XI SEX 119 



The attempt to discover a simple explanation of the 

 nature of sex has led us to assume that certain combina- 

 tions between gametes are incapable of giving rise to zy- 

 gotes which can develop further. In the various cases 

 hitherto considered there is no reason to suppose that 

 anything of the sort occurs, or that the different gametes 

 are otherwise than completely fertile one with another. 

 One peculiar case, however, has been known for several 

 years in which some of the gametes are apparently incapa- 

 ble of uniting to produce offspring. Yellow in the mouse 

 is dominant to agouti, but hitherto a homozygous yellow 

 has never been met with. The yellows from families 

 where only yellows and agoutis occur produce, when bred 

 together, yellows and agoutis in the ratio 2:1. If it were 

 an ordinary Mendelian case the ratio should be 3 : i, and 

 one out of every three yellows so bred shoxild be homo- 

 zygous and give only yellows when crossed with agouti. 

 But Cuenot and others have shown that all of the yellows 

 are heterozygous, and when crossed with agoutis give 

 both yellows and agoutis. We are led, therefore, to sup- 

 pose that an ovum carrying the yellow factor is unpro- 

 ductive if fertilised by a spermatozoon which also bears 

 this factor. In this way alone does it seem possible to 

 explain the deficiency of yellows and the absence of homo- 

 zygous ones in the families arising from the mating of 

 yellows together. At present, however, it remains the 

 only definite instance among animals in which we have 



