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AN ANALYSIS OF THE EFFECTS OF SELECTION
BY A. H. STURTEVANT
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
WASHINGTON, 1918
89-
CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 264
PRESS OF GIBSON BROTHERS
WASHINGTON, D. C.
QH
37-/
PLATE 1
STURTEVANT
1. Dichaet male (5-bristled) 2. Extended female. 3. Wild-type female.
(Drawings by Miss E. M. Wallace.)
ANANALYSIS OF THE EFFECTS OF SELECTION.1
INTRODUCTORY SUMMARY.
The present paper describes a series of experiments aimed at de-
termining the causes of the variability in bristle number observed in
Dichset, a mutant race of Drosophila melanogaster (ampelophila) .
These experiments are discussed under several headings, as follows:
(a) Selection of plus and of minus variants was carried out. Both
plus and minus lines were obtained and were used hi the further ex-
periments.
(6) A plus line and a minus line were crossed, and an increase in
variability was observed in F2.
(c) Linkage tests were made, and by this means it was demon-
strated that modifying genes were present in the selected lines.
(d) Evidence against the hypothesis of contamination of allelo-
morphs was obtained.
(e) This evidence, and that obtained by other investigators, is then
utilized in a general discussion of the selection problem, and of the
hypothesis of contamination of genes. The conclusions are drawn
that selection is usually effective only in isolating genetic differences
already present; and that genes are relatively stable, not being con-
taminated in heterozygotes, and mutating only very rarely.
DICRET.
The mutant character known as Dichset was discovered by Dr.
G. B. Bridges, July 3, 1915. In an experiment involving the sex-
linked characters sable, forked, and cleft there appeared a single
female that had wings extended and bent backwards near the base,
like those of the mutant bent (Muller, 19146). In addition it was
observed that this female had only 2 dorso-central bristles, instead
of the 4 usually present. When mated to a male having the mutant
character eyeless, this female produced 48 normal offspring and 46
"Dichset," thus showing the character to be dominant.
Bridges's unpublished data show that the Dichset gene is in the third
chromosome, approximately 5 units to the left of pink.
The data published by Muller (1916) give the locus as 9.7 from sepia
(the locus farthest to the left of those as yet discovered), and 11.0
from spineless, on the right. My own (unpublished) data give:
Sepia Dichset, 2®JL = 14.9 p. ct. Dichaet spineless, — = 13.1 p. ct.
louy v/oi
*I am indebted to Mr. J. W. Gowen for much advice and assistance in connection with the
statistical treatment of the present problem. He has done a part of the actual calculations,
but is not responsible for any arithmetical slips, as I have myself done all the checking.
3
4 AN ANALYSIS OF THE EFFECT OF SELECTION.
The averages, roughly weighted according to number of individuals,
are: sepia Dichset, 13; Dichaet spineless, 12. This agrees with the
data of Bridges on the position of Dichaet with reference to pink, since
that locus is about 8 to the left of spineless.
Bridges also found that homozygous Dichaets are not produced.
The gene, like that of the yellow mouse, acts as a lethal when homozy-
gous. The result is that when Dichaets are mated together they
produce two heterozygous Dichaets to one not-Dichaet. This dis-
covery has been verified by the experiments described in this paper,
and by other experiments carried out by Muller and by the author.
TABLE 1.
FIGS. 1 and 2. — Two types of bristle distribution
in Dichffita — a "3" and a "7." Small post-alars are
present in fig. 2. These are never counted in the totals.
Culture
No.
No. of bristles.
Total.
3
4
5
6
881
882
883
900
2715
1
9
23
9
32
7
20
29
11
22
15
27
30
11
13
3
56
83
31
67
25
1
80
97
84
262
2 and 7 bristles have also been ob-
served in unselected stocks.
As shown in plate 1, fig. 1, the wings of Dichaet flies are held out from
the body and are bent back near the base. The number of dorso-
central bristles (on the dorsum of the thorax) on the original female
was 2 instead of 4, as is usually the case in the normal fly (plate 1,
figs. 1 and 3). This has since been found to be a variable character.
The number of dorso-centrals varies from 0 to 4, and sometimes
one or more of the scutellars may be missing. In addition, the an-
terior post-alars above and just behind the wing-base are reduced or
absent. Plate 1, figure 1, and text-figures 1 and 2 show some common
types. The work reported in this paper has consisted in selecting for a
high and for a low total of scutellar and dorso-central bristles. Counts
from five unselected cultures gave the results as shown in table 1.
The normal flies occasionally show variations in bristle number,
but these are much rarer than hi the case of Dichaet. MacDowell
(1915) has given some data on the frequency of these variations, and
has also reported on very extensive selection experiments with them
(1915, 1917). These experiments will be referred to below.
I have made bristle counts on a few unselected not-Dichaet stocks,
with the results shown in table 2.
The normal flies have 8 dorso-central and scutellar bristles in most
cases, while the Dichaets range from 1 to 8. But the 8-bristled Dichaets
are still distinguishable from normals, even when their wings are not
AN ANALYSIS OF THE EFFECT OF SELECTION.
unfolded enough so that they can be separated on that basis. This
is because the anterior pair of dorso-centrals never, so far as I have
observed, becomes as large as the corresponding pair in normal flies.
The anterior post-alars are also reduced in 8-bristled Dichsets. This
TABLE 2.
1
r-V» •
1
1
f
>
1
0
9
d«
9
d*
9
d"
9
d1
9
d1
Wild:
Falmouth, Massachusetts .
Berkeley, California
0
0
0
0
0
0
1
0
186
95
118
104
11
0
2
0
0
0
0
0
318
199
Mitchell, South Dakota. . .
0
o
0
o
0
o
0
o
226
59
213
51
4
1
0
o
0
o
444
112
o
o
o
o
16
21
o
1
o
o
38
Pink band
0
o
1
f)
103
99
1
0
o
n
209
Black
0
n
0
o
26
38
n
o
o
o
64
Ebony
n
n
o
n
80
92
o
o
o
o
172
Blistered
0
o
o
o
114
67
o
o
o
0
181
White
n
o
n
o
74
77
9
o
o
o
153
separability is a matter of some importance, since, because of the
lethal effect of Dichset, any Dichset culture may produce normal
flies. However, the spread wings can be and are used for the separa-
tion in all but the rather rare instances of failure to expand properly.
SEXUAL DIMORPHISM.
Calculations show that there is a slight but significant sexual di-
morphism in bristle number in the Dichset races. Random selection
of plus and of minus selected cultures gave the totak shown in table 3.
TABLE 3.
Bristle number.
1
2
3
4
5
6
7
8
Plus 9 ....
4
490
668
1,702
81
fi
2,951
Plus c?....
3
25
436
684
1,527
53
8
2,736
Minus 9 . .
5
17
1,517
712
424
7
2,682
Minus d" . .
1
39
177
1,190
615
332
2
2,356
These distributions give the statistical constants shown in table 4.
The first three columns show that there is a slight difference in the
means, the females being higher in both cases. In the case of the plus
series the difference is doubtfully significant; in the minus series it is
larger and certainly significant. The last column gives the chance
6
AN ANALYSIS OF THE EFFECT OF SELECTION.
that differences as great as those observed between the two distribu-
tions are due to random sampling. These values were obtained by
Pearson's x2 method (Pearson, 1911). This column makes it quite
certain that there is a significant sexual dimorphism in both series,
and also brings out again the fact that the dimorphism is greater in
the minus series.
TABLE 4.
9 Mean.
d1 Mean.
Difference.
P
Plus
Minus...
5.468±0.010
4.583± .010
5.428±0.010
4.436± .012
0.041±0.014
.147=*= .016
0.0001
.0000000 +
Because of the information given by this table it has seemed de-
sirable to present the data for males and females separately. This
has been done in the Appendix; but since the dimorphism is slight,
the data have been lumped in the statistical treatment given in the
body of the paper. The data in the Appendix make it possible to re-
calculate the constants separately if it should seem desirable to do so.
EFFECTS OF ENVIRONMENT.
In any selection experiment it is obviously very important to have
some information regarding the influence of environmental conditions
on the variable character used. If the observed variations in the
character are largely due to environmental causes, it should be very
difficult to accomplish much by selection; but if the environment
plays little part in causing variability, selection should be very effective
in isolating different types, and on the multiple-factor view variability
should show a marked decrease after a few generations of inbreeding.
In the case of Dichset, it has been observed that as cultures grow
older the flies frequently have fewer bristles. In such cultures it is
usually observed that the later flies are also smaller and that the food
conditions in the bottle have become unfavorable. It is, therefore,
essential in such experiments that conditions be made as nearly uni-
form as practicable.
The data in table 5 show that under ordinary conditions there is
considerable environmental effect. Eight pairs from the regular series
were transferred to second bottles, after staying the usual period in
the first one. Offspring were thus obtained with identical pedigrees
and differing only in that they were reared in separate bottles. No
attempt has been made to make conditions different in the two bottles,
which constitute a random sample of the conditions under which the
experiments were carried out. Table 5 shows the results obtained.
(The actual data are in the Appendix; the first three columns of the
AN ANALYSIS OF THE EFFECT OF SELECTION. 7
table will enable the reader to find them.) The last three columns
give the results of an application of the x2 test to the data. The last
column, headed P, gives the chance (1.0 representing certainty) that
deviations from identity as great as those observed could have re-
sulted from random sampling. It follows that in at least three cases
(the fifth, sixth, and seventh) the results given by the two broods were
significantly different.
TABLE 5. — First and Second Broods from Same Parents.
Culture Nos.
Series.
Gener-
ations
mother
inbred.
X1
n'
P
First
brood.
Second
brood.
1,907
1,908
1,912
1,924
2,074
2,078
2,087
2,475
1,996
1,997
1,998
1,999
2,140
2,141
2,142
2,518
1331
1002 rev
4
6
7
7
9
11
11
*18
3.74
5.60
2.10
6.05
22.09
16.81
19:80
5.22
3
5
4
5
4
4
5
3
0.16
.23
.55
.19
.0001
.001
.0005
.075
1002 . .
1002
900
Test of crossbr. plus .
864
Test of 1002
1Fu and Fn were mass cultures in this case.
There is one possible source of error hi these data: It has been
shown by Bridges (1915) that the amount of crossing over in the sec-
ond chromosome of Drosophila varies with the age of the female.
My own unpublished data show that this is also true for the third
chromosome. In the present case, if the female parents of the flies
observed were heterozygous for many modifying factors, such a
change in linkage might result in the production of genetically differ-
ent first and second broods. However, the female parents in these
cultures were in every case from at least four generations of brother-
sister inbreeding (see table 5, column 4)1 and in the significant cases
for 9 and 11 generations. It is therefore very unlikely that they were
heterozygous for many modifying factors. The two broods from
these females must, then, be of the same genetic constitution, and the
differences between them can only be due to environmental causes.
It follows that hi the experiments recorded below a significant part
of the variability is not genetic, but environmental.
METHODS.
With very few exceptions, the flies recorded in this paper were bred
from pairs, and in pint milk bottles. The food used was ripe un-
cooked banana, fermented in a stock yeast-culture for from 12 to 48
JThree cases in which the female parents were hybrids have been discarded (see 2091-2143,
3064-3116, 3066-3118 pairs in Appendix).
8
AN ANALYSIS OF THE EFFECT OF SELECTION.
hours (usually about 24 hours). Paper toweling was added to absorb
surplus moisture.
The experiments were begun in New York City in February 1916,
and were carried on there until the middle of June, when the material
was moved to Woods Hole, Massachusetts, and continued there until
the end of September. All these flies were kept at room temperature.
The work was resumed in November, in New York, and continued
until the middle of May 1917. During these last six months the
flies were reared in a heated case that was regulated by a thermostat,
so that the minimum temperature was about 24°, the maximum being
about 26°, except when room temperature went a few degrees higher,
as occasionally happened. It is to be noted that the constant-tempera-
ture series run more evenly (see especially 1002 line), thus suggesting
that temperature influences bristle number.
In order that the data presented in the Appendix may be correlated
with this information, if it seems desirable to do so, the following
table is presented. Each culture received a serial number at the time
the parents were mated, and these numbers run consecutively through-
out all the author's recent experiments (on other problems as well as
selection). These serial numbers are recorded in the Appendix.
Therefore, it is possible to fix approximately the date on which a cul-
ture was made up, if we know the date on which a culture with a simi-
lar number was made up. The dates of all cultures are noted on the
record sheets, but it has seemed hardly necessary to present more than
the following "landmarks."
TABLE 6.
Culture.
Date.
Culture.
Date.
Culture.
Date.
884
Feb. 3, 1916
1507
June 7, 1916
2389
Sept. 16, 1916
1006
Mar. 24, 1916
1617
June 23, 1916
2423
Nov. 18, 1916
1100
Apr. 16, 1916
1830
July 14, 1916
2601
Jan. 13, 1917
1150
Apr. 22, 1916
2000
Aug. 1, 1916
2950
Mar. 17, 1917
1301
May 15, 1916
2250
Aug. 28, 1916
3078
Apr. 15, 1917
1401
May 28, 1916
SELECTION.
If the variations observed in the Dichaet character are due to modi-
fication of the Dichaet gene itself, selection should be as effective in
inbred stocks as in any other kinds. If multiple factors are responsible
for the variations, the method of breeding should affect the result.
If a stock is closely inbred while being selected, it will soon become
fairly uniform, so that selection should be effective for only a com-
paratively short time. But if a strain is subjected to some crossing
it will become uniform more slowly, so that selection should be effective
AN ANALYSIS OF THE EFFECT OF SELECTION. 9
longer. Moreover, there is a chance of combining more of the desired
modifiers in the same individual when crossing is done, so that this
method might produce more extreme results than the inbreeding
method. However, each time a cross is made some of what has been
gained may be hidden by dominants in the other stock; therefore
progress might sometimes be slower.
Accordingly, in these experiments parallel series have been carried
on. In one set selection has been accompanied by continuous brother-
sister matings; in the other, frequent crosses have been made between
individuals more or less closely related. The same method has been
followed in both the plus and the minus selected lines. The four
series will be considered in order: (1) inbred plus; (2) crossbred plus;
(3) inbred minus; (4) crossbred minus.
INBRED PLUS SERIES.
Two main lines of this series have been carried on. A few cultures
have been made from other sources, but none of these are sufficiently
extensive so that we need follow their histories here.
864 LINE.
Culture 864, from which this line arose, was produced by a female
of the constitution p , s ° from culture 847, and two males from
p SSK6 T0
the sepia, spineless, kidney, sooty, rough stock; 847 was the result of
mating four peach, spineless, kidney, sooty, rough males from stock to a
female of the constitution -^-£. This female TABLE 7'
p
Dorso-
centrals.
Total....
Offspring.
was descended from the Dichaet, ebony, peach,
spineless, kidney, sooty, rough, and other stocks.
Her pedigree is not now traceable in detail.
At the time culture 864 was counted, the scu-
tellar bristles were not observed. The dorso-
central bristles were recorded on 30 flies, as
shown in table 7.
The 3 (almost certainly a 7, according to the system later adopted),
a male, was mated to a 2 (6) female to produce culture 893. For the
details of the remainder of the pedigree see Appendix.
In the accompanying tables and curves the offspring of culture
893, above, are considered FI. Table 8 gives the data for this line
summarized by generations. In this and the following tables, n is
the number of individuals in the generation, M is the mean bristle-
number of the generation, %
se
N
SeD'ss
Total.
39
37
3
3
6
10
0
1
99
The mating of DichaetX Extended (or vice versa) gave the following
result: Dichset, 99; Extended, 69; normal, 102; total, 270. If we
suppose some of the flies classified as "normal" to be in reality Ex-
tended, this result approximates to the 1:1:1 expected if Dichaet-
Extended flies die. . The fact that the Dichaets are only about a third
of the total shows that half the Dichaet gametes have been eliminated
somehow. One of the Dichaets and a number (4 individual matings
and 2 mass cultures) of the Extendeds have been tested, and neither
sort has produced the other. It is, then, safe to conclude that Dichaet-
Extended flies die.
Culture 1379, in which Extended first appeared, was made up by
mating together two 8-bristled flies, the male from 1145, the female
from 1253. The latter culture gave among other offspring 5 sevens
and 2 eights. The other eight, in 1356, behaved normally, as did
one of the sevens (in 1357). Culture 1145, however, gave no seven
and only the single eight. Since 1379 gave a result indicating that one
parent was Extended instead of 8-bristled Dichaet, it seems probable
that the male parent, from 1145, was the mutant. In either case,
the Extended parent was produced by mating a 7-bristled Dichset
AN ANALYSIS OF THE EFFECT OF SELECTION. 33
female to a 6-bristled Dichset male, both parents being from the cross-
bred plus selection series.
It follows from the data presented above that Extended is an allelo-
morph of Dichset intermediate between Dichset and its normal allelo-
morph in its somatic effect, and that it arose in a fly heterozygous for
these two factors. It is, then, the kind of thing one would expect
contamination of allelomorphs to produce. On the other hand, it
seems at least equally possible to suppose that it arose as a mutation
of one or the other allelomorph, without the presence of the other or
the one having had any influence on the event. In any case, the
process must be an extremely rare one, for it has been detected only
once, in spite of the very large number of offspring of heterozygous
Dichset flies that have been observed and bred.
Since the Extended flies have more bristles than Dichsets, it may be
supposed that the fact that the former arose hi a plus-selected series
is significant. Such a supposition has actually been made by Castle
(Castle and Phillips, 1914, etc.) with regard to a similar case in hooded
rats. As has been pointed out by MacDowell (1916), a mutation in
the direction in which selection is being made has a very much better
chance of being discovered than has one hi the opposite direction.
Moreover, these mutations have been demonstrated only in an ex-
tremely small number of cases; and a very elementary knowledge of
the theory of probability will suffice to convince one that a considerable
number of cases must be established before one can conclude that muta-
tions are more likely to occur in one direction than in another. No
argument based on one or two cases, however well established those
cases may be, can carry any conviction.
"DICH/ETE INTERMEDIATE."
The Star Dichset stock in the Columbia laboratory was found to
have in it some flies that were indistinguishable from Extended. It
seemed possible that these flies were due to an independent occurrence
of the Extended mutation. Since the Star Dichset stock is kept by
mating (Star) Dichset flies together in each generation, the mutation
responsible for these "intermediates" must either have occurred hi a
Dichset fly (as did the Extended mutation), or have been in the stock
since it was made up. The fact that Dichsets are mated together in
continuing the stock seemed, however, to show that the character
was not true Extended, since, as we have seen above, Dicheet-Extended
flies always die. But the possibility remained that ' f intermediate ' ' was
another non-lethal allelomorph of Dichset. Accordingly, tests were
made as follows :
Matings of Dichset by Dichset gave some intermediates, showing
that the continuance of the character in the stock was not dependent
on the use of non-virgin females, and proving that the character was
not Extended.
34 AN ANALYSIS OF THE EFFECT OF SELECTION.
Matings of intermediates by intermediates gave both intermediates
and normals, showing that the character was either dominant or irreg-
ular in appearance.
Matings of intermediate to specks and to black purples of other
stocks gave only normals, showing the character to be recessive.
Mating together the Fi normals from the last type of matings gave
a few intermediates ; but these were in no case speck or black or purple.
This is the usual behavior of a second-chromosome recessive, due to
no crossing over in the FI male. Hence "intermediate" is a recessive
character, and lying in the second chromosome. Its occurrence in
the Star Dichset must have been only a coincidence, and can have had
nothing to do with the presence of Dichset in that stock. The differ-
ence between this character and Extended is a striking illustration
of the danger of arguments as to the identity of characters based on
similarity of appearance.
NOT-DICH/ETS FROM SELECTED LINES.
As has already been pointed out, Dichset flies almost always have
fewer bristles than have normals. All Dichsets are heterozygous for
the normal allelomorph. Therefore, in such an experiment as this
one, in which Dichsets are repeatedly mated together, one obtains
normal flies the not-Dichset genes in which have been associated with
Dichaet genes for many generations. The experiment is, then, suited
for a study of the question as to whether or not factors "contaminate"
their allelomorphs. If this contamination occurs, one might expect
the not-Dichset flies to show a tendency to have fewer bristles than
they normally have, and the Dichsets to have more. That Dichsets
tend to increase in bristle number is very improbable. The stock
has now been kept, always of necessity in heterozygous condition, for
more than 40 generations. There is no evidence that any progressive
change has occurred, though no selection has been used in keeping
the stock cultures. The modal class at present (5 bristles) is actually
lower than the class (6) of the original mutant.1
There are some data regarding the bristles of the not-Dichaets pro-
duced by selected Dichsets. Counts of these bristles have been taken
only occasionally (see table 24), but whenever a bristle number other
than 8 has been observed in such flies it has been noted on the record
sheet. Examination of these notes shows that in the minus-selected
series there are several records of 6 and 7 bristled not-Dichsets, but
none of numbers higher than 8. In the plus selected lines there are a
number of records of nines and tens, but no sixes and only 1 seven
(from 1190, an F6 of the crossbred plus series). The complete counts
taken of bristle numbers are given in table 24.
'It may be pointed out that the familiar yellow mouse and several similar cases in Drosophila
afford evidence of the same sort against contamination.
AN ANALYSIS OF THE EFFECT OF SELECTION.
35
There is no evidence for contamination. With the one exception
noted above, all the variations are in the direction for which the
Dichsets were being selected. On the multiple-factor view one would
expect this result, since it would seem likely that any modifier would
usually affect Dichsets and not-Dichaets hi the same direction. The
one exception, a 7 from 1190 of the crossbred plus series, is scarcely
surprising on this hypothesis, in view of the facts that unselected not-
Dichset races may produce sevens (see table 2), and that 1190 was prob-
ably not homozygous for a large number of plus modifiers. Since
this individual was not tested, it would perhaps be futile to argue the
case further.
TABLE 24.
Bristle Nos.
Culture.
Series.
Genera-
tion.
6
7
8
9
10
1277
864 plus
7
57
1285
Crossbred plus ....
7
35
i
1357
Crossbred plus ....
8
33
4
1810
864 plus
10
51
1811
1002 plus
7
16
1268
Crossbred minus. . .
6
13
1273
Crossbred minus. . .
7
33
1878
Crossbred minus . . .
10
15
1879
Crossbred minus. . .
10
20
1881
Crossbred minus. . .
10
23
1882
Crossbred minus. . .
10
31
1892
Crossbred minus. . .
10
10
1986
1331 (speck) minus
5
12
1996
1331 (speck) minus
5
i
34
2015
Crossbred minus. . .
11
88
It may be noted here that in the Star Dichset stock referred to above
(p. 31) there were found to be numerous not-Dichsets with 9 and 10
bristles. Unfortunately, no counts were made on these flies, and the
nature of the extra bristles was not determined. The stock has since
been " purified," to rid it of certain other mutations, and the extra-
bristled flies, formerly plentiful, have now disappeared. This stock,
as stated above, was continued by mating together (Star) Dichset
flies, without regard to bristle number. These extra-bristled not-
Dichsets therefore furnish evidence of the same type as that just dis-
cussed, except that the race was not selected for bristle number.
36 AN ANALYSIS OF THE EFFECT OF SELECTION.
GENERAL DISCUSSION.
THE SELECTION PROBLEM : QUESTIONS AT ISSUE.
It appears to the writer that the three questions below are the chief
ones at issue in the discussion of the selection problem:
1. Does selection use germinal differences already present, or differences
that arise during the experiment, or both?
2. In case it uses new differences, does it cause them to occur more
frequently, and does it influence their direction?
3. Are differences, already present or arising de novo, more likely to
occur in the locus of the gene under observation, or in other loci?
It is not, I think, questioned by any one that selection may effect
either gradual or sudden change in the mean character of mixed races,
or that it may even, occasionally, produce such an effect in pure races
if a mutation in the desired direction happens to occur.
1. Does selection use germinal differences that are already present, or differences
that arise during the experiment ?
Everyone who has bred animals or plants is familiar with the fact
that different strains, even when rather closely related, differ in all
sorts of minor points — size, proportions of organs, shade of color, resist-
ance to disease, fertility, temperament, rate and habit of growth —
in fact, in almost any respect that one investigates. This can only
mean that such strains differ genetically; and since the kinds of differ-
ences are usually so numerous, they probably usually have many
genetic differences — i. e., they differ in respect to many factors. In
any race not normally self-fertilizing or closely inbred, crosses between
individuals of different constitution must then be frequent. And
such crosses must, on the assumption that the original differences were
Mendelian, lead to the production of a population more or less hetero-
zygous for factors that produce minor effects on all sorts of charac-
ters. The assumption that the differences are Mendelian rests on the
observed facts, (1) that demonstrably Mendelian factors may produce
effects on practically any kind of character studied, and effects of
practically any observable degree; and (2) that non-Mendelian inher-
itance has never been demonstrated, except for a few cases of plastic
characters in plants and cases of infectious diseases.1 Other kinds
of inheritance may exist ; but the available data indicate that they must
be extremely rare. Therefore the chances are that any observed
difference between two strains is Mendelian.
If these conclusions be accepted, it follows that any strain not very
closely inbred is likely to be heterozygous for factors influencing many
characters. Selection for these characters will then be effective in
isolating favorable combinations of such "modifying factors."
K)ne may refuse to call these cases of inheritance if he chooses to define that term so as to
exclude them.
AN ANALYSIS OF THE EFFECT OF SELECTION. 37
Mendelian differences are still arising by mutation and may arise
in a selection experiment as well as anywhere else; and those that arise
in such an experiment are as likely to affect the character under ob-
servation as are any Mendelian differences taken at random. It is
therefore probable that selection sometimes makes use of variations
that arise during the course of the experiment, or, rather, that varia-
tions which may be available do arise.
The question is, what is the relative frequency of the two kinds of
available factor differences — those already present and those that arise
de novo? The answer is found by investigation of the data on selection
in inbred lines and in crossbred lines. In closely inbred strains there are
not likely to be many factor differences present when selection is begun,
while in crossbred lines these differences are likely to be numerous.
That selection is usually effective in crossbred lines is a well-known
fact, demonstrated many times with many different organisms. Not
many experiments have been carried out on closely inbred material,
but those of Johannsen (1903), MacDowell (1917), and the present
paper (p. 11) show that selection may be without effect in such lines.
In two of these cases selection was effective until the lines became highly
inbred. But mutations influencing the characters under observation
have been obtained in the selection experiments of Castle and Phillips
(1914), Morgan (Morgan, Sturtevant, Muller, and Bridges, 1915,
p. 205), Lutz (1911), and those reported in this paper (p. 3 1).1
Apparently, then, selection produces its effects chiefly through
isolation of factors already present, but occasionally available muta-
tions do arise during the course of the experiment.
2. Does selection cause mutations, or influence their direction?
The usual selection experiment consists hi breeding from individuals
that are extreme in some respect. This extreme character may be
environmental in origin, or it may be caused by germinal differences.
In the first case, no geneticist is likely seriously to maintain that selec-
tion will have any effect whatever. In case the extreme character
is germinal in origin, selection will of course be effective hi eliminating
certain genetic types. Moreover, given a combination of genes that
produce the character in a certain degree, we are evidently in a better
position to reach a further stage than if we have the character less well
developed. For how long a tail will be when it gains an inch evidently
depends on how long it was before it gained that inch. But it seems
incomprehensible that selection of individuals of a constitution favor-
1Evidence derived from forms that reproduce asexually is also available in studying this
question, for such reproduction commonly prevents recombination, and therefore gives results
comparable with those obtained from homozygous strains. Some of the evidence obtained from
studies on asexually produced Protozoa (e. g., Calkins and Gregory. 1913; Jennings, 1916; Middle-
ton, 1915) has shown that selection may be very successful in changing such forms. But it is
very doubtful if these animals are comparable with the Metazoa in the method of distribution
of their chromatin. It seems not improbable that in some cases recombination may here be
possible in asexual reproduction.
38 AN ANALYSIS OF THE EFFECT OF SELECTION.
able to the development of a given character can make more likely
the occurrence of factorial variations affecting that character, or
variations affecting it in a given direction. As a matter of fact, there
is no evidence for such a conclusion. The occurrence of mutations is
ordinarily such an extremely rare phenomenon that it would be very
difficult to obtain statistically significant data in the matter. More-
over, when one is selecting for a character, one is examining his animals
or plants for th;>,* character with unusual care, so that any mutations
in that character are very likely to be observed and tested, provided
they are in the direction in which selection is being- carried out. It
follows from these considerations that extremely careful controls are re-
quired before any data on these questions can have any significance.
3. Are variations more likely to occur in the locus of the gene under observation,
or in other loci?
In Drosophila over 25 different and independent mutant factors affect
the color of the eye. In mice there are 7 or more independent factors
affecting coat-color. According to Little (1915) there are 2 and prob-
ably 3 independently segregating factors that affect spotting in these
animals. There are at least 14 and probably more definite genes (in
different loci) that affect bristle number in Drosophila, not counting
the "modifying factors" studied by MacDowell and the writer.
In view of these and many similar facts, it is certain that changes
in a given character may be brought about by changes hi many differ-
ent parts of the germ-plasm. If selection of a given mutant race, say
hooded rats or Dichset Drosophila, is likely to cause or to isolate muta-
tions in the gene that differentiates that race from the normal type
(i. e., the hooded factor or the Dichaet factor) rather than in any other
factors, it follows that mutant allelomorphs must be more variable
than " normal" ones. For, by analogy with mice, hooded rats are
homozygous for the normal allelomorphs of several possible factors
affecting spotting; and Dichset flies are certainly homozygous for the
normal allelomorphs of at least 13 mutant factors that affect bristle
number. It may be true that mutant factors are on the average more
variable than their normal allelomorphs; but no evidence to that
effect is at hand; and owing to the great difficulty of statistical treat-
ment of the frequency of mutations alluded to above, such evidence
will be very difficult to obtain.1
In the absence of such evidence, it is more probable that variations
will appear in other factors, since there are many of them to vary,
but commonly only one that is responsible for the difference under
observation. That changes of the one factor itself may occur in selec-
tion experiments, however, has been shown by Castle (Castle and
Wright, 1916) and the writer (p. 31). It does not follow that selection
has caused these variations or that they are more likely to occur than
are variations in other factors.
'Evidence has been obtained by Emerson (1917), who used unusually favorable material,
that shows clearly that different allelomorphs may at times differ greatly in their mutability.
AN ANALYSIS OF THE EFFECT OF SELECTION. 39
CONTAMINATION OF ALLELOMORPHS.
When two races that differ in quantitative characters are crossed,
it is frequently observed that FI is fairly uniform, and that F2 shows
an increase in variability together with the production of forms inter-
mediate between the parent races and often different from the FI.
There are two current methods of accounting for these cases:
(1) The two races are assumed to have differed in a number of
Mendelian factors affecting the character in question. The observed
result is then explained as due to the recombinations of these factors.
(2) The two races are assumed to have differed in only one factor
affecting the character in question, and the new types observed in FI
are supposed to be due to "contamination" hi the FI hybrid, that is,
allelomorphs present in the heterozygote are supposed to have influ-
enced each other, so that they do not come out unchanged.
The fundamental principle of the first explanation — that more
than one factor may influence the same character — is admitted by
all Mendelians. But many of the adherents of that explanation are
unwilling to admit that "contamination of allelomorphs" has ever
been experimentally demonstrated. Let us then examine the evi-
dence that is brought forward hi support of that assumption.
The following quotations are the chief ones bearing on the ques-
tion that I have been able to find hi recent literature:
"The currently accepted explanation (of size inheritance), which its
supporters choose to call 'Mendelian,' rests upon the idea of game tic purity
in Mendelian crosses. It assumes that Mendelian unit-characters are un-
changeable and unvarying, and that when they seem to vary this is due to a
modifying action of other unit-characters (or factors) .... The idea
of unit-character constancy is a pure assumption. In numerous cases unit-
character inconstancy has been clearly shown, as in the plumage and toe
characters of poultry according to the observations of Bateson and Daven-
port, and the coat-characters and toe-characters of guinea-pigs in my own
observations. Unit-character inconstancy is the rule rather than the ex-
ception." (Castle, 19166, p. 209.)
" . . . .1 have shown in numerous specific cases that when unlike
gametes are brought together in a zygote they mutually influence each other;
they partially blend, so that after separation they are less different than they
were before. The fact remains to be accounted for that partial blending does
occur (1) when polydactyl guinea-pigs are crossed with normals (Castle,
1906); (2) when long-haired guinea-pigs are crossed with short-haired ones
(Castle and Forbes, 1906); and (3) when spotted guinea-pigs or rats are
crossed with those not spotted (MacCurdy and Castle, 1907). Davenport
has furnished numerous instances of the same thing in poultry; indeed, he has
shown that "imperfection of dominance" and of segregation are the rule rather
than the exception in Mendelian crosses in poultry." (Castle, 1916d, p. 253.)
" . . . . The English unit-character had changed quantitatively in trans-
mission from father to son. This seems to us conclusive evidence against
the idea of unit-character constancy, or 'gametic purity.'" (Castle and
Hadley, 1915.)
" . . . . We are often puzzled by the failure of a parental type to reappear
in its completeness after a cross — the merino sheep or the fantail pigeon, for
40 AN ANALYSIS OP THE EFFECT OF SELECTION.
example. These exceptions may still be plausibly ascribed to the inter-
ference of a multitude of factors, a suggestion not easy to disprove; though it
seems to me equally likely that segregation has been in reality imperfect."
(Bateson, 1914.)
Fractionation is referred to by Bateson in this same paper as prob-
ably due to imperfect segregation. Illustrations are Dutch rabbit
and Picotee and other sweet peas. (See p. 298.)
"Accordingly we seem limited to the conclusion that a slowly blending
gene is involved in the cross between early flowering and late flowering peas,
that the blending after one generation of heterozygosis may be small in
amount, but after three generations it is in the majority of cases practically
complete, so that the commonest ' constant ' class in the entire hybrid popula-
tion is one strictly intermediate between the modes of the parental varieties.
This interpretation is entirely in harmony with the observed modification
through crossing of many Mendelizing characters, as observed by Daven-
port, Bateson, and many others in poultry, guinea-pigs, swine, and other
animals, as well as in plants." (Castle, 19166, p. 215.)
Hayes (1917) states on the basis of his experiments with variegated
maize:
" . . . . One might conclude that certain heterozygous combinations
produce germinal instability which exhibits itself either as imperfect segrega-
tion, gametic contamination, or sporophytic variation."
In these quotations the following cases have been cited as evidence
in favor of contamination, and therefore calling for investigation :*
1. Polydactyl guinea-pigs (Castle, 1906).
2. Long-haired guinea-pigs (Castle and
Forbes, 1906).
3. Spotted guinea-pigs and rats (MacCurdy
and Castle, 1907).
4. English rabbits (Castle and Hadley, 1915) .
5. Poultry, plumage and toe characters
(Bateson and Davenport).
6. Merino sheep.
7. Fantail pigeons.
8. Dutch rabbits.
9. Picotee and other types of sweet peas.
10. Flowering time in peas (Hoshino, 1915).
11. Unspecified case in swine.
12. Variegated pericarp in maize (Hayes,
1917).
Before we can discuss some of these cases intelligently it is neces-
sary that we make sure what Castle means by the terms "gametic
purity" and "unit-character." Unless these terms are understood
in such a way as to eliminate from consideration the idea of recombina-
tion of independent factors there is, of course, nothing to discuss.
If by gametic impurity or inconstancy of unit-characters is meant that
recombination of modifying factors occurs, the existence of such phe-
nomena must be granted at once — this is, in fact, the main contention
of the school of "pure line" advocates or "mutationists." I think the
two following quotations from Castle are sufficient to show that there
need be no disagreement on the question of defining these terms:
"What we want to get at, if possible, is the objective difference between one
germ-cell and another, as evidenced by its effect upon the zygote, and it is
lThe rough-coated guinea-pig was formerly cited (e. g.. Castle and Phillips, 1914), but is now
never used. This is because Wright (Castle and Wright, 1916) has shown the results to be due
to multiple factors.
AN ANALYSIS OF THE EFFECT OF SELECTION. 41
the constancy or inconstancy of these objective differences that I am dis-
cussing. If these are quantitatively changeable from generation to genera-
tion, then change in the variability of the zygote composing a generation
might arise without factorial recombinations."1 (Castle, 1914a.)
"The head, the hand, the stomach, stomach-digestion, these are not unit-
characters so far as any one knows. But if a race without hands were to
arise and this should Mendelize in crosses with normal races, then we should
speak of a unit-character or unit-factor for 'hands/ loss of which or variation
in which had produced the abnormal race. But in so doing we should refer
not to the hand as an anatomical part of the body nor to the thousand and
one factors concerned in its production, but merely to one hypothetical factor
to which we assign the failure of the hand to develop in a particular case.
It is immaterial whether we call this a unit-character or unit-factor or use both
terms interchangeably " (Castle, 19166, p. 100.)
1. POLYDACTYL GuiNEA-PlGS.
The most extensive data on this case are apparently in the paper
(Castle, 1906) cited in the quotation already given. The extra-toe
character was at first irregular in appearance, but was improved by
selection. In five generations, without very close inbreeding, a practi-
cally uniform race was obtained. When crosses to normal were made,
the FI results varied from nearly all normal to nearly all polydactylous.
F2 contained both normal and extra-toed individuals. It is pointed
out by Castle in this paper that the results are very similar to those
obtained by Bateson from polydactylous fowls. Bateson's comment
on that case is given below.
In the absence of any definite data regarding F2 counts, the case
as reported is entirely explicable on the multiple-factor view. Castle
himself said of it, five years after the publication of the above paper:
"An alternative explanation is possible, viz., that the development of the
fourth toe depends upon the inheritance of several independent factors, and
that the more of these there are present, the better will the structure be
developed. The correctness of such an interpretation must be tested by
further investigation." (Castle, 1911, p. 101, footnote.)
So far as I have discovered, such further investigations have not
yet been reported, although five years later this case is listed as No. 1
among those that demonstrate contamination of allelomorphs.
2. LONG-HAIRED GUINEA-PIGS.
The reference given for this case (Castle and Forbes, 1906) seems
to contain the most recent and complete data regarding it.
Angora guinea-pigs appeared in a short-haired stock, apparently
as segregated recessives. On crossing to short and extracting, there
were produced some animals of intermediate hair-length, and some
unusual ratios. But similar intermediates appeared in another strain
of shorts, apparently uncrossed with angoras, thus making it highly
probable that we are dealing here with a factor already present hi the
Italics mine.
42 AN ANALYSIS OF THE EFFECT OF SELECTION.
race, and not produced by the cross of angora X short. The unusual
ratios are based on quite small numbers, and the authors admit that
there are difficulties in separation of the three classes, apparently
due to overlapping. Moreover, we are given the results only in total,
not from each mating separately.
Castle himself has said of this case: " ... a single unit-character
is concerned. Crosses in such cases involve no necessary change in
the race, but only the continuance within it of two sharply alternative
conditions." (Castle, 1911, p. 39.)
3. SPOTTED GUINEA-PIGS AND RATS.
The reference given for these cases is MacCurdy and Castle (1907).
I am unable to find in that paper any evidence regarding guinea-pigs
that bears on the question of contamination. Nothing but selection
experiments are reported. There is, so far as I am aware, no evidence
of significance in this connection in the more recent literature on
spotting in guinea-pigs.
The evidence referred to from rats is apparently that obtained from
crosses between hooded and Irish races. Hooded rats extracted
from such crosses had more extensive colored areas than the uncrossed
hooded rats. The data given by Castle and Phillips (1914) and ana-
lyzed by MacDowell (1916) show that this is true only when the hooded
race is a "minus" one. The "plus" hooded race becomes less pig-
mented when crossed to Irish (or to self) . MacDowell has shown that
these results conform very closely to the expectations based on the
multiple-factor view.
The later evidence on the case of the hooded rat is discussed else-
where in this paper.
4. ENGLISH RABBITS.
The data for this case are contained hi two papers (Castle and
Hadley, 1915a, 19156), in each of which the full presentation is made.
The spotting of the English rabbit is a dominant character and is
somewhat variable. A single heterozygous male, of the grade desig-
nated 2, was mated to a number of Belgian hares. 187 English young
were produced, of mean grade 2.43, and of these FI English, a buck of
grade 3.75 (only one FI English was of higher grade), was then mated
to the same Belgian hare females. 189 English young, of mean grade
2.92, were produced.
This case presents no difficulties for the multiple-factor view, since
no evidence is given that indicates the original English buck to have
been homozygous for all modifying factors, or that prevents us from
supposing the Belgian mother of the FI buck to have transmitted more
plus modifiers to him than were present hi his father. Under the
circumstances, it would have been very surprising if the two lots of
young had been of the same mean grade.
AN ANALYSIS OF THE EFFECT OF SELECTION. 43
5. PLUMAGE AND TOE CHARACTERS IN POULTRY.
We are referred to the observations of Bateson and Davenport
for these cases. In one instance it is stated that Davenport has shown
that " imperfection of dominance" and of segregation are the rule in
poultry. The question of imperfection of dominance is not apropos
in this connection. As Castle has said, regarding another case :
" . . . .if black is crossed with brown, the crossbreds are apt to develop
in their coats more brown pigment granules than do homozygous or pure
blacks. Nevertheless, we have no reason to question the entire purity of
the gametes, both dominant and recessive, formed by such cross-bred black
animals. It is the dominance, not the segregation, which is imperfect."
(Castle, 1911, p. 91.)
That FI results do not bear on the question has been shown by
Bateson (1909), who says with regard to polydactylous fowls:
"It might be pointed out that when, as in these examples, the abnormal
result is clearly perceptible in E\, no question arises as to the occurrence of
an imperfect segregation. The peculiarity is evidently zygotic, and is caused
either by some feature of zygotic organization, or by the influence of external
circumstances." (Bateson, 1909, p. 251.)
Moreover, in any case involving irregularities hi dominance, im-
perfect segregation in crosses between different breeds would be very
difficult to demonstrate.
6. MERINO SHEEP.
No reference to the data in this case are given. I have been unable
to discover anything more definite than a few general statements by
practical breeders regarding the effects of crossing Merinos.
Bateson admits, in the passage quoted above, that this and the
next case "may be ascribed to the interference of a multitude of
factors."
7. FANTAIL PIGEONS.
This case has been studied by Morgan (Morgan, Sturtevant, Muller,
and Bridges, 1915, p. 186). The fantail type did not reappear in the
comparatively small F2 generation, but individuals not far from the
fantail were obtained; and when the FI hybrids were mated to fan-
tails, several of the offspring fell within the range of the fantail race.
Bateson's "failure of a parental type to reappear in its completeness
after a cross" is, then, scarcely applicable to this case.
8 AND 9. DUTCH RABBITS AND CASES IN SWEET PEAS. FRACTIONATION.
These are the specific cases cited as illustrations of Bateson's theory
of "fractionation" or "subtraction stages," of which he states that
'it is to be inferred that these fractional degradations are the con-
sequences of irregularities in segregation." In the case of the sweet
pea, Bateson has pointed out that white flowers and the extreme dark
44 AN ANALYSIS OF THE EFFECT OF SELECTION.
flowers of the deep purple Black Prince were among the earliest varia-
tions to appear, while the intermediate forms have arisen later, as he
suggests by fractionation. It would seem to follow that they have
arisen in heterozygous forms, for otherwise the fact that the larger
variants appeared first would be of no significance. There is, I think,
no evidence to show that the later variations did actually arise in
heterozygous forms, either in sweet peas or in rabbits. These factors
are all inherited separately, and this fact would seem to rule them
out of consideration if one adopts the chromosome theory of inheritance
or if one appeals to multiple allelomorphs as evidence in favor of the
variability of genes. In short, we have no evidence regarding the
origin of these forms, and their present behavior seems to indicate
that they are not due to fractionation. The only evidence in favor
of such a hypothesis is the somatic appearance of the characters.
10. FLOWERING TIME IN PEAS.
Castle (1916a, p. 324) has summarized this case as follows:
"Hoshino (1) recognizes that gametic contamination results from cross-
ing early and late flowering varieties; (2) recognizes also that variation may
occur among the cross-bred families, as well as in different pure lines of the
uncrossed races, as regards the 'quality,' value, or potency of the same gene;
(3) although Hoshino does not refer to the fact, his observations show clearly
that genetic variation of a gradual or fluctuating sort occurs in at least one
of the varieties which he crossed.
" . . . . What I want to suggest is that in these several agencies we
have a sufficient explanation of the variation observed in Hoshino's F2, F3,
and F4 generations, without invoking a two-factor hypothesis (as Hoshino
has done), one factor being enough."
Castle's argument is that a difference in one pair of genes is sufficient
to account for the result, if contamination be assumed; and that one
difference is a simpler assumption than two. I have argued here that
such an assumption is not simpler, unless we can find positive evidence
that contamination ever occurs. In the present case, then, we must
turn to the evidence that led Hoshino to suppose contamination to
have occurred.
Hoshino crossed an early-flowering pea and a late-flowering one.
The FI was nearly as late as the late parent; F2, obtained by self-
fertilizing FI, approximated fairly closely to 3 late : 1 early, but the
two classes were somewhat more variable than the corresponding
parent varieties, and apparently overlapped slightly. Hoshino self-
fertilized 236 of these F2 plants and obtained 46 families that he
classified as constant, i. e., supposedly homozygous. This is a fair
approximation to the 1 in 4 expected if two pairs of genes are respon-
sible for the result. Hoshino shows that two pairs of genes will, in fact,
account for most of the results obtained. There are certain facts not
thus accounted for, but Hoshino shows (p. 265) that "secondary"
AN ANALYSIS OF THE EFFECT OF SELECTION. 45
modifiers (i. e., modifiers producing only small effects) will account
for all these facts, with a single exception. Three families were ob-
tained from F2 plants that must, on the two-factor view, have been
of the same constitution. These plants were heterozygous for one
pair of genes only. They produced, in F4, the same type of later
constant (homozygous) families, but differed slightly in the flowering
times of the earlier constant families produced. According to Ho-
shino's view, if the earlier types differed the later ones should have
differed in the same direction, because they must have received the
same " secondary modifiers." This objection is not valid, for specific
modifiers that act only in the presence of certain other genes are well
known (see especially Bridges, 1916), and are sufficient to account
for the differences observed. This argument is the only one that
Hoshino gives to support his conclusion that contamination must
have occurred. We must then conclude that the case does not furnish
positive evidence for contamination, since it is explicable without re-
course to that hypothesis.1
11. UNSPECIFIED CASE IN SWINE.
This case is cited by Castle (19166, p. 215), but no references or
authorities are given. It appears, however, from the legend under fig-
ure 93 (opposite p. 139) that the belted character is the one referred to.
The only data bearing on this case that I have found are presented by
Spillman (1907), and consist of information supplied largely by prac-
tical swine-breeders. Spillman himself interpreted the case as one in
which two factor-pairs are involved. The data also suggest the pos-
sibility that we are dealing with a case of "imperfect dominance" simi-
lar to those in poultry. At best, the data are meager and indefinite.
12. VARIEGATED PERICARP IN MAIZE.
The paper of Hayes (1917) referred to above should be studied
in connection with those of Emerson, particularly his full paper (Emer-
son, 1917), dealing with the same character. These two workers have
shown that there is a remarkable series of multiple allelomorphs in
this case, and Emerson has shown very clearly that some of these
allelomorphs mutate quite frequently — the only established instance
of the sort.
xWe are not here directly concerned with Castle's contention that Hoshino's results prove
the effectiveness of selection within a pure line. I can not, however, refrain from a few comments
on that contention. Castle states (1916a, p. 324), in connection with the differences in flowering-
time between the offspring of early and late flowering sister-plants: "From long experience in
studies of rats with such small differences as are here indicated I have no hesitation in concluding
that fluctuating variation of genetic significance is here in evidence." One wonders how ex-
perience in dealing with differences in pigmentation in rats can give an observer special ability
in determining by inspection the significance of three-tenths of a day diffeience in the flowering
time of peas. With respect to Castle's calculations from Hoshino's data, it may be pointed
out that the greatest favorable difference recorded, 1.27 days, is incorrect, and should read 0.26
day. In view of the fact that there is no guarantee that the material used was homozygous,
I have thought it scarcely worth while to recalculate all the differences, or to determine their
probable errors; but it is certain that the probable error of each difference is of the same order of
magnitude as the average difference itself, i. e., about 0.3 day.
46 AN ANALYSIS OF THE EFFECT OF SELECTION.
Hayes has, by selection from a mixed population, established four
different grades of variegation (including self-colored and colorless)
that breed true and that represent four allelomorphs. The two in-
termediate types, "mosaic" and "pattern," are the ones of special
interest in the present connection. When these two types were
crossed, the mosaic type was dominant, but there was an increase in
variability in FI and some individuals with more pigment than either
parent were obtained. The parent races had been selfed and selected
for about six generations before the cross was made. In view of the
great amount of heterozygosis that seems to be normally present in
maize, and the large number of chromosome pairs (20?), this seems to
be hardly sufficient to make certain that both races were pure for their
modifiers. The increased variability of FI is therefore not surprising;
and that phenomenon would of course be expected to be followed by
a still greater increase in variability in F2. Such an increase was, in
fact, observed, and is the chief basis for Hayes's conclusion that con-
tamination may occur. The data are not sufficient to demonstrate
that new allelomorphs arise more often in heterozygotes than in homo-
zygotes; and even if it be shown that they do so, it does not follow that
there has been contamination of allelomorphs. There are too many
unknown factors involved in the production of these new allelomorphs
for such a conclusion to be valid without very careful controls.
It appears from the foregoing review that the cases cited as illustra-
tions of contamination of allelomorphs or imperfect segregation are
all explicable on the multiple-factor view, or rest on extremely indefinite
data.
One series of data bearing on the question has been presented in
this paper (p. 32), and has been interpreted as giving evidence against
contamination. Three other cases have been worked out by Muller
(1916) and Marshall and Muller (1917). Muller kept three mutant
characters of Drosophila in heterozygous condition for about 75
generations. The factors were kept constantly in flies heterozygous
for their normal allelomorphs, so that the characters remained unseen
for a long time.
Muller extracted one of these characters (dachs) from this stock,
and measured the tarsi, using the length of thorax as a standard of
comparison. Dachs flies are characterized by shortened tarsi; and
the flies from the heterozygous stock were found to have tarsi actually
a trifle shorter than those found in a stock that had been kept pure for
dachs. This result was not very conclusive, chiefly because it was
based on a very few flies.
Marshall and Muller made much more extensive studies with the
whig characters, curved and balloon, derived from the same heterozy-
gous stock. They obtained a similar result; the wings were no nearer
AN ANALYSIS OF THE EFFECT OF SELECTION. 47
the normal than were those of curved and of balloon flies that had been
kept in pure stocks. These results, taken in connection with the data
presented above for bristle number in flies from lines heterozygous
for Dichaet, furnish definite evidence against contamination of allelo-
morphs in heterozygous forms.
CASTLE'S EXPERIMENTS WITH HOODED RATS.
Perhaps the best known selection experiment is that carried out by
Castle and various collaborators (Castle and Phillips, 1914, Castle
and Wright, 1916, etc.) with hooded rats. The theoretical conclu-
sions reached by Castle are not in agreement with those arrived at
by various other investigators, including the author, although for the
most part the data obtained are very similar. Castle's results have
been discussed by Muller (1914o) and MacDowell (1916), who have
shown in detail that all the data known to them were explainable on
the multiple-factor view, without recourse to such hypotheses as
contamination of factors or production of factorial variations by selec-
tion. One point has, I think, not been sufficiently emphasized by
them, namely, that the rat experiments are hard to evaluate properly
until we are in possession of more accurate data regarding the pedi-
grees. Since these two criticisms were written, Castle (Castle and
Wright, 1916) has given some additional data, which he has used,
in a reply (Castle, 1917) to MacDowelTs paper, as arguments against
the latter's conclusions.
With regard to the question of pedigrees, to take up these ques-
tions in order, the main point on which information is desired is:
How closely inbred were the rats, both before and after the beginning
of the selection experiment? The following quotations contain most
of the available evidence on this matter:
"Since the entire stock is descended from a very few individuals (less than
a dozen), and we have at no time hesitated to mate together brother and
sister, provided they varied in the same direction, but have always used the
most extreme individuals (plus or minus) which were available, to mate
with each other, it follows that very close inbreeding must have occurred
throughout the experiment." (Castle, 19146.)
"It is impossible for a colony of 33,000 rats to be produced from an original
stock of less than a dozen animals, with constant breeding together of these
which are alike in appearance and pedigree, and with continuous selection of
extremes in two opposite directions, without the production of pedigrees
which in the course of each selection experiment interlock generation after
generation and finally become in large part identical with each other. This
has been repeatedly verified in individual cases, but is incapable of a more
generalized statement or of demonstration in generalized form. At least I
am unable to devise such demonstration." (Castle,
Elsewhere (Castle and Phillips, 1914, p. 20) it is stated that part
of the original stock consisted in a mixed lot of trapped rats that "had
probably arisen by the crossing of an escaped albino rat with wild
48 AN ANALYSIS OF THE EFFECT OF SELECTION.
ones." We do not know where the rest of the stock came from, and
we do not know how the animals used to start the selection experi-
ments were derived from these sources. We do not know how many
individuals were used to start the selection experiment ; and we do not
know anything as to the relationship between the rats in the two series
(plus and minus). And, finally, we have only very indefinite data
as to what system of breeding was followed during the experiment.
All this information is very much needed, if we are to know how to
interpret the results. It is conceivable that each series was split up
into a number of separate lines, and that these have been crossed
from tune to time. Such a system would result in bringing together
modifying factors more slowly than would a system of very close in-
breeding. It is, of course, very improbable that any such system has
been followed; and such an assumption is by no means necessary for
a multiple-factor interpretation of the results. But definite informa-
tion is very desirable, as is indicated by an analogous case.
In connection with certain work that the writer has been carrying
on with Mr. J. W. Gowen, pedigrees of the two famous thorough-
bred race-horses, Sysonby and Artful, have been tabulated. These
pedigrees are both practically complete for 10 ancestral generations.
They constitute a fair random sample of pedigrees in the breed, for
Sysonby was of pure English blood, while Artful had many American-
bred ancestors. The two pedigrees show no name in common until
we reach the fifth ancestral generation. In that generation there are
three names that appear in both pedigrees. But by the time we reach
the tenth ancestral generation, approximately 90 per cent of the 1,024
names in Artful's pedigree appear also in the first ten generations of
Sysonby 's pedigree. And the result would certainly be even more
striking if the pedigrees were studied for a few more generations, or
if two English-bred horses were compared. Here, then, we have a
clear case of "interlocking" pedigrees. Yet in spite of the long in-
breeding (12 to 20 or more generations, with scarcely any out-crosses)
which the breed has undergone, there are still a large number of bay
or brown and of chestnut race-horses, besides a few grays and blacks.
Of the four Mendelian factor pairs (see Sturtevant, 1912) for which
the race was originally heterozygous, it has become homogeneous only
in that the roan factor has been eliminated.1 Clearly, selection for
any one of the colors now present would still be effective in eliminating
the others. The breed, which we may suppose to be inbred to some-
thing like the same degree as Castle's hooded rats, is still very far
from a "pure line."
The new data presented by Castle and not taken up by MacDowell
consist of two points: The crosses of extracted hoodeds (from plus
'Even in the early days roan race-horses were not at all common. Both roan and gray have
been selected against.
AN ANALYSIS OF THE EFFECT OF SELECTION. 49
race X wild) to wild, and the relations of the "mutant" series to the
selected series.
When the plus race was crossed to wild, and F2 hoodeds were ex-
tracted, it was found that in these extracted animals the mean grade
was lighter (less "plus") than that of their selected grandparents.
This, as MacDowell pointed out, is the expectation on the multiple-
factor view. But Castle now states that when these extracted hoodeds
are again crossed to wild, and hooded is extracted once more, the
twice-extracted hoodeds are about midway in mean grade between
their extracted grandparents and the uncrossed plus race. As he says,
the wild race might have been expected to bring these animals still
farther away from the plus race if modifying factors were involved.
Evidently it is very important that we know as much as possible about
the wild rats used in these experiments, in order that we may know
what they were likely to carry in the way of modifying factors. These
rats, we are told, all came from the same stock, which was trapped at
the Bussey Institution in large numbers and was reared for two gen-
erations in the laboratory. "In making the second set of crosses, the
extracted individual has, wherever possible, been crossed with its own
wild grandparent." An examination of the table given shows that
not more than 102 of the 256 twice-extracted hoodeds can have been
produced in this way, unless individuals of the same sex were mated
together. Just how many of the 102, and which ones, does "wherever
possible" include? How many wild rats were used hi the original
crosses? These questions are important, because it is evident from
a study of the data that the result emphasized by Castle is due almost
entirely to the descendants of one original plus-line female; 41 of the
73 once-extracted hoodeds were F2's from this female; and their mean
grade was 3.05, as against 3.3 for the remaining F2's, and 3.17 for the
generation as a whole. The twice-extracted hoodeds tracing to this
female were of mean grade 3.47, while those from the other original
hoodeds were again of approximately grade 3.3. Further data re-
garding the pedigree and other descendants of the mates of this female
and of her grandchildren are very much needed. Informr tion regard-
ing the ancestry of the female herself would also be interesting.
It should also be pointed out that this case, accepted at its face value,
is difficult to explain on the view that the hooded-rat results are pro-
duced solely by variations in the hooded factor itself. On that view
the changes brought about by crossing are usually referred to con-
tamination of the factors in the heterozygote. But that interpretation
leaves entirely unexplained the results of the first cross to wild. If
the hooded factor is contaminated by its allelomorph, the once-
extracted hoodeds should be darker than their grandparents, whereas
in reality they are lighter, as would be expected on the multiple-factor
50 AN ANALYSIS OF THE EFFECT OF SELECTION.
view. Castle has met this objection in the following manner (Castle
and Wright, 1916):
"This suggests the idea that that loss (of 'plus' character) may have been
due to physiological causes non-genetic in character, such as produce in-
creased size in racial crosses; for among guinea-pigs (as among certain plants)
it has been found that FI has an increased size due to vigor produced by
crossing and not due to heredity at all. This increased size persists partially
in F2, but for the most part is not in evidence beyond Fx. I would not sug-
gest that the present case is parallel with this, but it seems quite possible
that similar non-genetic agencies are concerned in the striking regression of
the first Fj and the subsequent reversed regression in the second Fj."
This comparison seems to me to be rather far-fetched, and I am
quite unable to understand the hypothesis of " non-genetic physiologi-
cal causes." That they are "physiological" is, of course, obvious;
but they depend for their appearance on the pedigree of the animal,
and they are persistent to F2, so why " non-genetic"? The results
from size crosses are entirely explicable on the basis of Mendelian
modifying factors, so why need one appeal to vague "non-genetic,"
yet transmissible, factors? And is not such an appeal, in principle,
an appeal to modifying factors? It certainly involves the assump-
tion that the grade depends on transmissible material other than the
hooded factor itself.
In the tenth generation of Castle's plus selection series there ap-
peared two rats of considerably higher grade than any individuals
of that series previously recorded. These individuals were shown
(Castle and Phillips, 1914, pp. 26-31) to differ from the plus race by
a single dominant factor. This has been taken by MacDowell to
indicate that a new modifying factor arose by mutation. But Castle
has now presented evidence indicating that the mutation occurred
in the hooded locus itself. When homozygous "mutants " were crossed
to wild rats, F2 consisted in self-colored rats and rats of the same grade
as the mutant series — no hooded individuals. (Castle and Wright,
1916.) Castle (1916) concludes from this evidence: "This serves
to confirm the general conclusion that throughout the entire series
of experiments with the hooded pattern of rats we are dealing with
quantitative variations in one and the same genetic factor." Now,
the "mutant" variation differs from the other results obtained by
Castle in two respects: It appeared suddenly, as a definite and very
slightly variable character, and it fails, when crossed to self, to give
normal hooded in F2. Because of the first point, it is probable that
it arose during the experiment as a new variation; because of the sec-
ond, it is probable that it is a variation in the hooded factor itself.
Since these conclusions as to its nature are based entirely on the points
in which it differs from the remainder of the results, it is difficult to
see how Castle's case for these results is in any way unproved. On
the contrary, if this is the behavior to be expected of a new variation
AN ANALYSIS OF THE EFFECT OF SELECTION. 51
arising in the hooded factor, then the "mutant" variation is evidently
the only case of that sort that Castle has reported.
GENERAL CONCLUSIONS.
That many characters may be influenced by more than one pair of
genes has long been recognized, and this is the essence of the multiple-
factor view. That genes exist which require the action of other genes
before they produce visible effects has also been long known. Further-
more, that there are genes which produce very slight visible effects
is now another commonplace. Given these three facts, and the
hypothesis (which is supported by much specific evidence) that most
races are heterozygous for a number of such genes is all that is re-
quired to complete the conception that is held by most adherents of
the view that multiple factors or modifying genes are responsible for
the results of selection.
In specific cases, the existence of definite modifying genes has been
demonstrated by Dexter, Bridges, Muller and Altenburg, and the
author. All other data hi question fit hi with the view that selection
ordinarily acts only by isolating modifiers.
Modification of factors by selection, crossing, fractionation, or
similar means is undemonstrated in any given case, and has been
shown not to occur hi other cases that are typical of the results usually
obtained. Factors do change, and more than two forms are possible
for certain loci; but there is no known method of inducing such changes^
and they are ordinarily quite rare and definite.
SUMMARY.
(1) Dichset is a dominant character, the gene being lethal when
homozygous (yellow-mouse case). The gene is in the third chromo-
some.
(2) Dichset flies are more variable in bristle number than are not-
Dichaets. This variability is partly environmental, partly genetic.
(3) Selection was effective in isolating both plus and minus Dichaet
lines.
(4) A cross between two separate inbred plus lines gave no increase
in variability and no increase in parent-offspring correlation. There-
fore the two lines were presumably of very similar constitution, though
independent in origin.
(5) A cross between an inbred plus line and an inbred minus line
gave the results characteristic of such crosses — increased variability
in F2 and increased parent-offspring correlation.
(6) Linkage tests demonstrated that modifying genes exist in the
selected lines. Several lines were shown to differ in one or more sec-
ond-chromosome modifiers, and at least one of these modifiers was
shown to cross over from the speck gene.
52 AN ANALYSIS OF THE EFFECT OF SELECTION.
(7) In one case at least one third-chromosome modifier was shown
to exist and to cross over from Dichaet, which must lie to the left of it.
(8) Two third-chromosome lethals were obtained. These were
shown to be new mutations, not due to fractionation of the Dichset
gene.
(9) A new allelomorph of Dichset, called Extended, appeared in a
plus selected line. It is argued that this mutation was not due to
fractionation of the Dichset gene, and was not influenced by the selec-
tion that was carried on.
(10) Another character, somatically indistinguishable from Ex-
tended, was shown to be due to a recessive second-chromosome gene.
(11) A study of unselected Dichaets, and of the not-Dichsets pro-
duced by long-continued mating together of Dichsets, is shown to fur-
nish evidence against the view that allelomorphs are contaminated in
heterozygotes.
(12) A general discussion of the selection problem is divided into
three parts: (a) an attempt is made to clear up certain current mis-
understandings and disagreements as to what questions are really at
issue; (6) cases cited as evidence for contamination of allelomorphs
are discussed in detail, and the conclusion is drawn that contamina-
tion is unproved and is an unnecessary hypothesis, with some direct
evidence against it; (c) certain specific objections are raised to argu-
ments made by Castle on the basis of his experiments with hooded
rats.
BIBLIOGRAPHY.
BATESON, W.
1909. Mendel's principles of heredity. 2d impression, Cambridge.
1914. Address of the president of the British Association. Science, n. s., 40.
BRIDGES, C. B.
1915. A linkage variation in Drosophila. Jour. Exper. Zool., 19.
1916. Non-disjunction as proof of the chromosome theory of heredity. Genetics, 1.
CALKINS, G. N., and L. H. GREGORY.
1913. Variations in the progeny of a single ex-conjugant of Paramecium caudatum.
Jour. Exper. Zool., 15.
CASTLE, W. E.
1906. The origin of a polydactylous race of guinea-pigs. Carnegie Inst. Wash.
Pub. 49.
1911. Heredity in relation to evolution and animal breeding. New York.
1914a. Multiple factors in heredity. Science, 39.
19146. Variation and selection; a reply. Zeitschr. Abst. Vererb., 12.
1916a. New light on blending and Mendelian inheritance. Amer. Nat., 50.
19166. Genetics and eugenics. Cambridge, Mass.
1916c. Report in Carnegie Inst. Wash. Year Book No. 15.
1916d. Can selection cause genetic change? Amer. Nat., 50.
1917. Piebald rats and multiple factors. Amer. Nat., 51.
and A. FORBES.
1906. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure
gametes. Carnegie Inst. Wash. Pub. 49.
and P. B. HADLEY.
1915o. The English rabbit and the question of Mendelian unit-character constancy.
Amer. Nat., 49.
19156. Same. Proc. Nat. Acad. Sci., 1.
and J. C. PHILLIPS.
1914. Piebald rats and selection. Carnegie Inst. Wash. Pub. 195.
— and S. WRIGHT.
1916. Studies of inheritance in guinea-pigs and rats. Carnegie Inst. Wash. Pub. 241.
DEXTER, J. S.
1914. The analysis of a case of continuous variation in Drosophila by a study of ita
linkage relations. Amer. Nat., 48.
EMERSON, R. A.
1917. Genetical studies of variegated pericarp in maize. Genetics, 2.
HAYES, H. K.
1917. Inheritance of a mosaic pericarp pattern color of maize. Genetics, 2.
HOSHINO, Y.
1915. On the inheritance of the flowering time in peas and rice. Journ. Coll. Agr.
Tohoku Imper. Univ., Sapporo, Japan, 6.
JENNINGS, H. S.
1916. Heredity, variation, and the results of selection in uniparental reproduction
in Diffltigia corona. Genetics, 1.
JOHANNSEN, W.
1903. Ueber Erblichkeit in Populationen und in reinen Linien. Jena.
LITTLE, C. C.
1915. The inheritance of black-eyed white spotting in mice. Amer. Nat., 49.
LUTZ, F. E.
1911. Experiments with Drosophila ampelophila concerning evolution. Carnegie
Inst. Wash. Pub. 143.
MAcCuRDY, H., and W. E. CASTLE.
1907. Selection and cross-breeding in relation to the inheritance of coat-pigments and
coat-patterns in rats and guinea-pigs. Carnegie Inst. Wash. Pub. 70.
53
54 BIBLIOGRAPHY.
MACDOWELL, E. C.
1915. Bristle inheritance in DrosophUa. I. Extra bristles. Jour. Exper. Zool., 19.
1916. Piebald rats and multiple factors. Amer. Nat., 50.
1917. Bristle inheritance in Drosophila. II. Selection. Jour. Exper. Zool., 23.
MARSHALL, W. W., and H. J. MULLER.
1917. The effect of long-continued heterozygosis on a variable character in Droso-
phila. Jour. Exper. Zool., 22.
MIDDLETON, A. R.
1915. Heritable variations and the results of selection in the fission rate of Stylonychia
pustulata. Jour. Exper. Zool., 19.
MORGAN, T. H., A. H. STURTEVANT, H. J. MULLER, and C. B. BRIDGES.
1915. The mechanism of Mendelian heredity. New York.
MULLER, H. J.
1914o. The bearing of the selection experiments of Castle and Phillips on the variability
of genes. Amer. Nat., 48.
19146. A gene for the fourth chromosome of Drosophila. Jour. Exper. Zool., 17.
1916. The mechanism of crossing over. Amer. Nat., 50.
1917. An Oenothera-like case in Drosophila. Proc. Nat. Acad. Sci., 3.
PEARSON, K.
1911. On the probability that two independent distributions of frequency are really
samples from the same population. Biometrika, 8.
SPILLMAN, W. J.
1907. Inheritance of the belt in Hampshire swine. Science, n. s., 26.
STURTEVANT, A. H.
1912. A critical examination of recent studies on color inheritance in horses. Journ.
Genet., 2.
DETAILED DATA.
TABLE 25. — INBRED PLUS SERIES. 864 LINE.
Genera-
tion and
culture
No.
Parents.
1
2
3
4
5
6
7
8
|
Grade.
Cul-
ture.
9
-
9
-
9
c?
9
c?
9
c?
9
*
9
e?
9
c?
9
0*
FI 893
F2 902
903
F3 926
F, 1006
1013
FB 1064
1081
1084
F, H53
6 1170
1191
F, 1239
7 1277
1287
1298
1299
1309
1318
1322
F8 1384
1390
1406
1420
1421
1422
1430
1431
1444
1459
1478
F'!SJ
1613
1629
1090
F10 1663
10 1763
1810
Fn 1887
1890
1944
1963
1982
Fio 2013
12 2027
2028
2029
2060
2061
2062
2087
2098
2105
2115
2123
21422
6
7
7
6
6
6
6
6
6
6
6
6
ft
6
6
6
6
6
6
6
6
6
6
7
6
6
6
a
6
e
a
7
7
6
6
6
7
a
'a
6
6
a
a
6
a
6
6
6
6
6
6
6
7
6
6
6
6
6
6
6
6
6
6
a.
6
6
6
6
a
6
6
6
7
6
6
6
6
G
I
6
6
6
G
6
6
6
a
6
6
6
6
6
6
6
6
a
6
6
6
6
6
6
864
893
893
903
926
926
1006
1013
1013
1064
1081
1081
1153
1170
1191
1170
1170
1170
1191
1170
1239
1277
1277
1298
1298
1299
1287
1309
1287
1287
1298
1390
1421
1459
1444
1478
1511
1613
1613
1763
1763
1810
1810
1810
1887
1887
1890
1890
1887
1890
1890
1887
1944
1887
1944
1963
1887
MO
107
lfifi
MO
113
4
5
5
a
7
7
20
26
8
7
a
17
6
18
27
' '4
17
17
32
17
28
10
7
17
14
28
12
21
9
7
51
70
73
108
152
48
26
75
12
21
87
17
92
68
124
33
69
60
47
22
25
24
22
95
92
60
21
27
13
60
15
55
53
10
21
11
37
111
57
58
65
29
23
95
47
34
78
70
65
7
50
30
71
25
15
44
1
1
1
2
19
25
7
23
24
a
1
...
9
5
21
1
11
4
20
fi
...
...
"a
14
28
1
4
20
4
19
2
17
a
16
9
17
4
4
13
7
3
1
4
4
16
11
10
G
a
16
3
10
13
20
3
14
11
12
9
4
I
14
6
6
a
3
10
2
6
]
5
6
17
10
6
14
I
6
12
3
C)
16
9
16
1
7
21
3
16
16
22
14
16
12
a
7
2
6
a
21
38
17
3
12
6
19
5
19
19
a
7
3
4
18
24
14
16
4
6
20
17
18
17
12
17
7
21
7
16
18
18
12
19
12
12
8
7
a
4
14
35
22
4
9
7
17
4
16
24
3
7
1
14
15
11
16
5
5
5
11
25
12
18
20
12
g
1
I
I
1
...
...
"i
10
8
6
6
1
5
1
1
"2
"i
3
4
1
13
1
3
a
i
2
16
1
2
...
...
i
i
.
*•••
...
5
1
7
f
3
m
1
3
1
2
i
1
i
8
4
20
2
4
4
4
14
4
1
a
a
16
18
8
9
14
1
2
11
' i
1
1
3
2
2
2
a
14
|
1
1
1
1
8
10
.a
4
1
4
8
6
4
i
4
4
a
1
13
a
11
11
]
9
r
i
i
1
4
13
1
10
1
9
1
11
15
11
4
4
..*
1
1
I
2
|
19
17
Offspring not separated for sex.
*2d brood of 2087.
55
56
AN ANALYSIS OF THE EFFECT OF SELECTION.
TABLE 25. — INBRED PLUS SEMES. 864 LINE— Continued.
Genera-
tion and
culture
No.
Parents.
1
2
3
4
5
6
7
8
1
Grade
Cul-
ture.
9
0"
9
tf
9
rf1
9
d"
9
c?
9
cf
9
cf
9
c?
V
cf
F1S 2132
" 2144
2146
2167
2180
2219
2221
2241
F14 2248
2293
2304
2356
2362
7
7
0
7
6
7
6
6
f,
7
8
6
7
7
7
7
6
6
6
7
7
G
8
7
7
7
2013
2027
2013
2060
2062
2098
2105
2029
2132
2167
2180
2219
2241
1
2
7
G
8
3
6
8
3
6
2
8
1
r,
19
23
20
21
16
12
14
27
14
I2fi
8
25
17
31
13
10
2
14
25
9
3
4
2
1
2
5
4
6
*1
2
2
i
2
2
3
15
64
53
72
44
50
18
37
60
38
28
31
18
?
?
1
1
2
1
1
3
7
3
1
1
ij
3
1
8
1
2
»i
1
2
4
9
8
12
6
1The original record sheet for 2304 has been lost, and the sexes are not noted separately on the
copy from which this count is taken.
TABLE 26.— INBRED PLUS SERIES. 1002 LINE.
Genera-
tion and
culture
No.
Parents.
1
2
3
4
5
6
7
8
H
Grade.
Cul-
ture.
9
d1
9
c?
9
c?
9
rf
9
d1
9
r
16
22
38
35
17
31
23
34
40
31
28
27
8
17
13
21
45
22
8
31
17
30
34
28
45
23
9
15
1
1
1
2
2
8
1
1
2
'3
1
3
1
'i
2i
53
58
97
198
64
172
47
107
120
111
87
98
23
47
1
2
c
9
8
6
4
6
10
5
1
3
10
2
'Unselected, or from inbred plus series.
This is probably the original extended mutant. Not included in totals.
AN ANALYSIS OF THE EFFECT OF SELECTION.
59
TABLE 28. — CROSSBRED PLUS SERIES — Continued.
Genera-
tion and
culture
No.
Mother.
Father.
1
2
3
4
5
6
7
8
I
Grade.
Culture.
Grade.
Culture.
9
c?
9
d1
9
*
9
rf
9
tf
9
d"
9
c?
9
tf
F6 H29
1130
1131
1146
1151
1171
1187
1188
1190
1196
1197
1204
1227
F7 H98
7 1203
1253
1254
1262
1269
1271
1284
1285
1293
1304
1324
1325
1326
1333
1345
1353
13052
F8 1334
8 1346
1351
1356
1357
1359
1360
1372
1373
13802
1425
1426
1427
1428
1429
1458
F9 1457
1492
1496
1497
1501
1538
1541
1612
F10 1581
1 1599
1709
1758
6
6
6
7
6
7
7
7
7
7
6
7
6
7
7
7
7
6
7
6
7
7
7
6
7
7
7
7
7
7
NotD'
8
7
7
8
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
8
7
1045
1074
1074
1074
10721
1099
1090
1101
1101
1100
1115
1116
1101
1101
1130
1146
1129
1151
1171
1190
1188
1171
1190
1151
1204
1171
1171
1227
1227
1204
1197
1227
1203
1196
1253
1253
1203
1203
1254
1196
1262
1271
1293
1285
1262
1269
1345
1334
1334
1359
1326
1356
1356
1326
1428
1457
1492
1458
1612
6
6
7
6
6
7
7
7
7
7
6
8
6
6
7
6
6
6
6
7
7
7
6
7
7
6
6
6
6
7
8
7
6
6
7
6
6
7
6
7
8
7
7
7
7
8
7
7
7
7
7
6
7
7
7
7
8
7
8
1074
1045
1041
1074
1081 l
1090
1100
1100
1090
1100
10811
1090
1115
1131
1099
1131
1115
1144
1129
1131
1171
1190
1151
1187
1171
1227
1190
1188
1190
1227
1090
1203
1204
1203
1227
1203
1204
1227
1204
1254
1090
1304
1304
1284
1293
1293
1285
1345
1351
1346
1356
1333
1359
1357
1426
1373
1373
1538
1538
•-
.*
2
3
2
•4
1
1
3
13
11
14
2
4
14
15
13
1
11
1
10
10
4
8
49
25
25
If
If
21.
14
48
r.o
20
19
40
03
20
1
52
24
25
9
17
28
11
25
43
12
14
25
53
23
fi
1
1
r>
0
'i
i
5
2
3
2
2
i
i
3
1
3
]
4
1
1
1
i
134
83
80
32
53
60
30
96
134
50
45
102
132
75
13
78
16
58
45
20
67
54
77
57
82
62
75
35
100
92
43
148
87
12
20
57
44
12
28
44
57
124
34
66
7
71
73
39
8
160
13
31
40
20
85
42
127
28
39
1
f
1
8
15
1
1
8
1
1
i
1
6
1
I
6
17
2
10
1
5
•2
••
:,
2
4
]
8
1
3
9i
1
5
22
8
24
21
8
25
20
35
13
29
28
40
14
39
38
10
41
32
t
6
18
17
6
1
19
6
39
12
29
4
29
28
14
4
24
t
21
1
8
23
19
24
11
23
20
18
20
34
33
9
42
22
6
9
21
16
8
6
17
8
33
15
10
1
27
87
18
1
4
5
2
1
3
2
i
2
i
i
3
*2
1
1
'3
*2
4
2
2
2
3
1
1
2
1
1
1
1
1
4
i
i
i
i
8
1
1
2
i
6
5
S
a
8
8
7
7
11
6
3
3
3
9
8
I
2
8
8
4
6
1
2
6
2
..
'..
i
4
6
5
*
1
9
1
6
17
7
7
2
8
11
6
9
7
8
17
11
7
8
6
17
9
••
'i
••
1
••
"i
"4
5
ft
I
1
1
*6
^
9
6
4
6
3
2
1
i
4
4
2
i
'2
2
12
14
15
6
ft
19
1
i
9
13
2
3
•I
3
2
2
8
2
1
2
1
18
0
24
2
16
7
48
5
12
IB
9
41
18
39
10
22
44
2
14
10
0
23
21
47
13
11
7
I
2
3
2
4
3
3
1
2
1
2
2
2
3
1
1
1
'i
i
i
i
i
e
2
2
1
1
i
15
1
14
••
i
1
2
^nselected, or from inbred plus series.
2The d71 in these cultures also was the father of 1204. 1305 is not included in the totals.
60
AN ANALYSIS OF THE EFFECT OF SELECTION.
TABLE 29. — INBRED MINUS SERIES. 900 LINE.
Genera-
tion and
culture
No.
Parents.
1
2
3
4
5
6
7
8
1
Grade.
Cul-
ture.
9
a1
9
c?
9
0^
9
14
11
23
5
G
19
14
6
19
IS
5
•2
15
14
9
13
12
19
16
s
4
5
1
is
1
11
3
3
1
"a
i
2
i
i
1
i
i
i
2
3
1
3
1
7
4
4
11
5
5
11
8
4
7
6
2
5
1
4
1
8
11
5
10
8
9
8
3
1
2
3
ill
::
1
4
3
4
3
2
...
1
3
i
AN ANALYSIS OF THE EFFECT OF SELECTION.
61
TABLE 30. — INBRED MINUS SERIES. 868 LINE.
Genera-
tion and
culture
No.
Parents.
1
2
3
4
5
6
7
8
1
Gra
9
de.
~c?
4
2
4
4
4
2
3
3
4
Cul-
ture.
9
ae
->
1?
11
7
6
-,
1"
1
56
1737 |
Sp
Sp
'
?
Sp
Sp
J Not-sp
1
1
8
10
A
5
.)
35
1937 <
4
1737
\Sp
i
4
10
3
S
4
1
26
f
Sp
Sp
/Not-sp
5
20
ft
5
4
1
41
1970 <
6
1737
\Sp
1
9
s
7
«}
1
26
864. Inbred
F
lu.
L
IK
,
Sp
Sp
1921 |
6
864
1763
1
^
o
f|
1 1
1 "i
in
45
Sp
Sp
/ Not-sp
4
8
1
7
r,
ft
3
35
2023
6
1921
\Sp
ft
14
1
?!
1
?
26
Sp
Sp
/Not-sp
1
4
ft
7
3
ft
ft
32
6
1921
\Sp
1
,
9
ft
1
1
1
1
23
Sp
Sp
f Not-sp
7,
1
3
6
3
15
6
1921
\Sp
7
1
7
1
6
Sp
Sp
( Not-sp
7
4
4
17
Is
45
2175
5
2023
\Sp
12
7
s
7
^
32
••
6
1921
/Not-sp
9
5
s
ft
7
30
Sp
6
Sp
192i
\Sp
f Not-sp
1
••
4
1
4
7
2
0
2
1
3
5
4
16
20
Sp
Sp
ISp
[ Not-sp not-ro
3
4
5
?
2
1
5
4
3
7
6
9
2
6
2
6
3
10
s
5
1
5
1
2
9
4
5
2
1
2
1
7
1
2
3
1
6
?
•
'
33
40
23
27
30
13
26
18
f
6
1002
2415
?
p,
1?
11
22
•>.r)
1
76
2433 |
1331
2414
1331
2431
Not-sp
B
[
0
7
j
27
2481 {
6
5
Sn
Sp
2433
2433
Sp
Not-sp1
Sp
i
3
2
4
o
8
4
4
10
c
4
2
8
g
1
6
n
ir
2
4
24
»40
33
f
6
2433
Not-sp
i
1
/|
1°
0
14
49
2488 {
1331
2432
Sp1
i
1
in
I
3
4
1
133
r
5
2471
Not-sp
ifi
13
ft
1°
7
1
47
2516 |
1331
2432
Sn
j
^
4
c
\
31
f
Sn sc
1331
2414
0
'\K>
in
11
14
4
1
61
2436 |
6
1002
2415
r
So
Sp
Not-sp
()
in
r
21
2480 |
3
2436
Sp
2
6
g
24751 I
4
Sp
'Sp
2436
2436
Not-sp
Sp
Not-sp
4
1
2
1
6
4
r
1
4
3
I
f
r
14
17
35
25181 <
2476 |
Sp
5
Sp
Sp
Sp
2436
Sp
Not-sp
Sp
<
f
1
4
13
8
9
14
12
1
]
37
22
21
2519 \
2607 <
5
Sp
Sp
6
'Sp
Sp
1002
2436
2548
Not-sp
\Sp
,
3
*
5
9
f
1C
1!
18
14
8
1C
35
28
39
2669 <
2698
SP
5
Sp
6
Sp
Sp
'Sp
Sp
2607
2607
'Not-sp
Sp
/Not-sp
\Sp
f Not-sp
1
"2
4
' -a
i:
1
13
1"
LI
j
i;
12
1 1
1
34
28
32
32
17
2699 |
6
2607
Sp
(
|.
25
2711
Sp
6
Sp
2607
/Not-sp
\Sp
;
If
]•
u
27
25
6
2607
/Not-sp
,
.
!
K
J'
oe
2682
Sp
Sp
\Sp
.
28
2665
2789*
2803*
5
Spro
Sp 5
Spro
Sp 5
Sp ro
Sp
1331
133i
1331
Sp
2607
2596
2669
2663
2711
2663
f Not-sp not-ro
1 Not-sp ro . . .
1 Sp not-ro
ISp to
J Sp not-ro
\Sp ro
f Sp not-ro
\Sp ro
:
j
|
|
(
i
i:
•!
I
23
29
19
32
21
49
13
16
12
6
1002
2570
2690
Spro
5
1331
2601
2633
/Not-sp
\Sp . . .
2
10
4
17
13
2704
Sp
6
Sp
2633
/Not-sp
|Sp
2
9
22
13
2811
5
Spro
1331
2704
2663
[Not-sp not-ro
1 Not-sp ro . . .
1 Sp not-ro
f
1C
f
|
18
34
15
(Sp ro
1C
23
'2475 and 2518 are two broods from same parents.
2 2789 and 2803 had the same male parent.
O 1 7 .0
UNIVERSITY OF CALIFORNIA LIBRARY
Los Angeles
This book is DUE on the last date stamped below.
pa
I WK from
Form L9-50m-ll,'50 (2554)444
A 000346966 5
UNIVERSITY r f CALIFORNIA
a oELES
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