uncrossed white.
If, in the several cases considered, crossing tends to mutual modi-
fication and assimilation to each other of the contrasted conditions
brought together in the cross, why does crossing of dark with tan
extend rather than shorten the range of variation, producing in F2
individuals darker than either uncrossed race and others lighter than
either uncrossed race? The answer to this question is perhaps to be
found in the imperfection of our scale of grades. The scale is a linear
one, whereas the variation is not entirely linear; for dark Dutch and
tan Dutch have the same modal grade, 3, yet are different in somatic
character, as has already been stated. Tan Dutch has a white head and
narrow collar, dark Dutch has a dark head and wider collar. In FI, if
pigment simply dominates whiteness, we may expect to get a dark head
and a narrow collar simultaneously, i. e., a condition with less white
than either parent possessed, which in general is the result observed.
Since uncrossed dark Dutch varies down to grade 1 and uncrossed
tan down to grade 2, an extension of the dark areas due to crossing
naturally carries the pigmentation in Fx down to grade 0 (self) in a
certain percentage of cases (1 in 6 observed). See text-figure 5.
In F2 the percentage of selfs is still larger, being about 1 in 3. There
are also found in F2 whiter individuals than either uncrossed race
contained. How these have arisen is indicated by the back-crosses of
FI with dark and with tan respectively. The back-cross with dark
(text-figure 5, FI X D) produces a monomodal group closely resembling
the FI group, but with slightly higher range. This shows that the
potent factor in lowering the FI range was the dark gamete, since it
is the only common factor entering into both crosses. On the other

18 HEREDITY IN RABBITS, RATS, AND MICE.

hand, the back-cross with tan (Fi X T) shows bimodal variation with
the modes at 0 and 5 respectively. This indicates that the gametes
formed by FI are really of two types, extracted dark and extracted
tan. The former uniting with pure tan produces a group like FI but
apparently of even slightly lower grade, since the modal condition is
now 0, not 1. The extracted tan gametes uniting with pure tan pro-
duce a group like pure tan, but of apparently higher grade, since the
mode is now on 5, not 3 as in pure tan. Hence extracted tan is poten-
tially of higher grade than pure tan, a conclusion supported by F2
from dark crossed with tan, for here we observe that extracted tan
meeting extracted tan produces zygotes of grade 5 to 8 or even higher,
whereas pure tan does not exceed grade 5. Yet, to return to the imper-
fection of our grading scale, these higher grades consist merely in
combining a wider collar with the same form of head markings as
are found in the tan series of grade 4 or 5. Hence it appears that tan
is regularly modified through its contact with dark in an FI zygote in
the way of acquiring a wider collar, whereas dark is modified by the
same agency in the way of acquiring a narrower collar. Yet there is
no indication that head marking and collar marking are due to dis-
tinct single genetic factors, but merely that they are qualitatively
different. This difference tends to disappear through mutual influ-
ence in the heterozygous condition, but the difference disappears more
rapidly in collar markings than in head markings; hence the extended
range of grades in F2. Because their collars become more alike the
extracted darks rank lower in grade and the extracted tans rank
higher; for it will be recalled that the uncrossed darks, though having
wider collars than the tans, were graded lower on account of their
dark heads. The collar changes, then, are actually blending in this
cross, as in all the others studied, but give the appearance of segrega-
tion with differences emphasized, merely because of the inadequacy
of our linear grading scale to record simultaneously changes in head
and in collar markings when these occur with unequal rapidity.

It has already been shown that we have conclusive evidence that
FI, from the cross of tan with dark (text-figure 5), produces two types of
gametes, not four types, this evidence being (a) the bimodal variation
seen in the back-cross of Fx with tan and (6) the bimodal variation
seen in the cross of FI with white. These results indicate that tan
and dark are to be regarded as allelomorphic but mutually modifying
conditions, as had already been found to be true for tan and white,
for dark and white, and for self and white. We have, then, a condition
of multiple allelomorphs in white-spotting patterns of Dutch-marked
rabbits, which includes the forms self, dark Dutch, tan Dutch, white
Dutch, and possibly many other types or conditions of white-spotting
which with sufficiently accurate observation might be distinguished
from each other.

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 19

The foregoing observations show unmistakably that the several
members of this allelomorphic series tend, as a result of crosses, to
become more like each other. This has been described as mutual
modification, but it should be expressly stated that in the light of
our experiments with rats "modification" need not be regarded as
change in the nature of a single gene, but merely as equalization of the
residual heredity additional to the single genes which produce mono-
hybrid ratios.

ENGLISH.

In November 1909 there were received at the Bussey Institution
four " English" rabbits, 1 male and 3 females, bred by R. W. Wills,
of Hornerstown, New Jersey. In terms of the grading scale shown in
plate 3, the male was of grade 2-j-; the females were of grades 2, 2-f,
and 3, respectively.

In matings of the male with each of the 3 females, there were pro-
duced both English and self-colored young, as shown in table 30;
of the former, 21 ; of the latter, 8. The self young were later found to
produce no English young when bred inter se. Hence it seems clear
that English is a dominant Mendelian character, that self is recessive
in relation to it, and that the 4 English parents were all heterozygous
dominants.

The question now arose whether homozygous English rabbits could
be produced and why English rabbits were not regularly bred in
homozygous form. We did not have long to wait for an answer to
these questions. Table 30 shows that the English young of our 4
original English rabbits fall into two groups quite different in appear-
ance. Of the 15 young which were graded, 5 were of grade 1 or 1^,
while 10 were similar to the parents in grade, varying from grade 2
to 3. The group of low-grade English was found to consist of homo-
zygous individuals which produced only English young in crosses
with each other or with selfs. The higher-grade group, twice as num-
erous in individuals, was found to consist of heterozygotes. These
are preferred by the fancier because of their much more striking color-
pattern. The homozygote is in appearance only an impure white
animal, but the heterozygote is beautifully mottled. It is therefore
clear why the fancier breeds heterozygotes. (See plate 3.)

Our original English buck, 2545, was also mated with self-colored
does of several different sorts, viz, gray, cream, yellow, sooty yellow
(tortoise), black, and black-and-tan. In regard to color inheritance,
these matings gave us such results as are already familiar through
earlier publications by Punnett, Hurst, and ourselves. We may
therefore confine our attention to the behavior of the English pattern
in crosses. Table 30, B, shows the results obtained; 26 English and
18 self young were recorded from these matings. No grade was
recorded for 17 of the English young. The others varied in grade

20 HEREDITY IN RABBITS, RATS, AND MICE.

from 1-^- to 4, the mean being 2.80. Evidently the heterozygous
English produced by these matings with unrelated does were much
more variable than those produced by matings of the original English
individuals with each other.

It now occurred to us to see to what extent this variability could
be carried farther in a plus direction by selection. Accordingly we
chose the grade 4 individual produced by 9 1492 as the starting-point
of the experiment. This individual had been recorded as c?2711.
He constitutes generation 1 of the selection experiment now to be
described, all animals produced in those experiments deriving their
English from him. He was mated with a black-and-tan doe, with
three black does, and six Himalayan (albino) does, all free from
racial white spotting (English or Dutch). These matings produced
56 young (table 31, B) equally divided between heterozygous English
and self. The English young were of higher grade than the English
young produced by 0^2545. (Compare tables 30 and 31.) They
ranged from grade 3 to grade 5, mean 3.89, as compared with a mean
of 2.80 for the young of c?2545, produced in similar matings.

Male 2711 was later mated to 5 of his heterozygous English daugh-
ters produced in the matings already described, and also to one of the
resulting grand-daughters. The character of their young is shown
in table 31, A. The mothers form generations 1-|- and 2^ of the selected
English race. They vary in grade from 3^ to 5. They produced three
classes of young: low-grade English (homozygotes), high-grade
English (heterozygotes), and selfs. Their respective numbers were
12, 11, and 8. The mean of the low-grade English group was 1.52,
that of the high-grade group was 3.93. which agrees closely with the
grade of the same group produced by matings with non-English (self)
does (table 31, B).

Grouping the mothers by grade, the relation shown

Mother.

3.50
4.75
5.00

Mean of

3.83
3. SO
4.25

young,

herewith is observed between grade of mother and
grade of young. This indicates that selection of
higher-grade parents would probably result in produc-
ing higher grade young.

A son of d" 2711, viz, c?50S6, generation 1J, grade 4-g-, was chosen
to succeed him in the selection experiment. This buck was mated
with 10 different heterozygous English does, 5 of which had also been
mated with his father. (See table 32.) There resulted 20 homozygous
English young, 44 heterozygotes, and 35 selfs. The mean grade of the
homozygotes was 1.38, that of the heterozygotes was 3.96, averages
not very different from those which characterized the young of cT2711
(table 31). Consequently no advance can be claimed as a result of
the selection of cT50S6. He was mated also with 3 black-and-tan
does (table 32, B), producing thus 10 English young of mean grade
3.15, a lower average than that given by c?2711 in matings with self

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 21

does, but it must be borne in mind that the mothers were not all
identical in the two cases. Black-and-tan does seemed in general to
give lower-grade offspring than does of the self black and Himalayan
races, with which c?2711 had been mated.

We next used as sire in the selection experiment cf 5375, generation
2|, grade 4£, a son of c? 5086 by his half sister. (See table 33.) He
was mated with 9 different heterozygous English does, all but one of
which had also been mated with his father. They produced English
young of somewhat lower mean grade than they had borne by the
father, c?5086. (See tables 32 and 33.) 17 homozygous English
young were of mean grade 1 .20; 29 heterozygous English young were of
mean grade 3.79; there were also 25 self j^oung. Again the higher-
grade mothers produced the higher-grade young. Hence there was
evidently material favorable for selection among the mothers, if not
among the fathers. This male was now discarded and replaced by
an own brother of slightly higher grade, viz, c?5555, generation 2-J-,
grade 4f . (See table 34.)

This male (d"5555) was bred more extensively than any of his
predecessors and produced higher-grade offspring. He was mated to
the same does as his father and grandfather and also to a number of
new ones which now became available. By all classes of does he pro-
duced higher-grade young than had any of his predecessors. He also
produced higher-grade young by his mates of higher grade than by
his lower-grade mates. (Table 34.) In his case, then, a second
advance had been made by selection in the male line and the necessary
variation was evidently present to make possible similar advances by
selection in the female line. Male 5555 produced 41 homozygous
English young, 125 heterozygous English, 2 ungraded English, and
65 selfs, or all together 168 English and 65 selfs.

The next sire tested was a son of cf5555; viz, 6^6370, grade 5,
generation 3, as regards selected ancestry. (See table 35.) He was
more advanced in grade and generations than any male thus far
tested and produced higher-grade young by females of the same
grade. Many of the older females had now been discarded, but
enough remained to form a standard of comparison between the
genetic properties of this male and those of his predecessors. The
mean grade of the heterozygous English young of this male was 4.66;
the grade of his homozygous English young was 1.79. The corre-
sponding figures for his father were 4.40 and 1.36 respectively.

Three other sons of cf 5555 were also tested by matings with sub-
stantially the same group of does, although tests in the case of the less
promising ones were terminated sooner. Male 6420 (table 36) was of
slightly lower grade than his father and was found to be probably
inferior to him and so was soon discarded. His heterozygous English
young were of mean grade 4.33.

22 HEREDITY IN RABBITS, RATS, AND MICE.

Male 6071 (table 37), although of higher grade, gave no better
results. His own brother, cf 6072 (table 38), born in the same litter
and graded the same, did much better. He was bred very extensively
and gave a record very similar to that of his father, ^6370, who was
of the same grade but had half a generation less of selected ancestry.
Male 6072 had 75 homozygous English young, of mean grade 1.97
(father's record 1.79); he also had 159 heterozygous English young
of mean grade 4.63 (father's record, 4.66). Increase in the grade of
the young with increase in the mother's grade is very clearly shown
among the young of this sire. (See table 38.)

The next male tested was 6964 (table 39) a son of cf 6071. He was
discarded after a set of matings which showed him probably not
better than his uncle, 6072, who was still in service. He had 21 homo-
zygous young of mean grade 1.49 (his uncle's record was 1.97) and 29
heterozygous English young of mean grade 4.68 (his uncle's record
being 4.63). Next was tested c?7699 (table 40), son of c?6072, who
shared with his half brother, c?6370, the position of best sire so far.
All were of the same grade, 5. This male was mated with all available
does and produced 354 recorded young. He has a better record than
any sire so far tested. By heterozygous does he has sired 75 homozy-
gous young of mean grade 2.31 and 149 heterozygous young of mean
grade 4.80.

Another male of the same grade and generation as the foregoing,
indeed his half-brother, being also a son of 6072, was tested, but
appeared not to be superior to 7699 and so was soon discarded. This
animal, 9532 (table 41), sired 16 homoz}rgous English young of mean
grade 2.53 and also 27 heterozygous English young of mean grade 4.73.

Three sons of the superior male, 7699, have since been tested, viz,
9806, 1212, and 534 (tables 42-44). The first one shows no probable
superiority over his father, but the last two are more promising, each
having produced a total of over 60 heterozygous English young with a
mean close to grade 5. In the case of their father the corresponding
group of young were of grade 4.80. The homozygous young produced
by their father were of mean grade 2.31; those produced by the sons
were of mean grade 2.87 and 2.95 respectively. Accordingly, as regards
both heterozygous and homozygous young, the sons have a distinctly
better record. This, no doubt, was due in part to the fact that their
mates were of higher grade or from more highly selected stock, but it
was not wholly due to this cause, for their half-brother (9806, table 42)
did not show the superiority which they showed, even when mated
with females of high grade and advanced generations. Hence we must
conclude that these two males, 1212 (table 43) and 534 (table 44), were
genetically superior to their father.

Table 45 shows the grade distribution of the young produced by a
homozygous English male, 1173 (plate 3, fig. 6), when mated with does

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 23

of the three categories used in testing heterozygous English males. It
will be noted that he produced only English young, however mated,
conclusive evidence of his homozygous dominant character. His hetero-
zygous English jroung were of mean grade 4.77 and 4.84 by homozygous
English and self does respectively. This male was a son of male 9532,
table 41, with whose genetic character his own was very similar, judg-
ing by the grade of their heterozygous English young. His line was
not continued.

Table 46 enables one to survey at a glance the summarized results
of this entire selection experiment. The course of the experiment is
followed only in the male line, because only in the case of the males
is the number of young large enough to show beyond question the
genetic properties of the individual. From the beginning of the experi-
ment fluctuation was observed in the grade of the young produced, and
this fluctuation was in part at least genotypic, since the higher-grade
mothers have given higher-grade young in matings with the same
male. That the fluctuation was also in part phenotypic is shown by
a comparison of the records made by different males of the same
grade when mated with the same group of females (tables 31-45).

The entire selection race derives its English character from c?2711.
This animal was a heterozygote deriving the English character in a
single gamete from cf2545, who was also heterozygous. Hence the
English character had evidently changed in transmission from father
to son, a sufficient refutation of the idea of unit-character constancy.
Whether the change resulted from a directly changed unit-factor (gene)
or from the introduction of one or more modifying factors is a matter
for further consideration.

The advances made in the male line seem to occur as five successive
steps corresponding roughly with generations of selected ancestry
(table 46). The first advance comes with the selection of the (single
gamete) male 2711, founder of the race; the next in the selection of
his grandson, 5555; the third in two sons of 5555, viz, 6370 and 6072;
the fourth occurs in the selection of two sons of 6072, viz, 7699 and
9532; the fifth is seen in 1212 and 534, sons of 7699. The direct line
of advance is through 2711, 5555, 6072, and 7699. The amount of
advance at each step, as indicated by the average grade of the young
of these males, is shown in table 47. The rate of advance has evidently
decreased as the experiment progressed.

That modification of the English pattern resulted immediately
from the cross with self individuals of an unrelated race is conclusively
shown in table 30. The original English male, 2545, produced by Eng-
lish mates heterozygous English young of mean grade 2.32; by self
mates he produced heterozygous English young of mean grade 2.80,
practically half a grade higher. By most of such mates the young
were more than a grade in advance of those produced by English

24 HEREDITY IN RABBITS, RATS, AND MICE.

mates. Since this is the direct effect upon the English character of
one dose of the self race, it might be supposed that two doses would have
a greater effect, so that if it were possible to lift the English character
bodily out of the English race and surround it with the complete resid-
ual heredity of the self race, an effect perhaps twice as great as that
actually observed in the cross might be expected. Accordingly an
advance of between 2 and 2^ grades may be attributed to the residual
heredity of the self race. Theoretically, if this residuum consisted of
a number of independent factors, then full effect would be secured
upon breeding with each other the highest-grade individuals of the
cross-bred race, repeating this process generation after generation
until each factor was present in a homozygous state. This is sub-
stantially the procedure which has been followed in the 5 full genera-
tions over which the experiment has extended. The advance realized
amounts to about 2f grades.

Allowing for the fact that one of our arbitrary "grades" may not
have the same genetic value as another, it seems probable that we
have secured something more than the effect of the residual heredity
of the self race employed in the original cross. This may have resulted
either from a process of elimination from the heredity complex of
factors which tended to low^er the grade of the English character or
from change in the heredity complex by some other process than addi-
tion or subtraction of factors — for example, by change in factors.

The important fact which this experiment demonstrates is the same
as that shown in the selection experiment with rats, that the single
characters which serve to identify our domestic races of animals
and which give value to them, even though they conform with every
criterion of unifactorial Mendelian heredity in transmission, do nev-
ertheless vary through minute gradations. By reason of the fact that
the residual heredity affects such characters, a cross into an unrelated
race can not be made, except with the possibility, or usually with the
probability, that the character or characters in question will be thereby
modified. This fact was formerly expressed in the statement that
"contamination" of unit-characters frequently follows upon cross-
breeding— a form of statement, however, which was challenged by
those who maintained that the gametes were "pure." Subsequent
investigation has shown beyond question not only that unit-characters
are frequently greatly modified by crosses, but also that they can be
modified by selection alone unattended by crossing.

Those who formerly maintained the doctrine of gametic purity now
shifted their ground, and while admitting that unit-characters might
change, insisted that single factors or "genes" could not change.
This is the doctrine of pure genes which Morgan has made so familiar.
This doctrine it is difficult either to prove or disprove. Pragmatically
speaking, it is of small consequence, since it is admitted (1) that single

INHERITANCE OF WHITE-SPOTTING IN RABBITS. 25

factors do sometimes change, leading to the formation of multiple
allelomorphs; (2) that the action of single factors is not limited to any
particular part of the organism, but may affect parts apparently
unrelated; (3) that the total number of factors concerned in the genesis
of even the simplest organisms must be very great; and (4) that in
what should theoretically be "pure lines" (asexually reproducing
organisms, Jennings) genetic changes are constantly occurring. Prag-
matically, then, genetic variability by minute gradations is a reality,
precisely as Darwin assumed it to be, and this fact allows races to be
altered steadily and permanently by selection, either natural or
artificial, as Darwin also assumed was the case. The hypothesis
that stable organic forms come into being only suddenly, by abrupt
changes from preexisting forms and not by gradual modification — this
hypothesis, the "mutation theory" as commonly understood, is not
substantiated.

RELATION OF DUTCH TO ENGLISH.

It remains to consider the genetic relations to each other of Dutch
and English spotting. Dutch, as we have seen, behaves as a reces-
sive in crosses with self-pigmented races; English, on the other hand,
behaves as a dominant. Dutch marking appears to result from a
simple deficiency of pigmentation, as if in development the pigment
supply failed at an extremity or at an embryological point of finishing-
off . When Dutch marking is reduced to its lowest point of expression
by selection or crossing, the only white visible is found at the tip of
the nose, or on the toes of a fore-foot, or as a spot in the middle of
the forehead. English spotting, on the contrary, appears to result
from some positive inhibiting force, some agency which uses up the
pigment-forming materials here and there in the epidermis and con-
verts them into an end-product not colored but white. That English
individuals possess all the agencies necessary for full pigment formation
is shown by the fact that English parents may produce fully pigmented
(self) young as recessives, which then produce only self young if mated
with each other.

We have seen that there occur different forms of Dutch spotting,
which apparently behave as allelomorphs, but which tend to become
less distinct, one from another, when they are associated in the same
zygote. It would seem probable that they represent quantitatively
different stages of reduction in the amount of some substance carried
in the germ-cell. But undoubtedly this substance, whatever it is, is
located in the chromatin, since the defect is transmitted equally
through egg and sperm. There are also qualitative differences among
the different forms of Dutch, as for example between "dark" and
utan" Dutch, in one of wrhich the white collar is more in evidence,
while in the other it is the head markings that are more in evidence.
Probably, then, the different forms of Dutch are variants of a single

26

HEREDITY IN RABBITS, RATS, AND MICE.

Dutch young.

English 3'oung.

Grade.

No.

Grade.

No.

0

3

If

2

1

2

2

15

2

3

2i

1

3

1

2|

2

2|

1

3

5

Total

9

26

"locus," in the terminology of the chromosome theory. But the physi-
ological and genetic behavior of English are so different that it would
seem improbable that they are variants of the same gene.

Nevertheless, when English is crossed with Dutch, the two appear to
be either allelomorphs or closely linked, as the following results show :
Heterozygous English rabbits of grade 5 were crossed with "white"
Dutch of grades 15 to 17. Two matings were also made of the
homozygous English buck, 1173 (table 33, and plate 3, fig. 6), whose
English character was of equivalent potency with that of heterozygotes
of grade 5. These matings produced 26 English and 9 non-English
(Dutch) young, which were graded (with no great exactness) as follows :

There was probably no real discontinuity
in the grouping of the English young, but
owing to the rough manner of grading them
the numbers heap up on the even grades,
fractional grades being neglected. The Dutch
young are similar in grade to the young pro-
duced by crossing "white" Dutch with self.
(Compare table 27.) But the English young
are much whiter than heterozygotes between
English and self. The latter are 4.75 to 5.00
in grade when produced by the same English sires. (Compare table
45.) But the English-Dutch heterozygotes in no case are of higher
grade than grade 3 and in the great majority of cases are no darker
than grade 2. In other words, they are of about the same grade as
English homozygotes. This means that a white Dutch gamete has
about the same whitening effect on English as another gamete of Eng-
lish would have. The pattern of the English-Dutch heterozygote is
indistinguishable either quantitatively or qualitatively from that of a
homozygous English animal. The Dutch is not at all in evidence except
as a whitening influence on the dominant English. Even in a single
dose it completely counteracts the darkening influence introduced into
the English race in the process of the cross with self and the subse-
quent 5 generations of selection.

Since it has been shown that both white Dutch and English segre-
gate from self in monohybrid fashion, much interest attaches to the
inquiry whether they segregate from each other in the gametes of
the Fi individuals. Two methods have been employed to test this
matter, one being to mate FI individuals inter se, the other to back-
cross them with white Dutch. If English and Dutch are allelomorphs
(invariably pass into different gametes) nothing but English or Dutch
young should be produced by either mating. If, however, English
and Dutch are not allelomorphs, then a certain number of gametes
should be formed by Fa individuals which are neither English nor
Dutch, but which are self, and a like number should be formed which

INHERITANCE OF WHITE-SPOTTING IN RABBITS.

27

carry both English and Butch, the genetic properties of which would
be to produce a very white (low-grade) English. These two new kinds
of gametes would respectively be as numerous as the simple English
and simple Dutch gametes, if English and Dutch are inherited inde-
pendently (in different chromosomes, for example). If English and
Dutch are linked in inheritance (are borne in homologous chromosomes
but not at the same locus), these two classes of gametes would represent
the "cross-overs." The existence of the gamete which bears both
English and Dutch would be difficult to demonstrate, since one char-
acter is dominant, the other recessive; but the existence of gametes
bearing neither English nor Dutch would be easy to detect, either in
the straight F2 generation or in the back-cross generation. Such
gametes uniting with each other would produce self individuals, or
uniting with a Dutch gamete would produce a Dutch of grade 3 or
lower. Now, among 47 F2 young neither of these classes of individ-
uals has appeared. The 11 non-English F2 young are all Dutch of
grade 7 or higher. An even better test for the existence of gametes
transmitting neither English nor Dutch is the back-cross with Dutch;
for in this case any such gamete would produce one and the same kind
of zygote, viz, Dutch of grade 3 or lower. No such zygote has appeared
in a total of 88 Dutch and 105 English young obtained in back-cross
matin gs. This indicates that English and Dutch are either allelo-
morphs or closely linked. The grade distribution of these back-cross
young is shown hi the table herewith.

It should be of interest to compare this
distribution with that obtained in the orig-
inal English-Dutch cross (page 26) as indi-
cating whether the contrasted characters
have been contaminated in the FI zygote.
The English back-cross young are of lower
grade (whiter) than the FI English. The
respective means are 1.36 and 2.25.

This whitening of the English is the result
of contamination from white Dutch in the
FI zygote. No English individual produced
in the back-cross is darker than the English
produced in FI. In both cases the darkest
English are of grade 3. This indicates ab-
sence of gametes transmitting neither English nor Dutch, for any
such gametes uniting with an English gamete should produce English
darker than grade 3. The FI Dutch (page 26) were self -white Dutch
heterozygotes. (Compare table 27.) None was darker than grade 3.
In this back-cross, any self gamete (neither Dutch nor English) which
might arise by cross-over should produce young equally dark, grade
3 or lower. But the lowest-grade Dutch recorded are of grade 6;

Dutch young.

English young.

Grade.

No.

Grade.

No.

6

2

1

52

7

1

2

25

8

1

3

2

9

4

?

26

10

6

11

4

12

13

13

1

14

15

15

14

1C

22

17

3

?

2

Total

88

105

28 HEREDITY IN RABBITS, RATS, AND MICE.

hence there is no evidence that cross-over gametes are produced as
often as once in 192 times. It, therefore appears that English and
Dutch are either very closely coupled or are variants of the same
locus. As regards the effect of the English cross on the grade of the
Dutch character, this is indicated in the grade of the back-cross Dutch
young, which range in grade from 6 to 17, average 13.88. Similarly
produced back-cross Dutch obtained from FI (white X self) X white
(table 17) range from 13 to 17 and are of mean grade 15.15. This
indicates that the English cross has darkened white Dutch even more
than self did in a similar cross (table 27). The superior darkening
effects of English over the self used in table 27 may be attributed to
the more highly selected character (for darkness) of the English used
in the cross and to the less highly selected character (for whiteness)
of the Dutch used in the same cross, but the difference is not great.

At any rate, from the grade distribution of the young produced in
the back-cross of FI (English X white Dutch) X white Dutch, it is
clear (1) that English and Dutch behave like allelomorphs, or closely
linked factors, since in nearly 200 cases studied no cross-over is
observed, i.e., no gamete transmitting neither English nor Dutch;
(2) that the segregated English and Dutch borne by the gametes of
FI individuals are mutually modified, the English (previously selected
for darkness) being made lighter, and the Dutch (previously selected
for whiteness) being made darker. In these mutual modifications we
are dealing probably for the most part with residual heredity, but
it is possible that quantitative variation of the English and the Dutch
genes is in part responsible, yet such an interpretation is not favored
by the results obtained from the critical experiments with hooded rats,
which point strongly to changed residual heredity as the correct
explanation of changed phenotypes, when only a single Mendelizing
character can be observed.

III. OBSERVATIONS ON THE OCCURRENCE OF LINKAGE

IN RATS AND MICE.

In publication 241 of the Carnegie Institution evidence was pre-
sented showing that the red-eyed yellow and pink-eyed yellow varia-
tions of the common rat (Mus norvegicus) are due to genes which are
linked with each other. Upon crossing with each other the two yellow
variations, which visibly differ in eye-color only, young are obtained
which differ from both parent races in coat-color as well as in eye-
color. These young are black-coated or gray-coated and have black
eyes. This result shows clearly that the two variations, which are
both recessive in genetic behavior, are due to independent genes.

In the F2 generation the two yellow varieties were recovered, each
with its distinctive eye-color, and certain individuals, which visibly
were pink-eyed yellows, were found from breeding tests to carry the
genes for red-eyed as well as for pink-eyed yellow. These double
recessives obviously had arisen by the process known as "crossing-
over," in which genes, although introduced in a cross by different
parents, yet later emerge together in the same gamete formed by an
F! individual. It is supposed that genes which behave in this way lie
in homologous chromosomes and that when crossing-over occurs a
gene (A) leaves the chromosome in which it originally lay and crosses
over into the homologous chromosome in which the other gene (B)
lay. Thus both A and B come to lie in the same chromosome and
at gametogenesis pass into the same gamete. From an examina-
tion of the proportion of the double recessive yellow individuals
found among the F2 yellows, it was concluded that cross-over gametes
(those which carry genes for both yellow variations or for neither)
represent about 18.5 per cent of all the gametes formed by FI individ-
uals. If no linkage occurred, such gametes would form 50 per cent of
the total.

To test more fully the strength of the linkage between these two
genes and to find out whether this linkage has the same strength in
both sexes, further experiments have been undertaken. A race of
homozygous double recessives (genetically both pink-eyed and red-
eyed) was built up from the F2 cross-over individuals and with this
race FI individuals were crossed. If we designate by r the gene for
red-eyed yellow and by p the gene for pink-eyed yellow, an FI individual
might be expected to form gametes of the four sorts PR, Pr, pR and
pr. Of these 4 combinations, Pr and pR would correspond with
those furnished by the parent races, red-eyed yellow and pink-eyed
yellow respectively; but the other two, PR and pr, would be new and

29

30 HEREDITY IN RABBITS, RATS, AND MICE.

could therefore arise only by crossing-over. A test mating of an FI
individual with a double recessive would give zygotes as follows, it
being understood that the double recessive individual produces only
one type of gamete, viz, pr. Non-cross-over gametes, Pr and pR,
would give zygotes Pprr and ppRr, visibly red-eyed yellows and
pink-eyed yellows respectively. Cross-over gametes, PR and pr,
would give zygotes PpRr and pprr, visibly dark-eyed (black or gray)
and pink-eyed yellow respectively. The pink-eyed yellows could not
be distinguished readily from yellows arising from non-cross-over
gametes, but the dark-eyed young could be distinguished immediately
at birth from all other classes. Since theoretically they would con-
stitute half the total cross-overs, it is evident that the simplest way of
estimating with accuracy the proportion of cross-over gametes is to
double the number of dark-eyed young observed in new-born litters.
This number divided by the total number of young would give the
percentage of cross-over gametes.

Following this procedure, we have reared from matings of FI indi-
viduals with double recessives a total of 1,714 young, of which 174
were dark-eyed. Doubling the number 174, we have 348 as the prob-
able number of cross-over gametes among the 1,714 FI gametes which
entered into the production of these young. This is a percentage of
20.3, a little higher than the calculation 18.5 of publication 241, based
on a study of a much smaller F2 population. The difference between
this figure, 20.3 and 50, the percentage expected where no linkage
occurs, would be a measure of the strength of the repulsion shown
between the genes for red-eyed yellow and for pink-eyed yellow
respectively, when they enter a cross in different gametes — that is,
each through a different parent, the condition realized in this cross.

But, on the chromosome theory, an attraction or "coupling" equal
in strength to this repulsion should occur between the same two
genes when they enter a cross together. Entering together, they
should tend to hold together, because they would lie in the same mem-
ber of a pair of chromosomes and so could pass out separately only
in consequence of a cross-over. This point, repeatedly verified in the
case of other organisms, was tested for rats by producing FI individuals
through a cross of double recessive yellow (pprr) with a pure non-
yellow individual (PPRR). In reality FI zygotes of this same sort
were being produced in considerable numbers in the matings already
described to test the strength of repulsion. Such were the 174 dark-
eyed young already mentioned. Each resulted from the union of a
pr gamete with a PR gamete, the relationship which would give the
expected coupling. Accordingly many of these dark-eyed young were
used instead of FI parents in the experiments to test the strength of
" coupling" between p and r. In these experiments, as in those to
test the strength of repulsion, FI individuals were mated with double

OCCURRENCE OF LINKAGE IN RATS AND MICE. 31

recessives. In both cases the FI individual was of the formula PpRr,
the double recessive was of the formula pprr.

The only difference in the two cases was that in one case the FI
arose from a union of Pr with pR, and in the other from a union of
pr with PR. But the importance of this circumstance is seen in the
different results obtained in the two cases. In one case (where p and
r enter the cross separately) 10 per cent of the young were dark, in
the other case (where p and r enter the cross together) more than
40 per cent of the young were dark. The exact figures for the repulsion
series have already been given, 174 dark young in a total of 1,714, or
10.1 per cent dark young. For the coupling series the figures are
1,255 dark young in a total of 3,032, or 41.3 per cent dark young.
To compare the strength of repulsion with the strength of coupling we
may estimate the percentage of cross-over gametes produced in each
case. Either sort of FI individual would produce 4 kinds of gametes,
PR, Pr, pR, and pr. But in the repulsion series PR and pr would arise
from crossing-over, whereas in the coupling series Pr and pR would
arise from crossing-over. In either case dark-eyed individuals would
arise only from the same type of FI gamete, viz, PR, but in the repul-
sion series this would be a cross-over gamete, whereas in the coupling
series it would be a non-cross-over gamete. While in the repulsion
series the number of dark-eyed young would measure half the total
number of cross-overs, in the coupling series it would measure half
the total number of non-cross-overs. Applying these criteria, we have
found in the repulsion series, as already stated, that the number of
dark-eyed young being 174, the probable number of cross-over gametes
is twice this, or 348, in a total of 1,714, which is 20.3 per cent.

Turning now to the coupling series, we find that the total number of
dark-eyed young is 1,255. Doubling this we have 2,510 as the probable
number of non-cross-over gametes. Deducting this number from
3,032, the total number of young, we have 522 as the probable number
of cross-over gametes, which is 17.2 per cent. This we may compare
with the 20.3 estimated for the repulsion series, and the earlier esti-
mate of 18.5 based on the census of an F2 population (publication 241).
These differences are not large enough to lead us to think that there
is any consistent difference between the strength of repulsion and the
strength of coupling between the same two genes. The chromosome
theory would not lead us to expect the existence of any such difference.
This case therefore fully accords with that theory. If we combine the
results obtained from both the repulsion and the coupling series we
have as the average linkage strength (either repulsion or coupling, as
the case may be) 18.3 per cent. This is based on a total of 4,746
young produced by the back-cross of FI with the double recessive. The
figures are large enough to have significance and agree remarkably
well (almost too well) with the estimate based on the F2 population,

32 HEREDITY IN RABBITS, RATS, AND MICE.

viz, 18.5 per cent. It is safe to conclude that the linkage strength of
red-eyed yellow with pink-eyed yellow is close to 18 per cent.

We may pass now to the question whether the linkage strength is
the same in spermatogenesis as in oogenesis, whether it is the same
among the gametes formed by FI males as in those formed by FI
females. A priori we might well expect it to be different in the two
cases, since in Drosophila crossing-over has been found to occur only
in females, whereas in the silkworm it has been found to occur only
in males. In publication 241 the fact was demonstrated that crossing-
over does occur in both sexes of the rat, but we were at that time
unable to state what its relative frequency was in the two sexes. Our
back-cross series of matings give data for such a determination. (See
table 48.) It will be observed that the estimated percentage of cross-
over gametes is somewhat higher for females than for males in both
the repulsion and the coupling series and that the difference is greatest
where the numbers are largest, viz, in the coupling series. This
would suggest that crossing over occurs more readily in oogenesis
than in spermatogenesis, but I doubt very much whether such is the
case when all other conditions are the same. Summaries made for the
concluding period of our experimental work, when conditions had been
more carefully controlled and the procedure of taking the records had
been best standardized, show no appreciable differences in the case
of the two sexes. For this period, in the coupling series, FI females
gave 187 dark and 277 yellow young, or 19.3 per cent cross-over
gametes. Simultaneously, FI males of similar parentage gave 133
dark and 197 yellow young, or 19.4 per cent cross-over gametes. In
the repulsion series only Fj males were at this time being used to
any great extent. They produced 29 dark young and 269 yellow
young, which by the method of calculation already explained indi-
cates 19.4 per cent cross-over gametes, a remarkably close agreement
with the results given by both sexes in the coupling series at this
same period.

Whether external conditions have any influence on the percentage
of cross-overs we are unable to state, but this seems doubtful in the
case of a warm-blooded animal such as the rat. That individual or
age differences may occur among FI animals affecting the percentage
of cross-overs is a possibility we have considered carefully, but with
only negative conclusions. The indicated percentage of cross-overs
varies in the case of particular F! males from 0 to 44 per cent, but this
variation appears to be the result of random sampling rather than of
consistent differences in genetic behavior. Several males showing
extremely high or extremely low percentage of cross-overs were trans-
ferred to new breeding-pens and mated with other double recessive
females. Their indicated percentages of cross-over gametes before
and after the transfer showed no consistency with each other, and

OCCURRENCE OF LINKAGE IN RATS AND MICE. 33

so we are forced to conclude that individual differences as regards the
production of many or few cross-overs have not been shown to exist.

In publication 241 evidence was presented indicating that albinism
in rats is probably due to a gene which is linked with the genes for red-
eyed yellow and pink-eyed yellow. This idea is now fully established
and we are able to give provisional estimates of the linkage strengths
involved, although the investigation of this matter is still incomplete.

When red-eyed and pink-eyed rats are crossed with each other, or
either sort is crossed with an albino, the FI young produced are dark-
eyed and dark-coated (either black or gray, according as the agouti
factor is absent or present). But the FI young are not quite as dark
in color as wild rats. This shows that all three variations are reces-
sive and complementary, but that the allelomorph of each is a little
less effective in producing pigment when in heterozygous form than
when in homozygous form (as in wild rats, or in Irish or in hooded
rats). FI individuals from the cross of albino with pink-eyed yellow,
when bred with each other, produce an F2 generation of three apparent
types, viz, (1) dark, in eye and coat color; (2) pink-eyed yellow;
(3) albino. If no linkage occurred we should expect these three
classes to occur in the ratio 9 : 3 : 4; but, as was pointed out in
publication 241, linkage would tend to equalize the numbers of pink-
eyed and albino young, and such a tendency has been recorded.
Further, if no linkage occurs, but if pink-eyed yellow and albinism
segregate quite independently of each other, then half the albino
gametes formed by FI individuals should transmit pink-eyed yellow
and half should not transmit it; conversely, half the gametes which
transmit pink-eyed yellow should also transmit albinism and half
should not. If less than half the gametes which transmit one character
transmit the other, the two show repulsion.

To test the matter, 45 F2 albinos have been mated with homozygous
pink-eyed individuals. Of the 45 so mated, 17 have produced both
pink-eyed and dark-eyed young, one has produced only pink-eyed
young, and 27 have produced only dark-eyed young. The 17 are
evidently heterozygous for pink-eye as well as homozygous for albinism,
their formula being ccpP. The one which produced only pink-eyed
young is probably of the formula ccpp. The 27 which produced only
dark-eyed young are of the formula ccPP.

We may now consider what was the nature of the gametes which
produced these 45 individuals. A gamete which furnished both
albinism and pink may be called a cross-over gamete; one which
furnished albinism only must be regarded as a non-cross-over gamete.
The 27 albinos which did not transmit pink-eye evidently arose each
from the union of two non-cross-over gametes. This accounts for
2 X 27 or 54 non-cross-over gametes. The 17 individuals which
were heterozygous for pink evidently received each a single non-cross-

34 HEREDITY IN RABBITS, RATS, AND MICE.

over gamete. This makes a total of 71 such gametes. Cross-over
gametes were represented singly in each of the 17 individuals which
were heterozygous for pink and doubly in the one which was homo-
zygous for pink. This makes a total of 19 cross-over as against 71
non-cross-over gametes, which is 21.1 per cent cross-overs. This is an
indicated linkage strength a little less close than that between pink-
eyed and red-eyed yellow, in which case the percentage of cross-over
gametes was estimated at 18.3. For with no-linkage giving 50 per
cent cross-overs, it is evident that the linkage strength increases as
the percentage of cross-overs decreases until (when cross-overs cease)
linkage becomes complete. If we measure the strength of linkage by
the difference between the observed percentage of cross-overs and
50 per cent (the percentage of cross-overs when no linkage occurs),
then linkage between red-eyed yellow and pink-eyed yellow is 31.7 and
that between pink-eyed yellow and albinism is 28.9, as provisionally
determined.

The linkage between red-eyed yellow and albinism is much stronger
than the linkage in either of the cases just discussed. Tests have
been made for the presence of the red-eyed yellow gene in 160 F2
albinos and for the presence of albinism in 57 F2 red-eyed yellows
derived from the cross of albino with red-eyed yellow. Only a single
cross-over has been detected, and even that is not beyond question.
One of the F2 albinos, a male, when mated with a pure red-eyed yellow
female, sired a litter of young, all of which were dark-eyed except one.
This one proved to be a yellow but died, as did the father, before
additional breeding tests could be applied. If this yellow individual
was really sired by the albino male (and not accidentally introduced
from some other cage, a remote possibility), then that male evidently
carried yellow as well as albinism and in his genesis a cross-over
gamete must have functioned. Each of the other F2 albinos and the
F2 yellows tested manifestly arose from the union of gametes neither
of which transmitted both yellow and albinism, since as mated they
produced only dark-eyed young (4 or more each). On these assump-
tions the experiments thus far show that only one gamete out of 434
formed by FI parents can have been a cross-over gamete, which
apparently gives less than one per cent of cross-overs.

The experiments are being continued with the hope of finding addi-
tional cross-overs and of thus securing a double recessive race, which
will make possible a more accurate determination of the linkage
strength. The information already presented shows that on the
chromosome theory the genes for albinism and for red-eyed yellow are
extremely close to each other in the same chromosome and that the
gene for pink-eyed yellow, while lying in this same chromosome, is
at some distance from the genes for albinism and red-eyed yellow.

OCCURRENCE OF LINKAGE IN RATS AND MICE. 35

In mice it has been shown by Haldane et al. that the genes for albin-
ism and for pink-eye are probably linked with each other. This fact,
interesting in itself, is made doubly so by the consideration that
characters apparently identical in nature with these two are also
linked with each other in rats. Since mouse and rat are species grouped
by systematists in a single genus, it should be of interest to compare
their genetic constitution as fully as possible. With this idea in mind
we had already undertaken to study the linkage relations of albinism
and pink-eye in mice before the appearance of the paper by Haldane
et al.1 This investigation was undertaken by Mr. L. C. Dunn while
acting as my assistant. Upon his entering military service, I took
over the experiments. It is a pleasure to acknowledge Mr. Dunn's
important part in the work.

We began, as in the rat experiments, by crossing pink-eyed with
albino individuals. Dark-eyed FI young were produced exactly as
in rats. These bred with each other produced an F2 generation of
dark-eyed young, pink-eyed young, and albinos, in a 9 : 3 : 4 ratio
manifestly modified by linkage. Pink-eyed F2 individuals were tested
for the presence of albinism and albino F2 individuals were tested for
the presence of the pink-eye gene as a first step toward the production
of a race of double recessives needed to ascertain the proportion of
cross-over gametes formed by FI individuals. The simplest way of
making the tests was found to be the mating of F2 albinos with F2
pink-eyed individuals. This afforded simultaneously a test of both
parents. For if the pink-eyed parent carried albinism, 50 per cent of
the young would be albinos, otherwise none would be albinos. But
if the albino carried the pink-eye gene, 50 per cent of the young pro-
duced would be pink-eyed. If both these contingencies were realized
in the mating, 25 per cent of the young would be pink-eyed and 25 per
cent albinos. All other young, as in a cross of pure pink-eyed with
pure albinos, would be dark-eyed.

If no linkage occurred between pink-eye and albinism, it would be
expected that half the F2 pink-eyed individuals would carry albinism,
and also that half the albinos would carry pink-eye. Any smaller
proportions than these of pink-eyed carrying albinism or of albinos
carrying pink-eye, among the F2 individuals, would indicate linkage.

Linkage is very clearly shown by the tests made. Among 63 F2
pink-eyed which were tested, 18 produced each one or more albino
young in litters otherwise dark-eyed, while 45 produced no albinos
but only dark-eyed young. In the genesis of the 18 parents men-
tioned, it is evident that 18 cross-over gametes had united with 18

1 In fact, I have not yet had access to the paper by Haldane et al. but know it only as cited
by others. Our copy of the journal in which it appeared is probably at the bottom of the ocean
and we have been unable as yet to replace it.

36 HEREDITY IN RABBITS, RATS, AND MICE.

non-cross-overs. But in the production of the 45, only non-cross-over
gametes had functioned. The total gametes involved accordingly are
18 cross-overs and 18 + (2 X 45) = 108 non-cross-over gametes;
total 126. As 18 is 14.28 per cent of 126, the indicated percentage of
cross-overs is 14.28 per cent.

Among 75 F2 albinos which were tested, 20 produced pink-eyed
young (as well as dark-eyed ones), while the remaining 55 produced
only dark-eyed young. Reasoning as before, there were evidently
involved in this case 20 cross-over gametes and 20 + (2 X 55) = 130
non-cross-overs, total 150. But 20 is 13|- per cent of 150; hence the
indicated percentage of cross-overs is 13-g-.

Combining the tests of F2 pink-eyed and of F2 albinos, we have in
tests involving 276 FI gametes an indicated percentage of 13.76
cross-overs.

The pink-eyed parents, which in the course of these tests had been
found to carry albinism, were now mated with each other, and the
albino young which they produced when so mated were used in
building up a race of double recessives, for all albinos so produced
must of necessity be homozygous for pink-eye. These double reces-
sives were next mated with FI dark-eyed animals obtained by the
original cross of pink-eyed with albino, or with dark-eyed individuals
of similar genetic constitution which had resulted from the test
matings. The interpretation of the results obtained from these back-
cross matings is the same as that given by similar matings in the case
of rats. The FI parent would form gametes of the four sorts CP, cP,
Cp, and cp, of which cP and Cp would represent the original com-
binations found in pure albinos and pure pink-eyed respectively, and
so would be non-cross-overs, but CP and cp would arise only by crossing-
over. Of the four types of gamete, CP alone would produce a dark-
eyed zygote, if mated with a double recessive, cp. But this is one of
the two cross-over types. Hence the number of dark-eyed young
produced in mating FI animals with double recessives should indicate
half the total percentage of cross-overs. By matings of the sort just
described, 3,142 young have been produced, of which 222 were dark-
eyed. Doubling this number, we have 444 as the probable number of
cross-over gametes in 3,142 gametes produced by the FI parents, an
indicated percentage of 14.13. This agrees very well indeed with the
13.76 per cent indicated by the test-matings of F2 pink-eyed and
albinos. It seems safe to assume, therefore, that the average cross-
over percentage is close to 14 per cent. For the corresponding char-
acters in rats, the indicated percentage of cross-overs is considerably
higher, viz, 21.1, but it should be borne in mind that the estimate is
based on a much smaller series of observations in the case of rats and
that further observations may alter it materially.

TABLES.

37

TABLES.

TABLE 1. — Classification of generation 17, plus-selection series.

Grade

Grade of offspring.

of

Totals.

Means.

parents.

3|

4

4i

4*

4f

5

5^

5*

5f

44

2

2

1

5

3.95

4i

1

2

3

3

9

4.22

4|

4

1

5

4

1

. .

1

. .

12

4.44

4f

4

11

7

4

4

1

. ,

31

4.47

4f

1

7

7

11

6

2

4

38

4.49

4f

5

12

26

17

13

8

6

1

88

4.46

41

1

8

10

13

5

1

3

2

43

448

5

9

5

10

12

10

10

4

5

65

4.56

5|

3

2

13

6

2

5

3

3

2

39

4.61

C 1
°4

1

3

8

1

2

1

16

4.30

5f

2

1

1

1

5

4.55

4.81

23

46

96

75

44

32

22

11

o

351

4.48

TABLE 2. — Classification of generation 18, plus-selection series.

Grade

Grade of offspring.

of

Totals.

Means.

parents.

ql
02

Q3
^4

4

4J

4^

4|

5

5i

5§

41-

3

4

5

3

1

2

1

19

4.10

4f

9

9

9

3

2

32

3.34

4i

3

8

10

7

5

1

1

35

4.57

4-i

5

10

22

27

12

4

5

1

86

4.45

41

2

3

13

7

6

8

6

3

48

4.64

41

5

8

14

5

2

1

35

4.21

5

7

11

25

16

12

9

3

1

84

4.43

5*

2

4

11

14

2

7

2

1

43

4.50

5|

1

2

7

5

3

4

5

27

4.61

5*

2

3

2

2

2

11

4.82

4.80

3

26

55

114

97

49

42

25

9

420

4.46

TABLE 3. — Classification of generation 19, plus-selection series.

Grade

Grade of offspring.

of

Totals

Means.

parents.

Ql
02

03
^4

4

4i

4*

4f

5

5|

5£

4i

2

4

1

0

4.21

4|

3

8

15

25

23

9

7

4

1

95

4.34

4i

4

4

3

2

. .

13

4.56

4f

3

4

5

2

1

2

. ,

17

4.50

4f

4

6

15

20

8

8

3

1

65

4.49

41

1

6

12

10

8

8

2

47

4.76

5

3

1

4

7

3

3

1

t g

22

4.47

5i

3

6

3

2

14

4.53

4.66

3

16

27

65

78

38

31

18

4

280

4.49

38

HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 4. — Classification of generation 20, plus-selection series.

Grade

Grade of offspring.

of
parents.

3f

4

41

4^

4f

5

51

51

5f

1 otals.

Means.

4i

1

2

3

3

9

4.22

4f

1

5

5

2

1

1

15

4.50

4*

2

9

3

3

2

19

4.42

4f

1

4

1

1

7

4.57

41

1

1

3

3

3

1

2

14

4.77

4J

1

3

1

2

3

1

2

13

5.02

5

7

2

9

4.61

5|

1

2

3

4.89

5£

3

3

4.75

4.66

1

6

21

28

13

11

7

3

2

92

4.61

TABLE 5. — Classification of generation 18, minus-selection series.

Grade of

Grade of offspring (minus).

parents.

Totals.

Means.

oi

9i
Z2

2f

3

31

34

<S2

03
•**

4

-2f

3

26

29

2

1

61

-2.63

-2|

3

5

28

9

4

49

-2.78

-3

2

18

37

9

2

68

-2.72

-3&

5

19

13

5

4

1

47

-2.94

-31

1

15

8

8

1

1

34

-2.97

-3f

12

8

6

1

1

28

-3.00

-3*

3

1

2

3

9

-3.14

05
°8

t t

4

9

4

5

6

1

. .

29

—3.03

-31

1

1

1

o

5

-3.45

-3.09

8

59

152

55

34

16

4

2

330

-2.84

TABLE 6. — Classification of generation 19, minus-selection series.

Grade of

Grade of offspring (minus).

parents.

Totals.

Means.

2*

•^4

2|

93

Z4

3

31

ol
02

4

-2f

1

2

2

5

-2.80

-2J

5

11

3

1

1

21

-2.79

-3

3

4

16

7

3

2

1

36

-2.85

-3*

3

11

15

7

1

37

-2.95

-31

t

t t

7

2

2

. .

11

-2.89

-3!

2

7

2

2

13

-3.08

-3*

2

1

3

-31

2

1

3

-si

1

1

-3.10

3

15

50

38

17

6

1

130

-2.89

TABLES.

39

TABLE 7. — Classification of generation 20, minus-selection aeries.

Grade of

Grade of offspring (minus).

parents.

Totals.

Means.

2

21

^

2i

3

31

3£

-2|

1

8

9

3

21

-2.67

-2f

1

2

1

1

. .

5

-2.80

-2*

1

. .

6

21

6

6

40

-2.81

-3

1

2

4

7

-2.86

-3|

5

1

6

-2.87

-2.81

1

2

15

39

14

7

1

79

-2.78

TABLE 8. — Classification of generation 21, minus-selection series.

Grade of
parents.

Grade of offspring (minus).

Totals.

Means.

2

21

ol

-2

2f

3

3i

•}i

02

— 25-

-2l
-21
-3

-3*

1

2
1

7
4

O

1

2
1
1
1
2

3

1
1

2
3

14

7
3
1
10

-2.61
-2.61

-3.11

-2.58

1

3

14

7

3

o

5

35

-2.74

TABLE 9. — Classification of extracted hooded third Fz young produced by a
cross of plus-selected with wild rats.

2d F2 grand-parent.

2

2i

2^

2f

3

3i

3*

Totals.

Means.

O

9208, +2f

1

1

9 9922, +3

2

1

O

1

1

o

9

cf 63, + 3^

2

1

6

9

Weighted mean, 3.22.

2

1

2

2

1

2

9

19

3.04

.64

NOTE. Standard deviation of first Fs was 0.73, that of second Fs was 0.50 (tables
141 and 145, Publication No. 241).

TABLE 10. — Classification of extracted hooded first Fz young from a cross of

minus-selected with wild rats.

Hooded grand-parent.

9l

•

If

ii

H

1

3

1

\

_i

0

i i

i

3

1

U

1*

If

2

21

2i

2j

3

"3
+j
o
H

<D

a

83
0>

3

<T

920,331, -2f, gen. 15|
Q 20 48^ 23 o-pn l^3

•j

1

1

1

4
4

1

2

1
i

8
1

2

1

•

4

1

1
1

1

2

2

1

1

29
16

+ .46
— .41

O 90 S'SQ — f>3 trpn Ifi

9

9

I

5

O 9f) Q97 93 trpn 1 T1

9

9

«

1?

1

1

2

1

1

53

— .65

Q *>0 4SO — 9T eon 15-

5

1

1

3

s

1

IS

-.49

Total ....

2

7

1C

7

1

?fi

1

10

2

19

3

1

S

1

7

3

3

2

1

1

121

-.38

1.25

40

HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 11. — Classification of extracted hooded second Fz young from the cross of

minus-selected with wild rats.

From
original
hooded.

1st Fj grand-parent.

If

14

H

i

3

1

2

_!

0

+ 1

2

3

4

1

11

ii

H

2

21

91
^2

21

Totals.

Means.

a

9 20 480

9 1698, —If.

?,

?,

1

1

4

i

1

1

1

14

1.11

9 20 327

9 1715, —If

1

1

4

<>

1

o

3

1

1

1

17

.80

9 20 327

91563 — lj

1

1

3

1

1

?,

q

.67

9 20 480

9944, +li

1

1

2

?:

2

1

Q

1.64

Weighted mean, —1.11

1

i

1

7

s

2

5

5

6

4

5

1

1

2

49

1.01

.92

TABLE 12. — Classification of extracted hooded third F2 young from a cross of

minus-selected with wild rats.

w

a

From 1st F2.

From 2d F2.

1

U

U

If

2

21

2£

2f

3

31

3*

C3

"S

ti

V

a

H

^

9944, — li. .

9 1924, +2

3

1

3

2

2

9

8

8

4

1

41

2.52

9944, +1^

9 1925, +5

1

2

1

2

2

3

3

6

3

23

2.51

9944, +l£ . .

c?1926, +1^

1

1

1

5

3

13

2.68

91563, -l£

92008, -H

1

2

1

1

2

7

2.25

91698, -If

c?2048, +lf

1

2

1

1

1

6

1.79

91563, -l£

c72068, +3

1

1

2

4

2

4

14

3.05

Weighted mean, +1-62. . . .

6

1

5

8

6

5

14

19

21

14

5

104

2.55

.66

TABLE 13. — Grade distribution of Fi young sired by the standard-bred Dutch <?SOS7
(grade 7) mated with does transmitting the self (unspotted) condition.

Mothers designated (E) were English marked and transmitted the self condition in only
half of their gametes. The English young of such mothers are omitted from this table. Mothers
designated (H) were Himalayan albinos.

Parents.

Grades of
young.

Totals.

Mean
grade.

0

1

2

3

cf3037(72) X 9 2651 (E) . ...

•

1

1
4
5
4

1

1
1

1
2

1
3
5

9

4

> 2.17

!• .95

J

" X 9 2688(E)

X 92687(H)

X 9 2830(H) . .

3

" X 92835(H)

Totals

3

15

3

3

24

1.25

TABLE 14. — Grade distribution of the back-cfoss young produced by F\ does
(table IS) mated with the standard-bred Dutch buck &30S6 (grade 9).

Parents.

Grades of young.

Totals.

Mean.

1

2

3

4

5

6

7

cf 3036(92) X 9 5001(1)

2

4

2
1

1

1
4

3

2

9
5
6

....

" X 9 5032(1)

X 95003(3)

Totals

2

4

3

1

5

5

20

4.60

2 The number in parentheses indicates the grade (pi. 1) of the animal.

TABLES.

41

TABLE 15. — Grade distribution of F2 young from the cross indicated in table IS.

Parents.

Grades of young.

Totals.

Mean
grade.

0

1

2

3

4

5

i-?1 5002 (31 X Q 5001(1)

3
3
1
14
4
2

2
3
5
3

3

1
5
1
3

2

1
1
2

2
1

8
8
13
25

8
8

2.45
1.34

" X 9 5032(1)

X 95003(3)
d" 5029(1) X 95001(1)
" X 9 5032(1)

7
1

" X 9 5003(3)

Totals

8

27

16

12

4

3

70

1.80

TABLE 16. — Grade distribution of young produced by F2 does (table 15) or back-cross does
(table 14) mated with the standard-bred buck, SOS6 (table 14).

Parents.

Grades of young.

Totals.

Mean
1 ower
group.

Mean
higher
group.

1

2

3

4

5

6

7

,s

9

10

11

12

13

14

15

16

17

d"3036(9) X 9 5150(5) F2

9

S

9

1

8
22
18
6
12
7
14

[4.94

|:

14.55

X 9 5153(5), F2..
X 9 5170(6), BC
X 9 5166(4), BC
X 9 5158(5), BC
X 9 5159(6), BC
X 9 5169(6), BC

1

1
3

4
1

11

5

1

3

5
4
1

2

1

1
1

2
2
?

3

4

1

1

2
3

1
3

2
1

3

2

1

1

Totals

1

4

5

24

14

16

6

2

4

1

1

2

6

1

87

5.06

14.55

TABLE 17. — Grade distribution of young produced by second back-cross does recorded in
table 16 and the same standard-bred buck, 3036 (tables 14 and 16).

Parents.

Grades of young.

Totals.

Means,
lower
group.

Means,
upper
group.

1

2

3

4

5

li

7

8

9

10

11

12

13

14

15

16

17

cf 3036(9) X 9 5536(7), 2BC

9

9

4
21

7.50
8.00

15.17

" X 9 5590(9), 2BC

1

2

1

5

2

3

1

1
1

3

2
2

Totals

1

2

3

7

2

3

1

3

25

7.89

15.17

TABLE 18. — Grade distribution of young produced by F2, back-cross and second back-cross
does (tables 14-16) mated with a back-cross buck, &5167, grade 7 (table 14), son of
<?5003(3).

Parents.

Grades of young.

Totals.

Means,
lower
group.

Means,
upper
group.

2

:;

4

5

6

7

8

9

10

11

12

13

14

15

16

17

d" 5167(7) X 9 5150(5), F2

1

1

4

1
1

5
4
9
6
6
8
6
5
4

• 5.11
• 4.00

4.85

14.40
15.67

' X 9 5153(3) F2

1

X 9 5166(4), BC...
X 9 5158(5), BC...
X 9 5601(6), 2BC

1

1

2
2

1
2
1

3

^

1

2

1

' X 9 5645(6), 2BC

9

1

1

X 9 5920(6), 2BC. .
X 95933(6),2BC. .
X 9 5936(5) 2BC

1

1

1
1

2
4

9

1

•>

—

Totals

2

.->

7

21

7

2

1

3

4

1

53

4.73

14.87

42

HEREDITY IN RABBITS, RATS, AND MICE.
TABLE 19. — Variation of the uncrossed "white" Dutch race.

Parents.

Grades of
young.

Totals.

Means.

15

16

17

c? 6175(17) X 95945(15)

1

1
11
25

7
1
7
3

4

1 16.17

•16.12

16.71
17.00
16.75

X 97934(15)

2
3
2

4
15

4

5

7
1
1
6
3
3

X 96703(16)

X 97003(16)

X 97185(16)

X 97313(17)

1

cf 9218(17) X 9 9222(15)

" X 9 9217(16)

1

Totals

9

24

26

59

16.25

TABLE 20. — Variation of the uncrossed "dark" Dutch race.

Parents.

Grades of young.

Totals.

Means.

1

2

3

4

5

6

7

c?670l(5) X 97641(2)

2

5

2

6

1

3
1
2

4
3

8

1
4
1

1
1

1

1

18
3
12

4
26
39

27
28
10
5

j 3.05
1-3.35

•3.35
•3.20

X 97642(2)

X 9 8034(3)

4

10
10
6
8
3
3

2
3
5
7
9
8
3
1

X 9 7644(3)

1

7
15

8
2
4

' X 9 7684(3) . .

3
3

X 96989(4)

X 97685(4)

X 9 8290(4)

X 95153(5)

' X 9 6707(5)

Totals

8

44

50

39

21

9

1

172

3.30

TABLE 21. — Variation of the uncrossed "Ian" Dutch race.

Parents.

Grades of
young.

Totals.

Means.

2

3

4

5

cf 5757(3) X 97393(3)

2
8
3
2
3

2
1
1

4
1
3

1

1

4
11
6
3

7
n

7

3.50
2.91

•3.40

ci*7142(4) X 99275(2)

2

2

X 9 9608(3)

X 9 6424(3)

" X 9 8881(3). .

" X 9 9044(3)

0* 6240(4) X 9 8881(3)

1

3

Totals

5

21

12

2

40

3.27

TABLES.

43

TABLE 22. — Grade distribution of the FI young produced by the cross of "white"

with "dark" Dutch.

Parents.

Grades of young.

Totals.

Means.

5

6

7

S

9

10

11

0^6175(17) X 96666(5)

1
1

1

1
1
3

4
3

2

2
1
1
1
2

1
1

1

1

10
6
5
5
2

1
!>7.2S

J7.28

X 95153(5)

X 96705(5)

X 95170(6) ....

X 96038(8)

6

Totals...

o

9

7

o

1

1

28

7.28

TABLE 23. — Grade distribution of the FI young produced by the cross of "white" with

heterozygous "dark" Ditfch.

Parents.

Grades of young.

Totals.

Means,
lower
group.

Means,
higher
group.

4

5

6

7

8

9

10

11

12

13

14

15

16

17

cf6175(17) X 95158(5)... .
X 95940(5).. . .

1

1
2

1
1

2
1

1

3

4

1

10
11
6
5
5
12
14
16
15
6
5
5
3
4
8
5

J6.33
6.82

J7.57

j 7.22
j 7.50

7.25

15.55
15.67

16.25
15.50
14.50

14.72

3

1

X 95920(6).. . .

1
1

3

1

9

1

X 95601(6).. . .

1

" X 9 5939(6) . . .

1
2
4

9

1

s

X 96570(6)
X 96643(7).. . .

2

3

1
1
1

2
2

O

1

2
6

4
2

1

1

3
3

1

1
1

1
1

2

1
1

" X 96891(7).. .

X 96031(8)... .
X 97118(8).. . .

1

o
1

1

1

2
1

2

cf 7007(14) X 95158(5) ..

1

1

1

1

X 95940(5)

1

1
1

1

1

" X 95601(6)... .

1

1

X 95920(6)...

1

1

1

1

" X 95645(6)

1

1
1

1

1

1

1

9

1
1

2

X 96570(6)

Totals

5

7

13

13

15

8

2

2

1

2

8

18

24

12

130

7.04

15.56

TABLE 24. — Grade distribution of the young produced by a cross of tan Dutch with

self animals heterozygous for tan.

Parents.

Grades of young.

Totals.

Means,
Dutch
young.

0

1

2

2

4

c?7142(4) X 96000(0)

6
16
2

2
3
5

7
3

1

1

1

1

5

2

'2

1

6

8
2

1
2
1
2
2

1
3

3
2

13
31
6
6
8
10
12
5

2.86
2.60
2.50
3.75
2.20
3.20
3.20
3.00

X 96119(0)

X 96122(0)
X 96124(0)

X 9 6380(0)

X 97529(0)

X 97677(0)

' X 98063(0)

Totals

44

3

11

24

9

91

2.83

44

HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 25. — Grade distribution of the FI young produced by the cross of
dark Dutch with tan Dutch.

Parents.

Grades of
young.

Totals.

Means.

0

1

2

d*7142(4)T X 97641(2)D

2

3

1
5
2
5

2
2

5
3
5
2
9

....

X 9 6058(4) D

c?G701(5)D X 97209(2)T

c?5757(3)T X 9 5170(6)D

X 95939(6)D

2

Totals

4

16

4

24

1.00

TABLE 26. — Grade distribution of the F\ young produced by the cross of white

Dutch with tan Dutch.

Parents.

Grades of young.

Totals.

Mean.

6

7

8

9

10

11

12

d"5757(3)T X 97003(16)W.. ..

1
1

6
3

••

1

••

••

3

8
4
3
5

• • * •

X 97185(16)W

cf 6175(17)W X 96424(3)T

X 96539(4)T

2

2

1

Totals

4

11

1

1

3

20

7.70

TABLE 27. — Grade distribution of the FI young from crosses of dark Dutch with

self and of white Dutch with self.

Parents.

Grades of
young.

Totals.

Means.

0

1

2

3

(Dark X self.)
cf 6701(5) X 97413(0)

3
9

1

4
9

X 98012(0)

Totals

12

1

13

.08

(White X self.)
cf 6175(17) X 96133(0)

5
2

4
2

2

X 97123(0)

4
11
3

X 97124(0)

X 98265(0)

Totals

18

13

2

33

1.51

TABLES.

45

TABLE 28. — Grade distribution of the F? young from the several crosses made between the
three types of Dutch and between white Dutch and self.

Cross.

Grades of young.

Totals.

Means.

0

1

2

3

4

5

6

7

8

9

Id

11

12

13

14

15

16

17

White X self

11

45
1

22
1

14

2

1

8

9
12

7
11

5
15

12

17

5
6

13

8

4
5

14
4

5

7

18
14

5

3

•2->
9

8
4

Id
4

4

12
2

8

1

10

5
1

4

6
6

4

4
13

1

o

2

191

56+25

130
275

White X dark (tables 16-18) .
Whitp V tan

5.82and
14.40

Dirlr V tin

89

84

25

TABLE 29. — Grade distribution of young produced by other crosses of Fi animals.

Cross.

Grades of young.

Totals.

Means.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

F! (white X self )X white
FI (white X self) X self

6S

2
^6

3
^

4

11

11

14

9

3

1

1

14

24

13

6

58+58

94
59
143
73
25
76
112

o.loand
15.15

Fi (dark X self) X dark .
Fi (white X dark) X dark
FI (white X tan) X white
Fi (dark X tan) X dark.
F! (dark X tan) X tan. .
Fi (dark X tan) X white

5

36
3

9
12

7
11

2
36

33

30

12
1

2
2

3
5

1

8

8

6

13

3

18

7

2

4
19
1

12
9

1

6
1

5

3
10
10

13

23

14
14

4

5

4

7

1

1

3

4

12

8

10

3

5

1

TABLE 30. — Grade distribution of young sired by the original English male, 2545, grade 2\.

A. MOTHER ENGLISH.

Mother.

Grade.

Self
young.

English young of grade —

Total
English.

1

H

i*

U

2

5>i

Z4

oi

^2

2|

3

?

2649
2650

2651

2
3
2f

1
5
2

2
2

i

1

2
1

2

1

1

1

1

4
2

3
13
5

Totals . . .
Means

8

4
l.i

i
35"

4

2

1

1

2

6

21

2.32

B. MOTHER SELF.

Mother.

Self
young.

English young of grade —

Total
English.

H

i!

2

9l

2*

o3

2?

3

3|

3|

31

4

t

1443
1492
2053
2502

2867
2912
2916

2
5
2
5
3
0
1

3
3
5
o

3

1

o
o

5
5
3
3
5
2

1

1

1

2

1

1

1

1

Totals . . .
Mean . . .

18

2

1

1

1

2

1

1

17

26

2.80

46

HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 31. — Grade distribution of young sired by English male 2711, grade 4, generation 1,

ancestor of all English in the selection series.
A. MOTHER ENGLISH.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

Means,
higher
group.

3
4

1

11

14

U

2

2|

24

2*

3

3|

34

3!

4

4i

44

4|

5

?

5051
5052
5206
5083
5084
5402

34
34
4f
5
5
5

If

14

24

14
14
14

2
2
0
1
2
1

1

1

1

1

1

4
4
6
4
3
5

J3.83
3.80

4.25

1

1

1

i

1

1

1

1

1
1

•

2

1

1

1

1

2

3

1

V_

1

2

Totala .

Means .

8

4

2

V

52

1

1

2

2

2

1

3

1

3

26

1.

3.93

B. MOTHER SELF.

Mother.

Self
young.

English young of grade —

Total

English.

0

ql

^4

34

3f

4

41

44

4f

5

2765
2770
2840
2862
2867
2878
2947
2948
2983
3019

1
1
4
4
3
0
4
4
4
3

1

2
1

1

1
1

2

3
5
3
2
5
3

3
3
1

1
1

2
1

1
1

1

2
2

1
1

1

2

1

1

1

Totals ....
Mean . . .

28

3

5

4

2

5

3

5

28

3.89

TABLE 32. — Grade distribution of young sired by English male 5086, grade 4%, generation 1 4-

A. MOTHER ENGLISH.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total.
English.

4

3

4

1

11

14

If

2

21

24

93

3

•u

•• i

^i

'2

31

4

41

44

4f

5

5051
5052
5188
5206
5053
5083
5084
5101
5102
5398

34

34
4^
4:i
5
5
5
5
5
5

14

i4
24

24

14
14
14
14
14

1 a
•I*

2
8
2
2
2
6
3
3
6
1

1

2

1
1

i
i

1

1
2

1

1

2

1

2

1

9
18
1
4
4
6
3
12
5
2

2

1

1
1

2

'2

1

1

2

1

2

1

1

1

2

1
1

1

1

i

1

1

1

2

i

1
1

1

1
1

1

3

1

1
1

1

Totals .
Means .

35

1

3

5

4

— -.-

Li

4

8

3

-•

3

1

5

8

8

4

5

6

1

64

3.96

TABLES.

47

TABLE 32, continued.
B. MOTHER SELF.

Mother.

Self
young.

English young of grade —

Total
English.

1 1
•ij

1*

U

2

oi

2|

2f

3

ol
^4

3^

3f

4

4|

4|

4f

5

4146
4147

4148

3

2
3

1

1

1

1

1

2

1

1

7
1
2

1

Totals. .. .
Mean ....

8

1

I

1

2
3

3

1

1

10

15

TABLE 33. — Grade distribution of young sired by English male 5375, grade 4%, generation 2\.

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means.

1

3

1

U

iHf

2

->}

2^

^

3

3i

3£

03
<J*

4

4i

4^

-if

5

5051
5052
5188
5205
5206
5083
5084
5102
5398

3|
3£
4£
4i
4f
5
5
5
5

1*
1*
2*
2*
2i
If
1*
1*
If

2
2

1
1
1
2
5
9
2

1

1

•

i

1

1

1

1

1

1

6

8
4
5

4
1
6
8
4

J3.40

j 3.85
3.75

4.11

?!

1

1
1

1
1

1

1

i

1

i

^

i

1

1

1

1

9

1

)

1
1

1

3

1
9

3

1

1

?,

Totals . .

25

1

7

i

— \^

1.1

2
50

2

2

-^

1

1

•2

1

3

9

s

2

2

46

Means .

3.79

TABLE 34. — Grade distribution of young sired by English male 5555, grade 4f, generation 2\.

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Tota

Means,
higher
group.

i

2

3

4

1

U

U

If

2

21

21

2|

3

31

1

3f

4

i

*!

4f

5

Eng-
.' lish.

5557
5051
5052
5561
5701
5752
5993
5188
5205
5672
5769
5952
6074
5206
5793
5801
5988
5084
5102
5398
5951

3

4
4
4

4~

A 3.

4|

5*
5
5
5

1

3
3
2

1

•

•

1

1

1

2
1

1

c

2

1

1

1

1

7
L 11
3
1
11
5
2
19
3
L 5
3
5
5
22
8
15
4
15
12
9
3

4.04
4.47

>4.44

1

1

1

1

2

1

]

1

2

3
1

2
1
1
1

1
1
5

1
9

1
6
2
1
2
6
2
3
1
12
2
7
1

1

1

2

1

2

1

1

3

3

1

2

1

1 1

1

1

1

1

1

1
1

2
1
2

1

2

1

1
3

2

4
2

2
3
6

4
2
2

2

2 .
2 .
1 .

2 .
2 .
1 .

2

1

1

1

1

2

4

1

1

1
1

1
1
1
1

2

3

1

1
1

1

1

1

1

•

2

1

Totals .

Means.

65

1

1

12

9

7

6

4

1

1

2

7

7

18

16

26

29

192

168

1.36

4.40

48 HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 35. — Grade distribution of young sired by English male 6370, grade 5, generation 3.

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means,
higher
group.

1

U

U

U

2

9i

•^4

9i

Z2

2f

3

•u

•J4

•Jl
"2

3f

4

41

4|

4f

5

5752
5993
5188
5672
5769
6074
6089
7193
6815
6369
6417
6693
0788

4
4
41
4^
4f
4|
41
4^
4|
5
5
5
5

3
3

9i

^2

3
3|
31
3^
41
3f
3
3
41
4?

4
o

0
0
0

4
2

0
3
1
1
1
3

4
1
1
1
2

4
7
7
1
7
2
7
4
17
4
3
7
2

•4.67
4.63

3

2

2

1

2

i

1

1

3

1

1

1

1

i

1

1
2

4
o

1
1
2
1

2
5

1

1

2

1

1

1

1

1

2

1

1

1

1
1

2

1
1

Totals .
Mean. .

21

1

3

3
1.

2

-v~~

79

5

2

2

1

1

8

15
66"

11

18

72

4.

TABLE 36. — Grade distribution of young sired by English male 6420, grade 4 I, generation 3.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

1

11

U

U

2

ol

-"4

2i

2f

3

31

•}i

02

3|

4

41

41

-i!

5

5993
5188
6074
5102
6417
6841

4
41
4*
5
5
5

3
2k
3§

1*

3

41

1
0
0
1
1
1

1

1

o

4
5
7
5
3
5

o

9

i

1

3

2

1

1

1

1

1

1

1

i

i

1
2

1

1

Totals .
Means.

4

o

2

2

1

— v—

1.8

1

1

1

0

1

5

£

1

— -w —

t.s;

3
J

5

2

— *

29

0

TABLE 37. — Grade distribution of young sired by English male 6071, grade 5, generation 3%

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means,
higher
group.

1

!i

LI

L|

2

21

21

2f

;;

31

3^

•?3
°4

4

41

4^

4i

5

5051
57f)2
5769
5891
6074
5206
5793
5988
6073
6189
5084
5951

ol

02

4
41
41

•u

4f
H
4!
43
4|
5
5

U

3
3|
3*
3*
21
3*
3

34

«}2

3

U
3

1
1
0
2
1
0
o

3

1
1
o

2

2
1

3

5
6
4
3
5
4
5
7
4
4
4
3

3.75
4.56
4.83

J4.25
•4.47

J4.12

i

1

1

1

2
2

1

i

1

1

1

i

1

1

1

1

o

1

1

9

4

2

1
1

1

1

2

1

9

1

1
1

1

1
1

1

2

Totals.
Moans.

16

4

3

::

7

;

2

3

v—

5

2

5

— v^

4.3

3
9

6

8

- _

54

1.61

TABLES.

49

TABLE 38. — Grade distribution of young sired by English male 6072, grade 5, generation S$.

A. MOTHER HETEROZYGOUS ENGLISH.

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means,
higher
group.

3

4

1

11

li

If

2

-M

2^

2|

3

3i

3^

3|

4

1.1

l|

if

5

51

5752
5701
5993
5188
5769
5891
6074
6079
6089
6452
6795
5206
5801
5988
6189
6264
7450
7817
8813
5084
5102
6188
6369
6416
6417
6622
6693
6841
7476

4
4
4
4|
41
4£

4*

4£
42-
4i
4^
4f
4f
4|
41
4f
4!
4|
4|
5
5
5
5
5
5
5
5
5
5

3
3|
3
2J
3|
3*

oi
02

4

ol
02

4
3£
2*
3
3
3
3!
41
3^
4f
11

li
3
3
3
3
3f
41
41
41

2
3
2
0
4
3
2

1
7
2
3
3
4
6
4
6
5
1
0
2
9
1
2
2
4
1
0
2
0

1

1
1
1

1
1

1

1
1

2

1

4
9
3
5
12
3
2
4
S
12
12
1
11
15
13
5
1
3
4
7
16
7
10
22
5
5
12
19
4

1 4.50
j 4.42

•4.58
•4.62

•4.69

•

1

1

1

1

1

i

1

1

1

1

1

1

2

2

1

1

1

3

1

1

i

1

1

1
1
4
4

2
2

1

1

3

1

1

1

1

2

5
1

1

1

i

1

1

1

1

• i

o
3

1

2
3
1
1

2
2
3

1

1
3
4
1

•>

1

i

1

4

1

1
1

i

2

2

1
1

4
1
1
4
3
3
4
7
4

1

2
2
1

2
5
1
1

1

1

2
2

1

1

1

1

1

1

1

2

1

4
3
4
1

1

2
1

2
2

•

•

2

1

o

1

1

?

1

1

1
2

1

4

1

1

2

1

1

8

Totals . .

81

1

2

7

1C

15

9

8

5

4

3

13

15

35

56

36

1

234

Means .

1.97

4.63

B. MOTHER HOMOZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

1

li

ii

U

2

21

2i

2f

3

31

ol
02

03

3?

4

41

4*

4|

5

5733
7535
7814
8704

2

ii
il

if

3
4
3*
4|

0
0
0
0

1

3

1

2

1

8
1
14
5

1

1

2

i

i

2
?

1

1

1

1

1

2
1

2

Totals

1

2

i

i

4

1

4

2

1

1

O

3

4

1

28

Means. . .

2.11

4.17

C. MOTHER SELF.

Mother.

Self
young.

English young of —

Total
English.

ol
02

3J

4

4|

4|

4|

7123

6

1

1

1

3

7878

3

1

••

• •

2

1

4

Totals ....

9

1

1

1

2

2

7

Mean ....

4.32

50 HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 39. — Grade distribution of young sired by English male 6964, grade 5, generation

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means,
higher
group.

3
4

1

If

ii

1 5
>-l

2

2-

-4

21
2

2f

3

3i

•Ri

02

3f

4

+1

4^

4|

5

6074
7193
7475
7356
7817
5102
6417
6693
6841

4^
4^
4f
4|
4f
5
5
5
5

3*
4J
4i
4|
3f

If

3

4f
4|

4
2

1
2
1
4
1
2
4

1

1

4
1

2
1

1

8
4
3
2
8
11
6
6
2

•4.64

•4.78

•4.65
n

1

1

1

1

2
2

1

1

1

3
?

1

1

2
1

2
1

1

1
1

2

1

2

1

1

2

1

2

Totals . .

21

2

•

4

4

4

~~N/

1.

1

49

3

2

1

_^

2

v

1

4

8

~v —

.68

10

8

— •>

50

Means. .

TABLE 40. — Grade distribution of young sired by English male 7699, grade 5, generation

A. MOTHER HETEROZYGOUS ENGLISH.

Mother.

Grade.

Gen-
era-
tion.

Self
young.

English young of grade —

Total
Eng-
lish.

Means,
higher
group.

1

]}

,!

IT

?,

91

91

93

3

qi

ji

V 3

4

41

4+

4-

^i

?

9449
6089
5988
6795
7300
7356
7389
7475
7817
9350
5084
6369
6416
6417
6622
6841
7325
7476
7906
8257
8259
9349
9363
9091
9535

3!
4!

4|
4|
4!
4f
4f
4|
4|
5
5
5
5
5
5
5
5
5
5
5
5
5
5

32
4|

s|

ii

3
3
3

03

"4

3|

4
4f

4|

5*

2
2
1
4
0
0
0
8
1
2
2
4
3
4
5
3
0
3
3
0
7
0
0
3
7

1

1

1

3
4
8
6
10
3
3
17
6
17
9
10
11
10
9
20
3
8
9
16
15
10
5
2
4

4.50
4.62

•4.66

•4.80

U.81

1
1

1

1

1

1

1

3

1

1

1

1

1
1

1
2

1

1
1
1
1

1

1

9

1

1

1

2

1

3

2

4
1

;;

1
3

1

1

1

1

1

2
1

1

1

1

1

1

1

1

1

1

1

5

5
3
2

3

6

2
2
1
1
3

4
1

1

2

1

1

1

2

1

9

1

1

2

1

1

1

1
1

3

1

1

3

1

1

1

1

1

1

1
3
2
4
1
3

5
2
5
6
6

1

1

1

2

2

I

1

1

2

1

1

2

1

1

1

1

3

1

1

. >

d

1

1

"

s

Totals .

G4

i

5

5

6

14

10

~^v-

2.2

<3

11

13

2

2

3

s

6

17

43

56

8

224

Means .

1 4. SO

TABLES.

51

TABLE 40, continued.
B. MOTHER HOMOZTOOUS ENGLISH OR SELF.

Mother.

<o

•a

cj
O

I

t- C

a I

I*

M

23 S
13 3
CO O

r*>

English young of grade —

"3 3J
"o "3>

HW

aT t- x

S-Sg

(U GO O
glS tt

U

1*

i 3

1 4

2

21

2*

2|

3

31

•u

02

3|

4

41

4*

4|

5

5j

Horn. Eng., 8704
Self, 7124
" 8251

If

41

3

0

1

1

1

1

2

3

8

1
6

1
1

11
16
3

4.78
J4.74

1

1

2

TABLE 41. — Grade distribution of young sired by English male 9532, grade 5, generation 4-

Mother.

o

•73
03

O

Gener-
ation.

^ g1
"3 5

02 S

>»

English young of grade —

_ ,£3

03 M
- —
O fct
EH C
' H

?F <-> o
p o B

03 fe P

^ o 2

§- M

oT u •

fl Ol ft

3X3
<B M O

S2 &

U

1*

U

o

2|

21

03

Z4

3

•*i

^4

3^

3f

4

41

^

4f

5

51

Het. Eng. 6795
" 7475
" 9362
" 953S
" 6369
" 6622
" 8257
" 8259
" 9535
" 9592

4f

4f
4f
4f
5
5
5
5
5J
51

3£
4|
5
4£
3
3f
4f
4f
31
41

3

1
2
1
1
0
2
0
0
0

1

1

1

1

2
3
1

5
4
5
1
4
5
4
6
4
5

•2.46

2.30
•2.85

4.69

4.70

4.94

2

1

1

1

1

1

2
1

1
4

1

i

2

2
1

4
1

1

2

1

1

2

1

1

1

Totals .

10

1

i

2

2

o

4

1

2

1

1

5

16

1

43

6
3
2

2.53

1.75
\

4.73

4.30
4.80

Means

i

2.

53

2

•5
1

L7:

1
1

!

1
2

2

Horn. Eng. 7814
Self, 8251
8265

U

2|

3
3

r

TABLE 42. — Grade distribution of young sired by English male 9806, grade 5, generation

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

2

21

9l
Z2

2f

3

^i
•^i

^i

02

3f

4

41

4^

41

5

6844
9871
6369
6622
456

3|
4|

5
5
5

41
4
3
31
41

2
2
1
1
1

1

1

1
1

2
4
1

1
2

5
9
3
5
4

3

1

9

1

1

1

1

1
1

Totals. . .

7

1

V—

4

1

— ^— _

2.52

1

3

'

1

*-

1

V

4.J

4
$6

10

^

26

Means. . . .

52 HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 43. — Grade distribution of young sired by English male 1212, grade 5, generation 5,

A. MOTHER HETEROZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

Means
higher
group.

2

21

2^

2*

3

Ql
•J4

ol
32

3f

4

41

4?

4f

5

51

9943
455
539
1360
6795
7475
8257
452
9372
230
6841
537
6622
112
1222
596
1613
1860
6369
9363
111
597

4^
4f
4f
4f
4f
4f
5
5
5
5
5
5
5
5
5
5
5
5
5
5
51
51

51

41
51
5|
3|
41
4f
5|
4

4|
41
51
3f
5f
51
5f
4f
6i
3
5
5f
5|

2
1

1

4

5
3
4
7
5
4
5
1
5
5
4
3
2
7
1
3
7
3

•4.97

•4.97
J5.15

1

9

1

1

2

7
4

1
1

1

f>

?

9

1

1

2

2

1

1
2

1

1

1

1

1
2
1

4
3
3
1

1

2

1

1

1

1

9

4

2

9

1

1

2

1

1

1

1

1

1

5
2

1
1

1

1

2

1
3

3

8
2

1

1

?

1

Totals . . .

29

7

1

5

4

2

2

3

2

11

s

43

— ^ —

L9i

7

— •

5

87

Means . .

2.87

B. MOTHER HOMOZTGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

English young of grade —

Total

English.

If

2

21

2-1

2!

3

?i
^4

3*

03
•J4

4

41

4!

4f

5

8682
9953
9684
9970

i 3
-1*

2^

Z4
01

^4

Ql
02

4|
5
4J
4|

1

1

4

1

1
1

9,

3

2

11
3
3
6

1

1

1

1

3

Totals

1

1

1

1

4

'

2

1

7

5

23

Means. . . .

2.59

4.75

TABLES.

53

TABLE 44. — Grade distribution of young sired by English male 634, grade 5 \, generations.

A. MOTHER HETEROZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

Self
young.

English young of grade —

Total
English.

Means,
higher
group.

2

21

..i
AI

21

3

ol
&4

ol
02

0,3
J4

4

41

4£

42-

5

51

9684
9943
7475
9871
360
455
584
1360
539
6369
9363
7906
258
452
230
456
537
1222
6841
112
9091
2325
111
597
9592

31

4*
4f
4|
4 f
4f
4|
4|
4f
5
5
5
5
5
5
5
5
5
5
5
5
5|
51
51
61

4£
5|
41

4

5|
41
5i
5|
51
3
5
4
5|
5f
4£
41
51
5J
41
5|
5J
5*
51
5f
41

2

1
2
o

2

2

1
1

1

1
3
4
2
4
6
2
5
5
5
7
2
6
3
2
4
I
2
2
5
5
6
4
3
5

•4.87

•5.02
•5.06

1

9

1

1

1

1
1

1

1

1

2

3

3

1
2

1

1

1

2

1
2
1

1

2

1

1

5

2

1
2

1

o

2
1
1

2
1
1

1

1

1

o

1

1

9

9

1
1

1

1

3
3
3
2

1

1
2

1

2

1

1

1

1

1

3

1

Totals .

21

5

*- —

6

11

3

4

3

- -*

1

1

*•-

4

9

—^ —

.97

42

9

94

Means. . .

2.95

B. MOTHER HOMOZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

English young of grade —

Total

English.

11

U

If

2

2i

4

2^

2f

3

3|

3*

3!

4

41

41

4f

5

7814
8682
9953

ii

i 3
-1*

2f

3|
4f
5

1

1

1

1
3

1
1

1
1

6
5
3

1

?

Totals

1

1

1

1

'

9

4

v —

4

2

— v —

.69

2

— -

14

Means .

— '

2.3'

54

HEREDITY IN RABBITS, RATS, AND MICE.

TABLE 45. — Giade distribution of young sired by homozygous English male 117S, grade 3\,

generation 5.

A. MOTHER HETEROZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

English young of grade —

Totals.

a

If

2

21

2|

21

3

31

•}i

02

o3
•JJ

4

41

4^

4|

5

51

9684
9970
7300
455
584
6369
8257
1222

3|

3|
4f
4|
4f
5
5
5

4|

4^
4^
41
5£
3
4f
51

i

1
3

2
1

1

1

1

1

5

8
8
5
8
5
3
4

•>

2

1

2

2

3
1

1
3
1

1

2

1

1

2

1

3

1

3

Totals . . .

i

4

5

4

2

4

2

2

2

2

2

7

8

1

46

Means . . .

2.42

4.77

B. MOTHER HOMOZYGOUS ENGLISH.

Mother.

Grade.

Gener-
ation.

English young
of grade —

Totals.

2

21

2J

2f

3

7814
9953

1*
2f

3£
5

1

2

1

2

4

4
6

Totals . .

1

2

1

2

4

10

Mean

2.65

C. MOTHER SELF.

Mother.

English young.

Totals.

41

4|

4|

5

51

7123

2

4

2

..

8

7124

1

2

3

9

1

16

8251

.

2

o

9318

5

5

Totals . . .

1

4

9

16

1

31

Mean. . . .

4.84

TABLES.

55

TABLE 46. — Summary of young of selected heterozygous English males by heterozygous

English females.

Homo-

Hetero-

Un-

Sire.

Grade.

Gener-
ation.

Self
young.

zygous
English

Mean
grade.

zygous
English

Mean
grade.

graded
English

Total
English

Total
young.

young.

young.

young.

2711

4

1

8

12

1.52

11

3.93

3

26

34

5086

4|

1*

35

20

1.38

44

3.96

. .

64

99

5375

4J

2*

25

17

1.20

29

3.79

46

71

5555

4f

2|

65

41

1.36

125

4.40

2

168

233

6370

5

3

21

18

1.79

54

4.66

72

93

6420

4i

3

4

10

1.80

19

4.33

t t

29

33

6071

5

3|

16

22

1.61

32

4.39

54

70

6072

5

3£

81

75

1.97

159

4.63

1

235

316

6964

5

4*

21

21

1.49

29

4.68

50

71

7699

5

4|

64

75

2.31

149

4.80

8

232

296

9532

5

4J

10

16

2.53

27

4.73

. .

43

53

9806

5

5|

7

10

2.52

16

4.86

. .

26

33

1212

5

5

29

26

2.87

61

4.98

87

116

534

5J

5

21

32

2.95

62

4.97

94

115

Totals . .

407

395

817

14

1,226

1,633

Percent .

24.9

24.2

50.0

0.9

75.1

TABLE 47. — Line of advance in the selection experiment with heterozygous English males.

Sire.

No. of
homo-
zygous
young.

Mean of
homo-
zygous
young.

No. of
hetero-
zygous
young.

Mean of
hetero-
zygous
young.

Advance,
homo-
zygous
young.

Advance,
hetero-
zygous
young.

2545

5

1 05

9

9 80

2711

12

1 52

11

3 93

47

1.13

5555

41

1.36

125

4.40

— .16

.47

6072

81

1 97

159

4 63

.61

.23

7699

75

2 31

149

4 80

34

.17

1212 and 534 .

58

2 91

123

4 97

60

.17

Total advance . .

1.86

2.17

Average advance . . .

.37

.43

TABLE 48. — Relative frequency of cross-overs among the gametes formed by FI rats of the

two sexes.

Series.

FI parent.

Dark-eyed
young.

Red-eyed or
pink-eyed
yellow
young.

Total
young.

Percentage
dark-eyed
young.

Percent
cross-over
gametes.

Repulsion. .

Female

101

837

938

10.8

21.5

Male

73

703

776

9.4

18.8

Coupling

Female

699

1 046

1 745

40 0

19.8

. *

Male . . .

556

731

1 287

43.2

13.5

56 HEREDITY IN RABBITS, RATS, AND MICE.

BIBLIOGRAPHY.

CASTLE, W. E., and P. B. HADLEY.

1915. The English rabbit and the question of Mendelian unit-character constancy.

Proc. Nat. Ac. Sci. 1, pp. 39-42, 6 figs.

— and JOHN C. PHILLIPS.

1914. Piebald rats and selection. Carnegie Inst. Wash. Pub. No. 195, 56 pp., 3 pi.

— and S. WRIGHT.

1916. Studies of inheritance in guinea-pigs and rats. Carnegie Inst. Wash. Pub. 241,

192 pp., 7 pi.

HALDANE, J. B. S., A. D. SPRUNT, and N. M. HALDANE, 1915. Reduplication in mice.
Jour. Genet. 5, 133-135. [Reference from Detlefsen (1918) Genetics, 3, p. 597.]

KING, HELEN D.

1918. The effects of inbreeding on the fertility and on the constitutional vigor of the
albino rat. Jour. Exp. Zool. 26, pp. 335-378.

PUNNETT, R. C.

1912. Inheritance of coat-colour in rabbits. Jour. Genet., 2, pp. 221-238, 3 pi.

PLATE 1

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Grades 1-18 of Dutch rabbits.

PLATE 2

19

21

Fig. 1!). a "white" Dutch rabbit, 97934, grade 15. Fig. 20. a ".lark" Dutch rabbit,
c?67()l, grade o. Fig. 21. a "tan " Dutch rabbit, 9 747. grade :!.

PLATE 3

FIGS. 1-5. Photographs of English rabbits adopted as grades 1-5 in classifying the variations
observed in the English pattern. Those shown in figs. 1 and 2 were homozygous, those shown
in figs. 3 to 5 were heterozygous.

Fin. 6. A ''high-grade" homozygous English rabbit, d'HTS, grade 3J£, genetically comparable
with a grade 5 heteroxygous English. Compare fig. 5.

Mlil. WHOI LIBRARY

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