4 



Cytogenetic Correlations and 

 Crossing Over 



Genetic mapping is made possible by the occurrence of recombination 

 between linked genes. In succeeding chapters, we shall see how useful a 

 procedure mapping can be in the investigation of genetic material. In 

 this chapter, we are concerned with the process of recombination of 

 linked genes (crossing over) as it occurs in sexual organisms and with 

 some of the kinds of aberrant behavior of which chromosomes are cap- 

 able. Most of our information comes from the correlated chromosomal 

 and genetic analysis of breeding behavior in Drosophila and in the corn 

 plant, Zea mays (maize). A group of brilliant investigations in the 

 classical period of cytogenetic research, centered in the 1920 s and early 

 1930 s, established the ground rules of chromosome behavior as the basis 

 of the segregation and recombination of Mendelian factors. 



These investigations established many of the regularities of crossing 

 over, but did not elucidate the mechanism of the process. It now seems 

 likely that the mechanism will require analysis at the molecular level. 

 Research in that direction will be discussed in Chapter 7. Here we 

 shall consider what has been learned about crossing over from cyto- 

 genetic correlations. 



Historically, this area of research began with the demonstration that 

 linkage groups established by genetic mapping could be correlated with 

 the cytologically observable chromosomes. Bridges first showed that a 

 peculiar pattern of genetic segregation was the result of an aberrant type 

 of chromosome segregation called nondisjunction. The regularities of 

 sex-linked inheritance in Drosophila had been established previously by 

 Morgan. As shown in Figure 4.1, when a white-eyed female is crossed 

 with a red-eyed male, all Fj female progeny are red-eyed and all males 

 are white-eyed. In the Fg produced by brother-sister matings, both 



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