204 Basis of Sex Determination 



results of his crossings indicated that in this form there 

 are two types of eggs and one type of spermatozoa/ 



These observations on sex-Hnked heredity confirm 

 the idea that the sex chromosomes determine the sex. 

 The most extensive and conclusive experiments along 

 this line are those by Morgan on the fruit fly Drosophila. 

 In this form there are two kinds of spermatozoa and 

 one kind of eggs ; the egg has one X chromosome, while 

 one-half of the spermatozoa has an X the other a Y 

 chromosome ; the entrance of the latter into an egg gives 

 rise to a male, of the former to a female. 



While the eyes of the wild fruit fly Drosophila ampelo- 

 phila are red, Morgan^ noticed in one of his cultures 

 a male that had white eyes. This white-eyed male was 

 mated to a red-eyed female. The offspring, the Fj 

 generation, were all red eyed, males as well as females. 

 These were inbred and now gave in the F^ generation 

 the following three types of offspring : 



(i) 50 per cent, females, all with red eyes. 



, N ^ , i ( 2S per cent, with red eyes. 



(2) 50 per cent, males < ^ ^ . , , . 



( 25 per cent, with white eyes. 



The character white eye was therefore transmitted 

 only to half the grandsons ; it was a sex-linked charac- 

 ter. It is known from a study of the pedigrees of 

 colour-blind individuals that if the corresponding ex- 



'Doncaster, L., The Determination of Sex. Cambridge, 19 14. 

 » Morgan, T. H., Heredity and Sex. New York, 1913. 



