24 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
there must be a few which in reality were heterozygous for R in 
one parent. Since plants heterozygous for R have a somewhat lower 
rate of crossing over than those homozygous dominant for this 
factor, the standard deviations for the R R group are too high. No 
adequate estimate of the number of such ears can be made, but it is 
doubtful whether enough such cases exist to destroy a significant 
difference. It may be concluded, therefore, that the R factor in 
heterozygous conditions is not associated with a greater variability 
in rate of crossing over between C and Wx. 
Table 17. — Comparison of the variability of crossover rate between C and Wx 
in plants homozygous and heterozygous for the R aleurone factor 
Heterozygous 
sex 
Variability (o"eo) of crossover 
rate (per cent) 
Difference 
(R R-R r) 
Progeny 
RR 
Rr 
DIE 
Dh 416L3L1C5L3L1R23 
Female 
Male 
Female and 
male 
Female 
Male 
Female and 
male 
Female 
Male. 
6. 41±0. 48 
5. 44± . 33 
6. 50±1. 03 
6.90±1. 10 
5. 61=1=1. 20 
5.31±1.03 
5.77± .61 
9.05± .94 
5. 94±0. 56 
6. 07=fc . 39 
5. 50± . 79 
7.34=b .94 
7.85=1=1.32 
8. 33±1. 78 
7.53± .67 
9. 19=bl. 13 
0. 47±0. 73 
- . 63=b . 51 
1.00±1.30 
- .44=1=1.44 
-2. 24=1=1. 78 
-3. 02±2. 05 
-1.76=1= .91 
- .14=1=1.47 
0.64 
Dh 416L3L1C5L3L2R23 
1.23 
.77 
.31 
Ph 230L1 and L2R24 
1.26 
1.47 
1.93 
.10 
Mean 
- . 53=fc . 24 
2.19 
RATE OF CROSSING OVER BETWEEN THE C-Wx GENES 
With the linked genes C~Wx, crossover rates ranging from 8 to 
50 per cent have been found in sibs of inbred progenies. In these 
cases the number of seeds precluded the possibility of their being 
chance departures from a common mean. The situation with respect 
to a moderately variable progeny is presented graphically in 
Figure 1. 
Before a definite arrangement of the many characters of the mature 
plant could be attempted it seemed necessary to gain an insight into 
the factors that were causing the great differences between progenies 
and individuals, in the closeness of the linkage of the aleurone gene 
C and the endosperm gene Wx. It was first thought that the differ- 
ences were due to heritable mutations in the rate of crossing over, 
and as a first step in the demonstration of the correctness of this 
hypothesis progenies were grown from ears of widely different de- 
grees of linkage. The results were disappointing, in that the hy- 
pothesis was both supported and contradicted by the evidence. 
At the present time (now seven generations from the first hybrid 
generation) there is conclusive evidence that a strain has been iso- 
lated which will give with some consistency 35 to 39 per cent of 
crossing over between C and Wx, while another strain for which the 
evidence is not so conclusive will give from IT to 22 per cent of 
crossing over between these factors. 
