THE MECHANISM OF HEREDITY 



63 



togenesis. At the time when the homologous pairs of 

 chromosomes approach each other just previous to the 

 reduction of the chromosome number to half, they twist 

 around each other. Often they retain their individuality 

 as they pass to the daughter cells ; but sometimes they 

 break at various places, join their parts in a different 

 combination and pass to the daughter cells in their new 

 guise. The diagram will make this clear. 



will give 



Fio. 20. — Diagram to illustrate gamete formation in a dihybrid in linked inheritance 



The easiest way to determine the frequency with which 

 these breaks in linked characters occur, and a way which 

 gives it in terms of chromosome crossovers is to mate F^ 

 individuals back to the double recessive type. When the 

 Fi male in the cross just described is mated with a black 

 vestigial female, only two classes of offspring are pro- 

 duced ; half are black vestigial and half are gray long in 

 type. The F^ male produces only two kinds of gametes. 

 There is, therefore, no crossing over between the chromo- 

 somes of the male. 



On the other hand, when the F^ female is mated with 

 a black vestigial male, four types of offspring are 

 produced : 



Non-croseovers Crossovers 



Black vestigial Gray long Black long Gray vestigial 



41.5 per cent. 41.5 per cent. 8.5 per cent. 8.5 per cent. 



