If the female gametes had been all of the same number 

 the ratio would have been 



1 black vestigial, 

 5 gray long, 

 1 black long and 

 1 gray vestigial, 



but as the two latter appeared in about 17% of the cases 

 it is evident that the gametes do not exist in equal numbers. 



Instead of crossing an Fi male with an Fi female, the 

 crossing-over can be more readily made out by back-cross- 



HiJ 



^ 



B l\\ 







I 



ri'- > 



/H. 



^1 



Fig. 21. — Diagram of the Probable Mechanism of 

 "Crossing-over." Pairs of homologous chromosomes, 

 one from the father and the other from the mother, 

 are shown in synapsis; on the left they lie parallel to 

 each other and when they separate they remain as 

 they were before union: in the second column they are 

 shown crossing each other one or more times: in the 

 remaining figures are shown the results of the chro- 

 mosomes breaking at the points of crossing, thus in- 

 terchanging sections of the two chromosomes. Let- 

 ters indicate loci of homologous genes in the two 

 chromosomes of a pair. (After W^ilson.). 



ing an Fi female to a black-vestigial male. Four kinds of 

 flies are produced, viz., black-vestigial, gray long, black- 

 long and gray vestigial, according to the following diagram: 



12.? 



