ARCHITECTURE OF THE GERMPLASM 



different classes resulting in the F 2 generation from 

 hybrids is different in typical Mendelian segregation 

 and in cross-overs. 



Furthermore, the percentage of cross-overs varies in 

 different crosses. For example, when white-eyed yel- 

 low-bodied flies are crossed with normal wild-type red- 

 . eyed gray-bodied individuals, the resulting hybrids re- 

 semble wild red-eyed, gray-bodied flies. When such a 

 female hybrid is crossed back to a recessive white-eyed 

 yellow-bodied male, the offspring show only one per- 

 cent of cross-overs, that is, white-eyed, gray-bodied and 

 red-eyed, yellow-bodied individuals, and 99% of linkage, 

 that is, white-eyed, yellow-bodied and red-eyed, gray- 

 bodied (Fig. 72). 



Another percentage of cross-over, that between 

 white-eye and miniature-wing was found to be 33. It is 

 obvious that in any case the cross-over will never ex- 

 ceed 50%. 



Jennings has said: "The studies of 'crossing-over' 

 promise to bring us into closer touch with the actual 

 , details of the hereditary mechanism than any other 

 phenomena now under examination." 



5. How DO CROSS-OVERS OCCUR? 



In germ-cells before maturation, homologous mater- 

 nal and paternal chromosomes pair off and usually come 

 to lie side by side. This is the phenomenon of syndesls 

 or conjugation. During this temporary contact there 

 seems to be an opportunity for such an exchange of 

 parts as cross-over breeding demonstrates does actually 



