VARIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 37 
The records afford 13 progenies with from 4 to 49 reciprocal back 
crosses. The data are presented in detail in Table 24 and summa- 
rized in Table 25. The material includes 151 reciprocal crosses. 
Measurements of crossing over in the female involve 70,898 seeds, 
and the reciprocals measuring the crossing over in the male involve 
71,391 seeds. 
In 9 of the 13 progenies the crossing over was lower in the male and 
in 4 the crossing over was lower in the female. There is therefore 
little ground for assuming any general tendency. In 6 of the 9 
progenies in which the male has the lower rate the difference is 
greater than three times the probable error, and in 2 the difference 
is more than six times the probable error. Thus there is no ground 
for explaining these differences as chance variations. It is equally 
obvious, however, that at least 3 of the 4 progenies in which the 
females have the lower rate are not chance departures from equality 
or from the general mean. 
INDIVIDUAL VARIATIONS IN THE CROSSOVER DIFFERENCE 
It is possible to go further and show that variations in the cross- 
over differences of the individual plants of a progeny are not chance 
departures from the mean of the progeny. This is best exemplified 
in progeny shown in entry 10, Table 24, grown at Arlington, Va., in 
1923. In this progeny, crossing over in the male and female gametes 
was measured in 49 individuals. Crossing over in the males averaged 
2.78 per cent lower than in the females. The individual differences 
range from —13.6 to 14 and many depart from the mean to an extent 
that can not be ascribed to chance. Eight are more than four times 
the probable error, five are more than five times, and three are more 
than six times the error. 
Although all progenies are rather closely related, it is of interest 
to examine their pedigrees for evidence of genetic factors affecting 
crossover difference. Entry 9, Table 25, with a significantly lower 
average crossing over in the female, is a back cross between two sister 
plants of entry 8, which showed a significantly lower average crossing 
over in the male, and the heterozygous plant parent of entry 9 had a 
lower crossing over in the male of 5.9 per cent. Entries 10 and 11 
were from two ears on the same plant. No. 10 was pollinated by a 
sister plant from a white- waxy seed and No. 11 was selfed. The 
difference between these two progenies is nearly 10 times the probable 
error. 
Thus there is no evidence that the pronounced diversity in the 
relative frequency of crossing over in the male and female is the 
result of genetic differences. 
When we turn to possible environmental differences for an expla- 
nation of the diversity, the results are equally baffling. Entries 10 
and 11 that differ so profoundly were grown in adjacent rows and 
flowered simultaneously. 
Emerson and Hutchison (6) have reviewed the literature of cross- 
ing over in the two sexes in plants and report experiments with two 
pairs of linked factors in maize. 
