- THE INHERITANCE OF SALMON
oe COLOR IN MAIZE

A THESIS

PRESENTED TO THE FACULTY OF THE GRADUATE: SCHOOL
OF CORNELI, UNIVERSITY FOR THE DEGREE OF

‘DOCTOR OF PHILOSOPHY

BY

ERNEST GUSTAF ANDERSON

Published as Cornell University Agricultural Experiment Station
Memoir 48—November, 1921

THE INHERITANCE OF SALMON
SILK COLOR IN MAIZE

A THESIS. —™

PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF CORNELL UNIVERSITY FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

BY

ERNEST GUSTAF ANDERSON

)

Published as Cornell University Agricultural Experiment Station
Memoir 48—November, 1921

é 6A LEIF od

CONTENTS
PAGE
ESI teen hs, Mt ah ace ae, . ws en ate ent eee 539
Seepen SUK COMES... To Fpl tea Se bs vas bed eee oe a 540
Prouminary studies and indications: ............ 600-0 eee. de ee cane 541
Remote tOMINMCTICANCE,, is ¢ <5. 65 0s boy 3 oes tae ns hb oreo eeee 542
Interrelations of green, salmon, and brown silks............... 542
elation of salmon silks to pericarp color...: 02.20. 0.0.5....-45 542
Relation of salmon silks to the B and Pl factors for plant color.. 545
Relation of salmon silks ‘to the A factor... .....:.5........52. 546
Pemuon of salmon silks to the # factor: .........0. 00 6. e0.-..5 546
panMmnIy OE, TTOLILALICE), . << fag irs octerce Aw tgabthes the + Sot ater SAT
mer=erteinons-or Y., 61, and SM j6 oe. a eee ose oa wr elles 547
pePrehminary tests of linkage of Pl and Sm. i..0........0...5 0% 547
Construction and results of three-point tests.................. 548
AMiermiraimOsonne WAI ..c. 4: et oy ee eee «sve es rae we ... O49
@ecrecrice Ol CROSSING “OVER? fie iu de ks koe wx ae tORe aes 553
peeaney Or LITKAE SLUGIES.. i... a beers ke oe so ae Be 554
(DURES Sb YET i ae SIR ogre OM ro ee es Da as Ar 554.

535

THE INHERITANCE OF SALMON SILK COLOR IN MAIZE!
E. G. ANDERSON

At the Nebraska State Corn Show of 1908, a number of odd types of
corn were gathered together to form a,‘‘freak”’ class. Among them was
a “Bronze Pop Corn,” so named became of a light bronze color in
the pericarp. This ear was obtained by Professor R. A. Emerson, after
the exhibit was over, in order to study the inheritance of that pericarp
color. The plants grown therefrom were also characterized by brown or
brownish silks (Plate LITT). An outcross gave green silks in the F;. From
brown-silked segregates in the progeny of this cross, a true-breeding stock
was again obtained. This stock was used in crosses for a study of the
inheritance of pericarp color. In one small F, of only five plants, there
appeared three with green silks and one with brown. The fifth plant
had very brilliant salmon or orange-colored silks (Plate LII). This plant
was a dilute sun red with red pericarp. It was crossed with red, green,
and brown silk colors, and with a purple plant having brown silks. Fy’s
were grown and selfed to obtain F2 progenies. The crosses with red and
with green silks gave in F; red and green silks, respectively. The cross
with brown silks gave salmon.

In order to devote more time to studies on aleurone and plant colors °
and other problems, Dr. Emerson at this point requested the writer to
take up the study of these silk colors and their relation to other characters
in maize. In his further studies the writer has had the advantage of the
hearty cooperation and ever-ready suggestions of Dr. Emerson, and he
wishes to acknowledge his sincere gratitude for this help and encouragement.

NOMENCLATURE

The factors referred to in this paper, together with the factor symbols
used, are given in the following list:
A a— Anthocyanin pigment. A factor pair for pigmentation of aleurone,
sheaths, leaves, anthers, and so forth. (Emerson, 1918, 1921.)
B b— Brown plant color. A factor pair for leaf and sheath pigmentation.
(Emerson, 1921.)
1 Paper No. 83, Department of Plant Breeding, Cornell University, Ithaca, New York.
539

540 E. G. ANDERSON

Plpl—Purple anthers. A factor pair for pigmentation of anthers
sheaths, pericarp, and so forth. (Emerson, 1921.)
These three factor pairs interact to give the following plant color types
described by Emerson (1921):
A B Pl— Purple

A B pl — Sun red
A b Pl— Dilute purple

A b pl — Dilute sun red
a B Pl— Brown
a B pl — Green
a b Pl— Green
a b pl — Green

R’ R® 7" 9 r°*— Red aleurone. A series of allelomorphs affecting antho-
cyanin pigmentation in aleurone, sheaths, leaves, pericarp, anthers,
and silks. (East and Hayes, 1911; Emerson, 1918, 1921.)

Pp—Pericarp color. Two of a series of allelomorphs for pericarp
coloration. (Emerson, 1911.) The bronze type was so pale in
color that it could not be satisfactorily distinguished from white
(colorless). Only two symbols are used herein, P for red igi
and p for white or bronze pericarp.

Y y — Yellow endosperm. (Kast and Hayes, 1911; Emerson, 1921.)

Sm sm — Salmon silk color. Described in this paper.

DESCRIPTION OF SILK COLORS

The colors of silk in maize may be described as follows:

1. Green (Plate L). Silks light green, paler below husks; varying
from a pure pale green to yellowish green.

2. Red (Plate LI). Silks green, as above, with the addition of a red
anthocyanin pigment where exposed to light. The amount of red pig-
ment may vary from a slight trace in the hairs, to sufficient. to obscure
the green color, giving the silks a deep or dark red color. The darker red
silks frequently have some red below the husks. Emerson (1921) has
shown this color to be due to the R factor. Microscopic sections show
anthocyanin pigment in peripheral parts of the silks?

2The microscopic sections are prepared as follows: Pigmented tissue is fixed for from twelve to
twenty-four hours in a saturated solution of mercuric chloride in 95-per-cent alcohol, and washed with
95-per-cent alcohol without iodine. The usual paraffin method of embedding and sectioning is followed
and the preparations are mounted in balsam without staining. Sections from 15 to 25 micra in thickness
have proved satisfactory.

Memoir 48 PLatTEe L

GREEN SILKS

Silks of this type may be associated with any plant-color type. Purple husks are shown
here for contrast :
(Drawing by Carrie M. Preston)

> a
a

ae

a
av

\) ge ay =
' 5 2
a
Nig *

cates
Mead
a .

—— ee

Menmorr 48 PuatTe LI

RED SILKS

The red pigment develops only in the parts of the silks exposed to the light
(Drawing by Carrie M. Preston)

Memorr 48 Puate LI

SALMON SILKS

The color develops beneath the husks as well as in exposed parts of the silks
(Drawing by Bernice M. Branson)

Memorr 48 Puate LITT

BROWN SILKS

The color develops in both exposed and protected parts of the silks
(Drawing by Carrie M. Preston)

PD CS a BN VR Oy ar A

INHERITANCE OF SALMON SILK CoLor IN Maize 541

3. Salmon (Plate LIT). Silks light salmon-orange to salmon. The
color below the husks is similar to that of exposed parts. Microscopic
sections show only a faint brownish cast to the tissues thruout. the
silks.

4. Brown (Plate LIII). Silks orange-pink to pale salmon or salmon-buff
in both exposed and covered parts. Salmon and brown silks intergrade,
forming a continuous series. The lighter forms are difficult to distinguish

_ from the yellowish green silks of No. 1. Both salmon and brown silks

may have red anthocyanin pigment present, as in No. 2.

PRELIMINARY STUDIES AND INDICATIONS

_ Previous tests had shown salmon silk color to be recessive to green and
at least partially dominant to brown. Crosses of salmon and brown did
not give green color in the F,. This was taken to indicate that these
colors were recessive for a common factor. The anthocyanin pigment
present in red silks was shown to be inherited separately by the occurrence
of all combination classes (green-red, green, salmon-red, and salmon)
in the F. of red x salmon.

From observation of cultures previously grown, both salmon and brown
silks were known to occur on dilute sun red, on sun red, and on purple
plants. Their occurrence on brown or green plants had not been recorded.
Microscopic examination of the pigments of maize had shown the presence
of a purple-red anthocyanin pigment in purple, sun red, dilute purple,
and dilute sun red plants. When the A factor is recessive, no anthocyanin
develops (except traces in the shank and the inner husks of brown plants).
Instead, a yellow or brownish pigment may be formed. A similar rela-
tion holds with the pericarp pigments. Red pericarp color is due to an
orange-red or brick-red pigment. Its homolog with recessive A is yellow-
ish brown, similar in appearance to the brown plant-color pigment.
The quantity of pigment in salmon silks is so small that microscopic
sections gave little information. But the color of the salmon silks was
so similar to the color of thin sections of red pericarp as to suggest the
possible identity of the pigments. The brown silk might, it was thought,
be only a dilute form of salmon. These suggestions were further strength-
ened by the fact that the original salmon-silked plant had red pericarp
and that the brown silks had been obtained from a plant with bronze

542 E. G. ANDERSON

pericarp. Sections of this bronze pericarp showed a small amount of
orange pigment.
With these suggestions in mind as a working basis, experiments were
planned to test them.
ANALYSIS OF INHERITANCE

Interrelations of green, salmon, and brown silks

The indications just mentioned, regarding the relationships of these
silk colors, were all checked and corroborated by further tests. Crosses ;
of green with salmon gave green in the first generation, segregating green
and salmon, or green, salmon, and brown, in the second. The distinction
between salmon and brown was not sharp. With the small numbers used
in these tests, the numbers approach a simple ratio of 3 greens to 1 salmon
or to 1 salmon and brown.

Crosses between green and brown likewise gave green, segregating in
the F, into green and brown or in some cases into green, salmon, and brown.
In either case there was about 75 per cent of greens.

Crosses of salmon with brown gave salmon. The F2 ranged from salmon
to brown, with salmon predominating.

These results show that there is a common factor pair which differen-
tiates between green on the one hand and salmon and brown on the other.
This pair is herein referred to as the salmon-silk factor pair and is desig-
nated by the symbols Sm sm.

The difference between salmon and brown silks is not explained by these
simple tests, tho the occurrence of brown silks in the progenies of out-
crosses of salmon, and vice versa, is at least a strong indication of one or
more modifying factors.

Relation of salmon silks to pericarp color

In order to test the relationship of the salmon factor to the factor for
pericarp color, two series of crosses were made. In the first, a colored-
pericarp, green-silked plant (P Sm) was crossed with a light-bronze-peri-
carp, brown-silked plant (p sm). In the. second, two white-pericarp
green-silked plants (p Sm) were crossed with the original salmon-silked
plant, which had red pericarp (P sm). The F,’s were crossed with the
double recessive. The results are given in table 1:

INHERITANCE OF SALMON SILK CoLor IN Maize 543

TABLE 1. Reuation oF SALMON SiLKs To PERIcaRP CoLoR

I. Backcrosses of P Sm x p sm with p sm

Pedigree no. PSm|P sm| pSm| psm | Total
MEET For, 5s Ro TESS 9 6 11 20
co i 5 Ad 12 10
oS, oy Se aa ener 9 15 a
Me. encore ahaa kD ae. Me 4] 32 Al 39
MN Sa !e Saye uletar sins G0 08 64 55 79 76 274
Observed per cent of recombinations.......................-. 48.9
Per cent expected with independent segregation.............. 50.0+2.0
Il. Backcrosses of p Sm x P sm with p sm
Pedigree no. PSm|P sm | pSm | psm | Total
oD ANTS he 43 61 51 53
ee ae eo eens he al 28 31 38
OO Ra a ea 23 25 25 28
Lo. oe ee 43 40 50 40
Wie ies aos eked. «0 140 154 157 159 610
Observed per cent of recombinations..................-+.--- 48.9

Per cent expected with independent segregation.............. 50.0+1.4

The observed per cent of recombinations is 48.9 in both series, which
is in very close agreement with the expectancy for independent segrega-
tion of P and Sm.

In order to determine the possible relation of pericarp color to the dif-
ference between salmon and brown silks, a light-pericarp, brown-silked
plant was crossed with a red-pericarp salmon. This was backcrossed

544 E. G. ANDERSON

with a light-pericarp, brown-silked plant similar to the one parent. The
silk colors were noted during the summer. It was impossible to make
any sharp separations, for the colors varied from a deep salmon to a typical
or even light brown. The presence of red anthocyanin pigment added to
the difficulty, as did also the fact that the silks could not be noted at the
same stage. So they were only roughly classified, the classifications
from time to time not being entirely comparable. The notes were under-
scored for a number of good salmons and browns. The pericarp colors
were determined in the fall. The results are given in table 2:

TABLE 2. BackcrossEs OF p, sm x P sm WITH p sm

Ras White or
Silk color pericarp aeea
Salmon, underscored... > se «3. . he See oe 17 0
Salmon...) v2 Sao. Pa 2 oe a ee 167 19
PMN Ay sg oy coos oe acs So aes ee 25 10
palmion-browns:. 22. << 6.3 b. bol eee ee 33 11
Brown-salmoues. oo. css aan: das eee 10 24
BOWIE 2s. oer tisk Ro oe ee 24 65
BOW 5 tn fees SOR on ee eee i Li2
Brown, underscored: <5". spa a ee ee 0 44

It will be seen from this table that most of the red-pericarp plants had
been noted as having salmon silks, while the light-pericarp ones were
mostly noted as having brown silks. It is also significant that, of those
cases in which salmon was underscored, all had red pericarp. Like-
wise, of the cases in which brown was underscored, all had light pericarp.
Since the salmon factor pair Sm sm has been shown to segregate inde-
pendently of the factor pair P p for pericarp color, this variation can- -
not be due to the Sm sm pair. The conclusion is drawn that the intensity
of pigmentation of silks recessive for sm is largely a function of the inten-
sity of pigmentation of the pericarp or of some factor closely associated
with the factor for pericarp color. The former view is substantiated

INHERITANCE OF SALMON SitK Couor IN MaAIzE 545

by the fact that no selfed progenies from light-pericarp plants have ever
given any good salmon silks, while progenies from red-pericarp plants
have always given some salmon silks even tho the parents had been noted
otherwise. No brown silks have been found in families breeding true
for red pericarp.

Relation of salmon silks to the B and Pl factors for plant color

Several crosses made with salmon silks involved the B factor. Both
F, combinations, B Sm x b sm and B sm x b Sm, were backcrossed with
the double recessive. The results (table 3) show independent segregation
of these factors.

TABLE 3, ReELATION OF SALMON SILKS TO THE B anp P! Factors ror PLANT CoLoR

I. Backcerosses of B Sm x b sm with b sm

|
Pedigree no. BSm| Bsm| bSm| bsm | Total
=) 2 25 24 39 4]
Series civ gle ec es 8 21 19 28 35
MINE ao ye cw aew 6 ws oe 46 43 67 76 232
Observed per cent Aiecombmationae mone lsa keel oe 47.4
Per cent expected with independent segregation. ............. 50.0+2.2

II. Backcrosses of B sm x b Sm with b sm

Pedigree no. BSm | Bsm | b Sm | bsm | Total
PE Ae oe oe mist eae os 78 62 80 53 213
Observed per cent of Riuaitions. 89200 055 okaee oh 47.6

Per cent expected with independent segregation.............. 50.0+2.0

546 E. G. ANDERSON

One of the crosses of the original salmon-silked plant, A b pl sm, was
with a purple plant with green silks, A B Pl Sm, related to the bronze
stock. The progeny consisted of purple and sun red plants with green and
salmon silks, showing the parent to have been heterozygous for both
Pl and Sm. Two small plantings gave the following distributions;
PI Sm, 26; Pl sm, 7; pl Sm, 4; pl sm, 23; whereas equality of the four classes
would be expected if the factors were independent. This was obviously
a linkage relation. The factor Pl was known to be linked with a factor
Y for yellow endosperm (Emerson, 1921). Tests of the inkage relations
‘within this group are given in a later section of this paper.

Relation of salmon silks to the A factor

From an outcross of the original salmon-silked plant with one heterozy-
gous for brown silks and for the A factor, several plants were selfed. One
sun red plant was homozygous recessive for the salmon silk factor and
heterozygous for A and B. Thirty-four sun red and dilute sun red plants
had salmon or brown silks. Two others were first noted as green but were
presumably a light brown, both having white pericarp. Eleven green
plants appeared, all having green silks. Later observations on green and
brown plants of other families segregating for both a and sm have like-
wise failed to reveal any green plants with other than green silks. That
this is not due to linkage is shown by the linkage of Sm with the Pl factor,
which is known to be independent of A, and by the fact that the green
plants have green silks in families that are homozygous recessive sm.

Relation of salmon silks to the R factor

Two questions of interest arose regarding the relation of salmon silk
color to the Ff series of allelomorphs. The first was the relation of cherry
pericarp color to the intensity of color in salmon or brown silks; the second
was the possibility of the occurrence of salmon silks on green plants of
the constitution R? A b Plor R* A b pl.

To test the effect of cherry pericarp, a sun red with brown silks,
A B pl sm r’, was crossed with a dilute purple with cherry pericarp and
green silks, A b Pl Sm r™. Backcrosses gave a few plants with cherry
pericarp and brown silks. They were not noticeably different in silk
color from the white-pericarp plants of the same families. -

INHERITANCE OF SALMON SitK CoLor In Maize 547

. To test for the occurrence of salmon silks on green plants of the consti-
tution R’ A b Pl or R’ Ab pl, a dilute purple plant with salmon silks
was crossed with a green plant of the constitution R’ A b pl. Two of
the F; plants were selfed. Purple seeds only were planted. These
gave 44 dilute purples and dilute sun reds, of which 32 had green silks and
12 had salmon. There were 25 green plants, R? A b, 18 of which had green
silks; the other 7 had typical salmon and brown silks.

From these two tests, it may be concluded that salmon silk color is
not dependent on the F factor nor is it noticeably influenced thereby.
This is similar to the relation between red pericarp color and R? (Emerson,
1921).

Summary of inheritance

Salmon and brown silks are recessive to green silks by a single factor
pair, Sm sm.

This factor, Sm, is independent in inheritance from P (pericarp),
A (aleurone and plant color), B (plant color), and R (aleurone, plant color,
cherry pericarp, and red silk color).

It is linked with the factor Pl (plant color), and consequently also with
Y (yellow endosperm).

Dominant A is necessary for the production of salmon or brown silk
color; that is, the combination a a sm sm is green.

The intensity of pigmentation of salmon-brown silks is directly related
to the intensity of pigmentation of the pericarp.

The relation of the factors A, Sm, and P to silk color may be represented
schematically as follows:

A Sm P = Green a Sm P = Green
A Sm p = Green a Smp = Green
A sm P=Salmon asm P=Green
Asm p=Brown asm p = Green

LINKAGE RELATIONS OF Y, Pl, anp Sm
Preliminary tests of linkage of Pl and Sm

The first indication of the linkage of the Sm and Pl factors was observed
in the progeny of an outcross of the original salmon with a purple plant
having green silks. This plant proved to be heterozygous for both Sm

548 FE. G. ANDERSON

and Pl. To the distribution given on page 546 may be added the data
from a duplicate planting by Dr. Emerson:

Per cent of
crossing- —
Pl Sm: Piss phism ple over
BAD? Seu Roe aes 26 rs 4 23
From Emerson.......... 25 9 + 38
51 16 8 61 17.6

Two other backcrosses were then made, which the following year gave
the results:

Per cent of ©
crossing-
PlSm Plsm plSm_ pl sm over
73 og Oa eRe eee 76 24 20 93 20.3
DAD ees ec ee 60 + 4 66 6.3

Construction and results of three-point tests

In the meantime, crosses. were made to involve the Y factor for yellow
endosperm in addition to Pl and Sm, since Y and Pl were known to be
linked. To get a satisfactory three-point backcross test involved several
difficulties, as follows:

1. Yellow endosperm is not easily distinguished from white if brought
in only by the pollen. This is assumed to be due to the dominant Y’s
being represented only once in the triple-fusion endosperm nucleus. It is
therefore desirable that the F; plants should be used as female parents
in the backcrosses.

2. Brown silks are not readily separated from green. This difficulty
can be avoided only by having red pericarp in each plant. But the
presence of red pericarp obscures the color of the endosperm. So in ordér
to make endosperm separations possible, the female parent of the back-
cross must be free from red pericarp.

3. Purple and dilute purple plants usually have some purplish pigment
in the pericarp, which in some cases interferes with the classification of
yellow endosperm.

INHERITANCE OF SALMON SiLK Conor IN Maze 549

_ 4. The dominant A factor must be present in every individual where
silk color scparations are to be made.

5. Aleurone color must be avoided.

6. Presence of the dominant B factor, while not affecting accuracy,
would nevertheless facilitate note-taking by making all the plants of
two sharply differentiated classes, purple and sun red.

To avoid as many as possible of these difficulties and accomplish the
results within the shortest period of years, the following procedure was
put into effect: Crosses were made involving the factors Y, Pl, and Sm
in different combinations. In all of these crosses, pericarp color and also
the R factor for aleurone color were kept recessive. At the same time, an
attempt was made to find or isolate a stock of the triple recessive of the
desired composition. Tests of all available salmon-silk material revealed
two closely related families breeding true for red pericarp, white endosperm,
and recessive r. Both families consisted of sun red and dilute sun red
plants showing the B factor to have been heterozygous. These were used
the following year in the backcrosses. Their composition was y y pl pl sm
sm rr P P A A, some plants being homozygous and some heterozygous
for dominant B. Pollen of these plants was used on silks of the F;
crosses.

These backcrosses were made in 1918 and the progenies were grown in
1919. The results are given in table 4 (page 550). The percentages of
crossing-over are: Y-Pl, 28.9; Pl-Sm, 9.1; Y-Sm, 36.6; showing their
relative order to be Y-Pl-Sm.

While material for these tests was being built up, some rae less satis-
factory backcrosses were made by pollinating white-endosperm, brown-
silked, dilute sun red plants with pollen from crosses involving Y, Pl,
and Sm. These were grown in 1918. The results are given in table 5
(page 551).

A summary of the percentages of crossing-over is given in table 6
(page 552).

The chromosome map

From the totals of all the data obtained on these linkage relations, the
observed percentages of crossing-over are found to be 29.70 for Y-Pl,
10.01 for Pl-Sm, and 36.79 for Y-Sm. This shows their relative map
order to be Y-Pl-Sm. The distance from Y to Pl as observed is 29.7,

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INHERITANCE OF SALMON SILK CoLor IN MaizE

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TABLE 6. Summary or LinxaGe Data

Total Percentage of crossing-over
: number
Pedigree no. of
plants Y-Pl Pl-Sm Y-Sm

Ts a Se ee eRe Ee a op i310 eal Mees ace tas 17-5.) Ga ae
DER Ore 8 eo eit eevee aati s apace Sts eae ee em 2137p QD) TN
DATE os is sie a eo thes So ee 134 ; 6.0.21) :50 ae =

Totals 20-20 de eR ee ARS | oo ek oe 15.731 ee rg
TOES Fae ARE CW, oo Pee an ee 219 36.1 10.5 40.2 =
TAZ G i Ee io Xs eee Woke were eas: aha 273 34.8 9.5 40.7
Todas sce me aeons hae Thee a) the Roane eRe ivf 28.2 10.2 31.6
VATE fens aa a a ee aR AN LSE And GN 158 27.2 8.9 34.8
MEO SO reo oes byte heeds iece chats Saas 103 34.0 12.6 36.9

Totals 91S). seta. mtk ot eine et mie 930 32.47 10.11 37 .42
12 154) as eae Stroh AT Croe ee art Bsc SICA ohio C8 CAC 287 97.7 10.1 45.6
1454-6 Fic Sa Ses ie ac ahs eeay a etceetaemeea =. Mee meee 98 37.8 eet 41.8
WESTOP aah es Bow i a, occa oa ere a et Rod 496 31.0 7 5 38.5
VAG ORO x ote eer ceca, Stein, aeaakce wa bares 413 30.0 8.7 36.8
PAGS Once tk Meese «ore tithes «Suan Prete es ene 376 % 3 74 32.7
TAG TASH lace NOE cine eee at hank eee 187 19 3 6.4 25.7
TAGS SAO 52 Sate meets peneeic sk ee 318 23.6 6.3 29.9
TATA SS. toe 2 tae iain trol eo oN 355 34 1 76 41.1
VAT Gali sche eee a. 5 oe ee ee 143 25.9 13-3 37.8
Na S80 cr oo ee oto etere. Sci ee Se 236 22 0 14.0 35.2
1 EH IR RS OP) Goh Sin nh: ca See eet cic 2 184 97.2 18.5 37.0

Lotals LU 2k Fas sets ait Seca 3,093 28.87 9.09 36.60

Totals 1918-101... «2S iss ages We 4,023 29.70 9 32 36.79

Totals:of All datets. ...24 5 « Gaase shen a 1 ee 10.01.) eee
or approximately 30 units. Since in such long distances double crossing-
over may be expected, a corrected map distance should be 30 plus twice
the per cent of unobserved double crossovers between the two points.
But with the high amount of interference indicated by the small number
of observed coincident crossovers in the two regions Y-Pl and Pl-Sm, the
corrected value for these data is probably not much above 30 or 35. The
value 10 for the map distance between Pl and Sm is probably correct
for these data.

INHERITANCE OF SALMON SILK Cotor IN MaIzE Bos

It should be understood that the chromosome map is primarily a graphic
representation of the data on linkage relationships. Its correspondence
with actual positions on the chromosome itself is not implied, tho the
work of Morgan and his coworkers has given much evidence of at least a
correspondence between relative map order and the actual relative position
of the genes in the chromosome. .

The variability of the percentages of crossing-over shown in -table 6
is not greater than would be expected of heterogeneous data. Gowen
(1919) has shown crossing-over in Drosophila to be an extremely variable
phenomenon. Plough (1917) has shown it to be modified by temperature,
and Bridges (1915) by age of the individual. The subject of variation
of crossing-over in maize must remain for study with less difficult characters
than those involved in these experiments.

The distributions when the F;’s were used as pistillate and as staminate
parents give nearly the same averages, but the data are inadequate for
any conclusion except that the crossing-over is not widely different in
the two cases.

Coincidence of crossing-over

Coincidence of crossing-over in two regions of a chromosome is the
ratio of observed coincident (simultaneous) crossing-over to the calculated
expectancy. The expectancy is the product of the percentages of crossing-
over of the two regions. The actual calculation may be simplified, as
shown by Weinstein (1918). The derived formula is

Coincidence = ~~
ab
in which n = the total number of individuals,
'x = the number of coincident crossovers,
a and b= the total number of crossovers in the respective regions.

The coincidence of crossing-over in the two regions Y-Pl and Pl-Sm,
calculated from tables 4 and 5, is as follows:

From table 4:
at £50903 %-

———— = 0.26
893 x 281

Coincidence =
From table 5:
24 x 930 _

——— = 0.79
302 x 94

Coincidence =

554 E. G. ANDERSON

From combined data of tables 4 and 5:
45 x 4023 tds
1195 x 375

These values are entirely comparable with those listed by Weinstein
(1918) for Drosophila. From this and the similarity of all phases of
linkage and crossing-over, it is evident that the mechanism of crossing-
over in maize is not strikingly different from that in Drosophila except
in one respect. In Drosophila, crossing-over occurs in oogenesis only,
in spermatogenesis not at all. In maize the phenomena of crossing-over
are at least of the same order in both megasporogenesis and microsporo-
genesis.

Coincidence = 0.40

Summary of linkage studies

The factor Sm for salmon silk color is shown to be linked with the
factor Y for yellow endosperm and the factor Pl for plant and anther color.

The relative order of these three factors is Y-Pl-Sm.

The amount of crossing-over between Y and Pl is about 30 per cent;
between Pl and Sm it is about 10 per cent.

The observed coincidence of crossing-over in the two regions Y-Pl and

Pl-Sm was about 0.4.
LITERATURE CITED

Bripces, Catvin B. A linkage variation in Drosophila. Journ. exp.
zool. 19:1-21. 1915.

Fast, E. M., anp Hayes, H. K. Inheritance in maize. Connecticut
Agr. Exp. Sta. Bul. 167:1-142. 1911.

Emerson, R. A. Genetic correlation and spurious allelomorphism in
maize. Nebraska Agr. Exp. Sta. Ann. rept. 24:58-90. 1911.

A fifth pair of factors, A a, for aleurone color in maize, and
its relation to the C c and R r pairs. Cornell Univ. Agr. Exp. Sta.
Memoir 16: 225-289. 1918.

The genetic relations of plant colors in maize. Cornell Univ.
Agr. Exp. Sta. Memoir 39:1-156. 1921.

GoweEN, JoHN WuitremMore. A biometrical study of crossing over. On
the mechanism of crossing over in the third chromosome of Droso-
phila melanogaster. Genetics 4:205-250. 1919.

Pioucu, Harotp H. The effect of temperature on crossingover in
Drosophila. Journ. exp. zool. 24:147-209. 1917.

WEINSTEIN, ALEXANDER. Coincidence of crossing over in Drosophila
melanogaster (ampelophila). Genetics 3:135-172. 1918.

Memoir 41, Lysimeter Experiments—II, the seventh preceding number in this series of publications, was
mailed on November 16, 1921.

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