34 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
per cent white (G c R r Wx wx X ccrr wx wx) . It can not be shown 
that the crossing over in any of these classes is not a chance departure 
from that of the larger groups, and they are therefore included in 
the table. If they are excluded, however, and the comparison con- 
fined to the back-crossed ears giving 50 per cent and 62.5 per cent 
white, the individual comparisons become statistically significant. 
That the association between heterozygous R and a reduced cross- 
ing over is not peculiar to this hybrid is shown by the results from 
another distantly related hybrid, Ph 230, considered later in con- 
nection with the factor for sweet seeds. 
The mean crossing over in the several groups is shown in Table 
22. In this progeny the effect of heterozygous R appears to be con- 
fined to the formation of the female gametes. The difference be- 
tween the mean of the 20 back-crossed ears with homozygous R 
and that of the 33 ears with heterozygous R is 4.15±0.96 per cent, 
a difference of more than four times the probable error. 
Table 22. 
-Crossover values in the several gametic groupings of the hybrid 
Ph 230L1 and L2R24 
Nature of cross 
Gametic composition 
Num- 
ber of 
ears 
Number 
of seeds 
Percentage of 
crossovers 
Female heterozygous: 
50 per cent white. . 
62.5 per cent white 
Do 
75 per cent white.. 
Male heterozygous: 
50 per cent white. . 
62.5 per cent white 
f c E wx c r wx 
I CRWx X crwx 
CRWx^cR wx 
cr wx ^crwx 
CRWx~ Crwx 
cr wx cr wx 
C R Wx X c r wx 
c r wx c Rwx 
crwx* CRWx 
c Rwx C RWx 
10, 374 
9,933 
1,285 
5,641 
5,883 
5,654 
21. 59±0. 69 
17. 70± . 70 
15. 46±2 35 
21. 88±1. 03 
26. 19±1. 16 
26. 73± . 84 
In this hybrid it is evident also that the crossing over in the group 
giving 75 per cent white is not reduced, although R is heterozygous. 
There were 16 ears of this character with 5,641 seeds and a mean 
crossover of 21.88 ±1.03 per cent. This might be explained by 
assuming that to reduce the crossing over the heterozygous R must 
enter from both parents. But if this were true the group of back- 
crossed ears resulting from the combination G cRr Wx wx X G err 
wx wx should have a high rate of crossing over. Ears of this char- 
acter can.be detected by the low value of Q, which can only reach 
0.5 with perfect linkage. Because of the linkage between G and Wx 
this combination is relatively rare, but there were 4 among the 33 
back-crossed ears giving 62.5 per cent white, and their mean cross- 
over percentage is only 15.46 ±2.35, a rate even lower than that of 
the group to which they have been assigned. The small number of 
ears, however, makes the probable error so large as to leave open the 
possibility that they are a chance departure from the RR group. 
The difference is 6.14±2.45_per cent. 
