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 FI 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, i.e., lay in the 

 same chromosome at first, and w T hich 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- 



