52 
BULLETIN 1488, U. S. DEPARTMENT OF AGRICULTURE 
It appears that in these two progenies there is a small but signifi- 
cant difference in the rate of crossing over at the base and at the 
tip of the ear. This difference is confined to the formation of female 
gametes and is not found when the male is the heterozygous parent. 
It should be kept in mind that the results are based on the behavior 
of two closely related progenies. These same two progenies, how- 
ever, showed a striking difference in the relative frequency of cross- 
ing over in male and female. 
The most obvious difference in the conditions of fertilization of 
the base and tip of the ear where the plant bearing the ear is heter- 
ozygous is the greater promptness with which fertilization is effected 
at the tip of the ear. This difference suggests a differential mortality 
of female gametes as the cause of the lower crossing over at the 
base of the ear. It is difficult, however, to reconcile this explanation 
with the regular arrangement of the seeds on the ears measured and 
with the lack of correlation between crossing over and silk length 
considered later. 
LENGTH OF SILK NOT A FACTOR IN RATE OF CROSSING OVER 
In seeking an explanation for the greater percentage of crossing 
over on second ears as compared with first or upper ears of the 
same plants, the thought naturally occurred that the difference be- 
tween the two ears might be due to a difference in length of silks. 
Table 32. — Differences in crossover rate when measured on ea/rs ivith long and 
short silks 
[The differences are between pairs of ears pollinated simultaneously and measure crossing over in female 
and male gametes respectively] 
Silk length (inches) 
Difference in 
Designation 
crossovers 
Long 
Short 
Dif- 
ference 
(per cent) i 
i 5 
4 
1 
-4. 7 ±3. 64 
3 
1 
2 
2. 8 ±5. 02 
4 
% 
&A 
-0. 7 ±2. 91 
8 
% A 
VA 
-1. ±2. 37 
3 
l 
2 
-3. 6 ±3. 75 
8 
4 
4 
-1.6 ±2.44 
3 
1 
2 
-0.3 ±4.04 
J 3 
1 4 
1 
2 
3^ 
-1.4 ±2.86 
3. 5 ±3. 11 
4 
2 
2 
-3. ±2. 33 
6 
4 
2 
-1.9 ±1.61 
5 
1 
4 
-2. 5 ±1. 58 
3 
1 
2 
5. 3 ±2. 72 
4 
m 
2V 2 
0. 8 ±1. 95 
4 
i 
3 
-2. 6 ±2. 76 
I 5 
3 
2 
-10.1 ±3.41 
Mean . --- 
4.5 
1.7 
2.7 
-1.36± .51 
f 3 
2 
1 
4. 8 ±2. 02 
7 
2 
5 
-7. 8 ±2. 02 
3 
4 
H 
2 
2}i 
2 
5. 8 ±5. 14 
5. ±3. 86 
3 
2 
1 
5. ±2. 00 
3 
1 
2 
-1.7 ±5.45 
3.8 
1.6 
2.2 
1.01±1.71 
i Minus sign indicates those cases where the highest percentage of crossing over occurred with the long 
silks. 
