VARIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 
33 
for R. This group, although giving 50 per cent white, had a crossing 
over of 16.90± 0.805, which agrees well with 16.97, the average of the 
group heterozj^gous for /?, and differs from the average of the group 
with homozygous R by 2.5 times the probable error. Besides support- 
ing the results from the group with 43.75 per cent white, these results 
indicate that the differences are not due to the method of measuring 
the crossing over when one of the characters is dihybrid. 
Among the plants heterozygous for R back crossed on more than 
one individual, there were seven that were used both on plants homo- 
zygous dominant for R, resulting in 50 per cent white seeds, and plants 
heterozygous for R, giving 62.5 per cent white seeds. If the lower 
crossing over of the plants segregating 62.5 per cent ascribed to the 
heterozygous R was due to the method of measuring the crossover, a 
comparison of the two rates in these seven plants should disclose the 
fact. The data are presented in Table 21. 
The results show no consistent differences between the rate meas- 
ured when 50 per cent of the seeds are white and that with 62.5 per 
cent white. 
Table 21. — Percentage of crossing over in the male gametes of individuals tack 
Grossed on plants homozygous for R compared with the crossing over in the 
same individuals vjhen oack crossed on plants heterozygous for R 
Male parent 
Female parent 
cc R Rwxwx 
cc Rr wxwx 
Difference 
Cc Rt Wxwx 
Number 
of seeds 
Percent- 
age of 
cross- 
overs 
Number 
of seeds 
Percent- 
age of 
cross- 
overs 
(RR-Rr) 
Plant No. 5901 . 
1,523 
379 
2,471 
324 
170 
636 
510 
14.2 
26.1 
19.9 
17.0 
12.9 
16.9 
16.0 
1,157 
1,229 
328 
526 
229 
264 
713 
14.3 
23.4 
13.8 
22.6 
5.2 
22.3 
15.4 
—0. 1 ±0. 9 
Plant No. 5905 
2. 7 ±1. 7 
Plant No. 5955.... 
6.1 ±1.4 
Plant No. 5976 
-5. 6 ±1. 9 
Plant No. 5911 
7. 7 ±2. 
Plant No. 5918 
-5. 4 ±2. 
Plant No. 5909 
.6 ±1.4 
Mean _ .. 
. 96± . 99 
It follows that the reduction of crossing over associated with 
heterozygous R probably is somewhat larger than our results indi- 
cate, for there is no way of detecting all of the individuals hetero- 
zygous for R in the groups giving 50 per cent white. 
In the second progeny tne number of plants is much smaller, and 
in none of the groups taken individually are there significant differ- 
ences between the plants with R R and R r, but since none of the 
f roups departs significantly from the corresponding group in the 
rst progeny, they are combined in the table. In this second prog- 
eny, which was the result of selfing a plant of the gametic com- 
G R Wx 
position , there were four ears with the linkage between 
L crwx 
dominant and recessive, one back-crossed ear giving 25 per cent white 
(O cR R Wx wx x C err wx wx) and three back-crossed ears with 75 
