420 EVOLUTION, GENETICS, AND EUGENICS 



dealt with in the previous chapter, and were seen to be sex-linked. 

 Now let us assume that by the proper breeding experiment we have a 

 yellow-bodied white-eyed female (call her "yellow white" for short). 

 Mate her with an ordinary normal male with gray body and red eyes 

 (call him "gray red"). All the daughters are gray red like the father 

 (each having inherited an X-chromosome from him), but the sons are 

 yellow- white like the mother (having inherited her X-chromosome). 

 The Y-chromosome does not affect the result at all. The daughters, 

 in addition to receiving an X-chromosome from the father, receive 

 another X-chromosome from the mother; so they have two different 

 X-chromosomes. They are all phenotypically gray red because gray 

 and red are dominant over yellow and white. 



Now it is easy to test the composition of these hybrid females by 

 breeding them with double-recessive (yellow white) males. The result 

 is as follows: 49.5 per cent of offspring are yellow white, 49.5 per cent 

 are gray red, 0.5 per cent are yellow red, and 0.5 per cent are gray 

 white. Such a result as this could hardly be anticipated. If there 

 were no linkage, but entirely independent assortment, as would be the 

 case were the two pairs of genes in different chromosomes, we should 

 expect the dihybrid ratio of nine gray reds, three gray whites, three 

 yellow reds, and one yellow white. If, on the other hand, chromo- 

 somes retain their integrity when they separate after synapsis, we 

 would expect 50 per cent gray reds and 50 per cent yellow whites. 

 Why do we find the anomalous ratios that we do? Obviously the 

 chromosomes that pair in synapsis do not always part company with- 

 out being affected by the chromosomal embrace, but instead they seem, 

 at least occasionally, to undergo a mutual exchange of equivalent genes. 

 Thus in one case in a hundred the gray and yellow body allelomorphic 

 genes are traded without also trading the red- and white-eye genes, 

 and in exactly the same number of cases the red and white genes are 

 traded between chromosomes without the yellow and gray being traded 

 along with them. This mutual and perfectly equitable exchange of 

 genes between homologous chromosomes is called crossing-over, and 

 the percentage of crossing-over between any two allelomorphs is the 

 same each time the same breeding experiment is repeated under the 

 same conditions. In the case we have just described, the crossing- 

 over percentage is very small, only i per cent. Let us try another pair 

 of sex-linked genes. 



A female with white eyes and miniature wings is bred to a male 

 with red eyes and long (or normal) wings. The miniature-wing gene 



