LINKAGE 53 
17 per cent. of cases. Here too we see that now the 
factors tend to separate, whereas in the case of the 
other F, female they tended to stay together, since 
they lay in the same chromosome. In the present 
case, when the chromosomes interchange, the factors 
are brought together, and so the crossover classes 
are just the opposite in the two cases, as also are the 
non-crossover classes. Yet there is the same amount 
of crossing over shown in both crosses, so that the 
frequency of the double recessives and double domi- 
. nants in the first cross is exactly equal to the fre- 
- quency of the single recessive and single dominants in 
the last cross. Which classes shall have the high 
frequency and which the low does not depend on the 
nature of the factors themselves, therefore, but on 
which ones come from the same parent, 7.e., lay in the 
same chromosome at first, and which lay in opposite 
chromosomes. The amount of crossing over is seen 
to be independent of the way in which the factors enter 
an individual. Hence it is fair to infer that the 
process is not peculiar in any way to hybrids, but 
takes place in the same way and to the same extent 
in gametogenesis in pure homozygous stocks. This 
is also indicated by the fact, later to be discussed, 
that when several different allelomorphs of a factor 
may occur, all give the same per cent. of crossing 
over with other factors. 
Many other combinations, involving a large num- 
ber of different characters in the second group, have 
been studied and give consistent results. There is 
never any crossing over in the male; and, in the fe- 
