CROSSIXG-OVER 113 



these characteristics. One kind is black-long, the 

 other normal-vestigial. 



These four kinds do not appear in equal numbers, 

 but 83 per cent are like the grandparents and 17 

 per cent are the cross-overs. The result may be 

 stated in another way. The mutant characters that 

 went in together, black-vestigial and normal-long, 

 liave remained together (linked) in 83 per cent 

 of the grandchildren, while in 17 per cent of the 

 grandchildren there has been an interchange or 

 crossing-over. 



The results may be represented in terms of chro- 

 mosomes as follows. The gene for black is represented 

 by b and its normal partner by B ; the gene for ves- 

 tigial by V and its partner by V (fig. 49) . Black (b) 

 and vestigial (v) are represented in the figure as 

 contained in the same chromosome (here a rod) in 

 one parent, and gray (B) and long wing (F) by the 

 corresponding chromosome in the other parent, here 

 b}" the all-black chromosome. The daughter contains 

 one of each of these tw^o chromosomes. When her 

 eggs mature these two chromosomes separate and 

 some of the eggs contain one, some the other chro- 

 mosome. These two kinds of eggs as the sequel shows 

 represent 83 per cent of all the eggs. But in 17 per 

 cent of the cases there has occurred in some way an 

 interchange between these two chromosomes with 

 the result that the genes for black and long come to 

 be in one chromosome and normal and vestigial in the 



