THE STRENGTH OF LINKAGE 373 



Fig, 108. — Crossing-Over in a Cross Between Black Vestigial 

 and Gray Long. This experiment is like the last two except that 

 in this case a female hybrid is bred to a double recessive male. The 

 second line shows that four kinds of eggs are produced as if free 

 assortment had occurred. The difference lies in the fact that there 

 are many more eggs with the original combination of genes than 

 with the recombinations as is shown by the fact that in the second 

 hybrid generation there are about five times as many grandparental 

 types as there are recombination types. Crossing-over of genes in 

 the vinegar fly has occurred up to the present only in the female 

 and not in the male. Hence female hybrids are used when it is 

 desired to demonstrate crossing-over and males when linkage only 

 is to be shown. 



Morgan, Phys. Basis. ^ 



