70 



THE CHROMOSOMES 



definite regions. Thus in the female * Drosophila 

 melanogaster an examination of the genetic map 

 reveals the fact that the genes in Chromosomes II 



Chiasma 



frequency 



'50 



a No Localization 



WO Units 



-\ 1 l liMl|?* l l t k I 



I Chiasma 

 [frequency 

 =20 



b Partial Localization {Drosophila type) 

 ■50an/ts 



■ ■ ■ t*: 



*^' Chiasma 



frequency = 10 



Single Chiasma 

 near spindle attachment 



c Complete Localization {Mecostethus type) 

 Fig. 17. — Diagrams of the relationship between chiasma- 

 localization and genetic maps. Explanation in text. 



and III are crowded in the central region round tha 

 spindle attachment ; there is also a lesser degree of 

 crowding at the ends, the genes between the ends and 

 the central region being much more sparsely scattered 

 (Fig. 17). By a study of chromosome breaks it has 



* In Drosophila genetic maps only exist for the female sex, 

 since no crossing-over normally takes place in the male 

 {vide infra). 



