HEREDITY 281 



1. Yellow wings and white eyes, male and females) 



2. Gray wings and red eyes, males and females ) per C 



3. Yellow wings and red eyes, males and females { 



4. Gray wings and white eyes, males and females / ^ er Ce 



In the first and second classes, which make up 99 per cent of the 

 total of the F 2 , the characters have remained in their original 

 combination or linkage, a result that should have applied to all 

 of the F 2 flies, had the genes for yellow wing, white eye and 

 gray wing, red eye remained in their original locations in their 

 respective chromosomes in all cases. But since in 1 per cent of 

 the total F 2 flies, represented by classes 3 and 4, new combina- 

 tions of characters are present, the conclusion would seem to be 

 that an exchange of parts of chromosomes must have taken 

 place, provided, of course, the original premise that chromosomes 

 are the bearers of these genes is true. In Fig. 169, which por- 

 trays these results, the first pair of flies represent the parent 

 generation, the next pair, the F h and the last two groups the 

 F 2 . It will be noted that in the formation of the gametes of the 

 Fi female the assumption is made that an interchange of genes 

 between the 2 X chromosomes has taken place, giving the possi- 

 bility of four kinds of eggs instead of the expected two. Such 

 a recombination of genes is known as crossing over and is thought 

 to take place as a result of a twisting of the X chromosomes about 

 each other during synapsis, with a subsequent reciprocal trans- 

 location or formation of composite chromosomes, as illustrated 

 in Fig. 170. The result is 2 chromosomes, equivalent in size 

 but differing from the originals in their gene content. If one 

 assumes that such a recombination of genes occurred in 1 per 

 cent of the cases, the experimental results are explained. Cross- 

 ing over does not take place in the male. To detect and deter- 

 mine the amount of crossing over between two characters one 

 must cross a heterozygous female with a recessive male. 



Crossing over of non-sex-linked characters has also been 

 studied in Drosophila. The genes for such characters are borne 

 on the autosomes and therefore presumably are inherited from 

 both parents regardless of sex. The wild Drosophila has a gray 

 body and long wings. Black body and vestigial wings occur as 

 mutations. If a female having a gray body and long wings is 

 mated with a male having a black body and vestigial wings, the 



