120 



CHAPTER 9 



r \ vg + 



vg \ 



9* Ho* 



dW °" d ?? 



vg 



+ b 



A 



_ + F o x nip^ 



vg b 



vg b 

 vg b 



B 



? 



vg + _ 



x F. 



+ b ' 



a* 



F 2 40% 



40% 



10% 



10% 



vg b 



F 2 50% 



50% 



vg + 

 vg b 



+ b 

 vg b 



figure 9-4. Results of reciprocal crosses involving black body color (b) and vestigial 

 (vg) wings. 



by vg b/vg b $ ) is made with the Fi, 50% 

 of offspring are vg-\-/vgb (vestigial), and 

 50% are + b/vg b (black) (Figure 9-4B). 

 This cross produces no offspring with cross- 

 overs, so that linkage is complete for these 

 genes in the male Drosophila. (Had link- 

 age been complete in the female also, we 

 should not have had any evidence that vg 

 and b are separable and, therefore, two 

 genes instead of one. ) One finds, more- 

 over, that in Drosophila any genes showing 



incomplete linkage in the female are com- 

 pletely linked in the male; the male, there- 

 fore, does not undergo the process of cross- 

 ing over to produce crossovers. 2 It may be 

 noted that in animals in general, crossing 

 over is reduced or absent in the hetero- 

 gametic sex. For example, no crossing over 

 occurs in the females of birds. 



2 On rare occasions a special kind of "crossing 

 over" does occur in the male Drosophila but is 

 not of the kind that typically occurs in females. 



