THE PHYSICAL BASIS OF INHERITANCE 



297 



them like one of the male's parents, half like the other. Evidently the 

 male forms but two kinds of germ cells. 1 The total lack of assortment 

 in the male could be explained by adopting the Boveri-Sutton hypothesis 

 and supposing that in the first cross, B and V were in the same chromo- 

 some and b and v were in its mate; and that in the second cross, B and 



\m/ 



s 



O 



\#/- w 





Fig. 19.3. The backcross of a female dihybrid Drosophila, from gray-vestigial black-long, 

 and a pure recessive black-vestigial male. Note that black and long, and gray and vestigial, 

 are neither completely linked nor do they show independent assortment. Compare with 

 Fig 19.2. 



v were in one chromosome and b and V in its mate. But how are the 17 

 per cent of new combinations that are formed by the Fi dihybrids to be 

 explained? 



Morgan and his associates were already strong proponents of the 

 Sutton-Boveri hypothesis, and they consequently sought for some expla- 



1 This total lack of crossing over in the male of Drosophila appears to be peculiar 

 to this group and is not characteristic of organisms in general. 



