298 



THE CONTINUITY OF THE RACE 



nation that would make the new findings compatible with the evidence 

 that the genes were in the chromosomes. A clue was found in some then 

 recent microscopic studies on the details of early maturation, in which the 

 pairing chromosomes appeared to be twisted about one another at the 

 time they were "fixed" (killed) and stained. Morgan saw in these over- 

 lapping chromosomes a possible explanation for his breeding results. If 

 the chromosomes should break at the points where they cross over and 

 interchange with each other the sections included between the crossings 

 over, it would be possible to see how two or more genes could lie within 

 one chromosome and yet occasionally become separated to form the new 

 combinations that had been observed. Suppose, for instance, that we 

 imagine the conditions and sequences diagramed below, in which the 

 "first cross" in Drosophila is represented with the genes for body color 

 and wing length situated within the same chromosome: 



The homozygous parents Gray-long X black- vestigial 



The genes involved, both within the same chromosome B B b b 



pair 



' 



v v 



Gametes formed by parents 



The resulting dihybrid Fi 



1. Possible events during meiosis of oogonia 



Resulting gametes 



2. No crossing over during meiosis of sperma- 

 togonia 



B 



B b 



V v 



No Crossing Over 

 B b 



V v 

 B b 



Crossing Over 

 B b 



B 



V v 



B 



Resulting gametes 



B 



Here was a hypothesis that would explain the puzzling, partial, but 

 not completely independent assortment that had been found in one cross 

 in sweet peas and that was now being found in more and more crosses in 

 Drosophila. Such a hypothesis depended upon a number of unproved 



