HEREDITY 



283 



Fi flies all have gray bodies and long wings. Since the reciprocal 

 cross gives the same results, these characters are not sex-linked. 

 Gray body is dominant to black, and long to vestigial wings. 

 If a heterozygous gray-long female is mated with a recessive 

 (homozygous) black-vestigial male, the following four classes of 

 flies are obtained: 



83 per cent 



17 per cent 



1. Black body, vestigial wings, males and females) 



2. Gray body, long wings, males and females / 



3. Black body, long wings, males and females ) 



4. Gray body, vestigial wings, males and females J 



The first two classes show the characters in their original com- 

 binations which held for 83 per cent of the progeny. The new 

 combinations, present in classes 

 3 and 4 and representing 17 per |y 

 cent of the total, are the result 

 of crossing over of genes in the 

 formation of the germ cells of 

 the heterozygous female parent. 

 That crossing over takes place 

 only in the female is shown by 

 the fact that if the reciprocal 

 cross is made (black-vestigial 

 female X heterozygous gray- 

 long male), only the first two 

 classes are obtained, viz., gray- 

 long and black-vestigial. Limi- 

 tation of crossing over to the female, though true of Drosophila, 

 does not hold generally, since many forms exhibit crossing over 

 in both sexes. 



Chromosome Maps. — The percentage of crossovers differs in 

 the two examples just cited. Other characters whether sex- 

 linked or not show different crossover percentages, but the same 

 percentage under the same conditions is obtained from the same 

 pair of characters. This has been interpreted to mean that the 

 genes for different characters occupy different positions along the 

 length of the chromosome and that the nearer together the genes 

 are, the less the chance of their being separated in the crossover 

 process. The percentage of crossovers obtained in any given 

 case then becomes a measure of the distance separating the genes 



& 



1 2 3 



Fig. 170. — Diagram illustrating recom- 

 bination of characters in crossing over. 



