CHAPTER XI 



SEX {continued') 



The cases which we have considered in the last 

 chapter belong to a group in which the peculiarities 

 of inheritance are most easily explained by supposing 

 that the female is heterozygous for some factor that 

 is not found in the male. Femaleness is an addi- 

 tional character superposed upon a basis of maleness, 

 and as we imagine that there is a separate factor 

 for each the full constitutional formula for a female 

 is F/MM, and for a male ffMM. Both sexes are 

 homozygous for the male element, and the difference 

 between them is due to the presence or absence of 

 the female element F. 



There are, however, other cases for which the 

 explanation will not suffice, but can be best inter- 

 preted on the view that the male is heterozygous 

 for a factor which is not found in the female. Such 

 a case is that recently described by Morgan in 

 America for the pomace fly {Drosophila ampelophild). 

 Normally this little insect has a red eye, but white- 

 eyed individuals are known to occur as rare sports. 

 Red eye is dominant to white. In their relation to 

 sex the eye colours of the pomace fly are inherited 



