112 ORIGIN OF SOMATIC 



while half the females are homozygous and normal, 

 and the other half heterozygous and normal. 



T. H. Morgan 1 has observed a number of cases 

 of sex-linked inheritance in the mutations which 

 occurred in his cultures of Drosophila. The eye of 

 the wild original fly is red, one of the mutants has a 

 white eye, i.e. the red colour and its factor are absent. 

 When a white-eyed male is mated to a red-eyed 

 female all the offspring have red eyes. If these are 

 bred inter se, there are, as in ordinary Mendelian 

 cases, three red-eyed to one white-eyed in the F 2 

 generation, but white eyes occur only in the males, 

 in other words half the males are white-eyed. On 

 the other hand, when a white-eyed female is mated to 

 a red-eyed male all the daughters have red eyes, and 

 all the sons white eyes. This has been termed criss- 

 cross inheritance. If these are bred together the result 

 in F 2 is equal numbers of red-eyed and white-eyed 

 females, and equal numbers of red-eyed and white- 

 eyed males. The ration of dominant to recessive is 2 

 to 2 instead of the usual Mendelian ration of 3 to 1. 



According to Morgan the interpretation is as 

 follows : In the nucleus of the female gametocytes 

 there are two X chromosomes related to sex, in those 

 of the male there is one X chromosome and one Y 

 chromosome of slightly different shape. The factor 

 for red eye occurs in the sex-chromosomes, that is to 

 say, according to this theory, the sex-chromosome 

 does not merely determine sex but carries other 

 factors as well, and this fact is the explanation of 

 sex-linked inheritance. The factor for red eye then 

 is present in both X chromosomes of the wild 

 female, absent from both X and Y chromosomes 



1 A Critique of the Theory of Evolution. 



