50 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
cross sections of approximately 100 seeds each, and the crossing over 
in each section was measured separately. The fluctuations in such 
small samples were too large to detect any consistent differences at 
different levels, and these ears were included with those classified 
into tip and base by discarding the basal section and combining the 
remainder into two groups. 
The ears included back crosses between double-heterozygous plants 
and double recessives and self -pollinated double-heterozygous plants. 
Ears with both monohybrid and dihybrid ratios of white to colored 
were included, and since there are significant differences in the cross- 
over ratio of different individuals the mean ratio is of no significance. 
But the base and the tip of each ear are always of the same compo- 
sition and are measured in the same way, hence it seems a proper 
procedure to average the differences between the crossing over of 
base and tip, the differences being weighted by the reciprocals of 
their squared probable errors. 
The results show a difference between the rate of crossing over at 
the base and at the tip of the ear, with the higher rate occurring in 
the tip of the ear. Contrary to expectation, however, the difference 
is evident only when the female is the heterozygous parent. In the 
43 ears of the progeny LI, all of which were from heterozygous 
females, the percentage of crossing over in the tip exceeded that in 
the base by 1.29 ±0.49 per cent. In the progeny L2 the 26 ears with 
the female heterozygous showed a mean difference in the same direc- 
tion of 2.79 ±0.87 per cent. Combining the ears of both progenies 
having the female heterozygous, the difference is 1.80±0.44, which is 
4.1 times its probable error. In the 10 ears of the L2 progeny with 
the male parent heterozygous there was practically no difference 
in the crossing over in the base and in the tip of the ear. The mean 
difference was 0.98 ±0.40 per cent, the higher crossover ratio being 
in the seeds from the base of the ear. The individual ears are listed 
in Table 31. As a check on the reliability of the difference in cross- 
ing over, a similar comparison of the tip and the base of the ears 
was made with respect to the percentages of white and of waxy. 
The results showed that in the simple Mendelian characters there 
were no significant differences between the tip and the base of the 
ears. 
Table 31. — Differences in percentages of crossing over in tip and base 
Tip 
Base 
Ear No. 
Number 
of seeds 
Percent- 
age of 
cross- 
overs 
Number 
of seeds 
Percent- 
age of 
cross- 
overs 
Difference 
(tip— base) 
Dh 416L3L1C5L3L1R23, female heterozygous: 
5901 - - 
334 
210 
348 
288 
278 
326 
180 
294 
368 
59 
161 
27.8 
18.6 
34.2 
18.4 
31.6 
18.4 
30.6 
26.9 
18.5 
27.1 
13.0 
3D5 
323 
339 
332 
310 
348 
240 
323 
420 
66 
192 
24.6 
19.2 
25.1 
19.0 
31.6 
14.4 
26.7 
23.2 
23.1 
26.8 
10.4 
3. 2 ±2. 3 
5902-2 
—.6 ±2.3 
5905 
9.1 ±2.3 
5907 
-. 6 ±2. 1 
5910. J 
0. ±2. 6 
5922.- 
4. ±1. 9 
5923 
3.9 ±2.0 
5928.. 
3.7 ±2.3 
5929... 
- 4.6 ±1.9 
5931 
. 3 ±5. 6 
5935 
2. 6 ±2. 3 
