Crossing Over and Chiasma 



117 



^i vg 



+ ^ . + b / 



, 2i± (/c/an< 



?? 



vg b 

 vg b 



B 



? 



vg 



+ b 



..d' 



40% 



vg b 



50% 



vg + ^ ^ vg + 



+ b 



vg b 



FIGURE 16-1. Results of reciprocal crosses in- 

 volving black body color (b) and vestigial (vg) 

 wings. 



the dihybrids are ah/-\- +, however, this 

 ratio is not obtained. 



Thus far, two cases in Drosophila have been 

 studied and different crossover rates found in 

 each, one case involving the X, the other an 

 autosome. The question may now be asked: 

 What is the strength of linkage between a 

 given gene and several others located in the 

 same chromosome? This can be studied 



readily for certain X-linked genes. Females 

 are constructed with the genotypes shown in 

 Figure 16-4. The frequencies of crossover 

 combinations, as detected in their sons, are 

 shown in the other column of the Figure. 

 The recombination rates given are those 

 found between the gene for yellow body color 

 (y) in each instance and for white eye (m'), 

 crossveinless wings (cv), cut wings (ct), minia- 



