THE CHROMOSOME BASIS 



M P 



Fig. 9. — Maps showing the linear distribution of 13 

 genes and the centromere (cent) in the X-chromo- 

 some of Drosophila melanogaster. C is the map ob- 

 tained by arranging the genes according to the 

 frequencies of crossing-over (as judged by genetic 

 recombination) between them. M is the correspon- 

 ding map where the genes are spaced according 

 to the proportionate rates of occurrence of lethal 

 mutations between them. P is the cytological map 

 based on the positions of the genes in the polytene 

 chromosomes of the salivary glands, and S that 

 based on the positions of the genes in the mitotic 



chromosomes of the nerve ganglia cells. Only 7 of the genes have been localized 

 on the mitotic map. The large block of heterochromatin near the centromere is shown 

 black. It is hardly detectable in the genetic map, and is much smaller in the poly- 

 tene than in the mitotic chromosomes. 



cent 



47 



