32 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
Hutchison (9) found that crossing over in C-Sfa-Wx stock was 
approximately 80 per cent of that in an I-Shr-Wx stock and sug- 
gested that a crossover modifier might be responsible. The lower 
rate of crossing over applied equally to both regions of the chromo- 
some. The factors / and C are very closely linked, and the ap- 
parent distance between / and Wx was fouoid to be 26.18 units and 
that between C and Wx was but 21.88 units. The difference be- 
tween the two stocks is no greater than that found between many 
of our progenies where the / factor is not involved and may repre- 
sent merely progeny difference rather than the effect of a crossover 
modifier, but the point is of interest in view of the effect of the R 
factor. 
From the two progenies which contained plants that were heterozy- 
gous for i?, crossing over was measured in 140 individuals. These 
fall into six groups, the reciprocal back crosses and the selfed ears 
from each of the two progenies. In Table 20 the rate of crossing 
over in the individuals that are homozygous for R in each of these 
six groups is compared with the rate in those heterozygous for R. 
Table 20. — Comparison of the percentage of crossing over in plants homozygous 
for the R aleurone factor with those heterozygous for this factor 
Homozygous for R 
Heterozygous for R 
Crossover difference 
Nature of cross 
Num- 
ber of 
ears 
Num- 
ber of 
seeds 
Percentage 
of 
crossovers 
Num- 
ber of 
ears 
Num- 
ber of 
seeds 
Percentage 
of 
crossovers 
R R-R T 
D/E 
Dh 416L3L1C5L3L1R23 
Back crosses: 
Female heterozygous.. . 
Male heterozygous 
Self 
41 
60 
9 
9 
5 
6 
19, 759 
27, 725 
4,455 
4,710 
2,225 
2, 032 
20. 77±0. 54 
19. 14± . 37 
21. 48±1. 10 
24. 34±1. 79 
27. 18±1. 52 
23. 77±1. 25 
26 
56 
11 
14 
8 
5 
14, 258 
24, 202 
3,156 
6,763 
2,588 
1,477 
20. 19±0. 61 
16. 97± . 41 
19. 12± . 82 
21. 20±1. 17 
25. 44±1. 63 
24. 80±2. 17 
0. 58±0. 82 
2. 17± . 55 
2. 36±1. 37 
3. 14±1. 72 
1. 74±2. 23 
— 1. 03±2. 50 
0.7 
3.9 
1.7 
Dh 416L3L1C5L3L2R23 
Back crosses: 
Female heterozygous. _ 
Male heterozygous 
Self 
1.8 
.8 
.4 
Mean 
1. 75±0. 28 
6.2 
The differences between the mean crossing over of plants with R R 
and R r in the six groups, when treated as a population weighted by 
the reciprocal of the squared probable errors, show a mean percentage 
of 1.75 ±0.28, a departure from zero of more than six times the prob- 
able error. Five of the six determinations are positive in sign, the 
group with the smallest number of ears being the only exception. 
Plants with heterozygous R were identified by the resulting ratio 
of white to colored seeds. In back-crossed ears there are three com- 
binations that would give 50 per cent white : 
R 
c R wx c R wx 
C R Wx c R wx 
wx c r 
C R Wx c R wx' 
and 
c r 
wx w c R wx 
"X." 
C R Wx c R wx 
This made it impossible to detect all of the double heterozygous 
plants that were also heterozygous for R. There were, however, nine 
plants in the back-crossed group of the first progeny giving 50 per 
cent white, which by other pollinations were shown to be heterozygous 
