LINKAGE 



51 



taken place between black and vestigial in the pair 

 of homologous chromosomes. The numerical results 



Fig. 22. — Back-cros.s of Fi female (out of black by vestigial) by 



black vcsti<!;i;il tiialc. 



show that this crossing over takes i:)lace in a]:)out 

 17 per cent, of cases. In other words, the chances are 



