XI SEX US 



the nature of sex has led us to assume that certain 

 combinations between gametes are incapable of 

 giving rise to zygotes 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 incapable 

 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 pro- 

 duce, when bred together, yellows and agoutis in 

 the ratio 2:1. If it were an ordinary Mendelian 

 case the ratio should be 3:1, and one out of every 

 three yellows so bred should be homozygous and 

 give only yellows when crossed with agouti. But 

 Cu^not and others have shown that all of the yellows 

 are heterozygous, and when crossed with agoutis 

 give both yellows and agoutis. We are led, there- 

 fore, to suppose that an ovum carrying the yellow 

 factor is unproductive 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 homozygous 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 grounds 

 for assuming that anything in the nature of unpro- 

 ductive fertilisation takes place.^ 



1 For the most recent discussion of this peculiar case the reader is 

 referred to Professor Castle's paper in Science, December 16, 1910. 



