XIII CERTAIN COMPLICATIONS 151 



ratio 3:1. Further, it was found that no homo- 

 zygous yellows were produced from the mating of 

 yellow X yellow, though, on simple Mendelian 

 expectation, one out of every three from such a 

 mating should have been of this nature. Why 

 were no homozygous yellows formed ? A suggestion 

 was made that there was some form of selective 

 fertilisation of such a nature that a " yellow " sperm 

 refused to fertilise a "yellow" ovum. It was, how- 

 ever, pointed out that "yellow" ova manifested no 

 antipathy to "non- yellow" sperm: that if this 

 hypothesis were true all of the " yellow "' ova would 

 be fertilised by " non-)ellow " sperms, of which there 

 was a superabundance. Hence all of the "yellow" 

 ova would give heterozygous yellows, while of the 

 " non-yellow " ova one half would give heterozygous 

 yellows, and the other half non-yellows. And as 

 "yellow" and "non-yellow" ova are produced in 

 equal numbers, the net result of a mating between 

 two yellows should be yellow : non-yellows in the 

 ratio of 3 : I. But the many hundreds of mice bred 

 from yellow x yellow had established the fact 

 beyond question that the proportion of yellow to 

 non-yellow was 2:1. The hypothesis naturally fell 

 to the ground, and it was then suggested that the 

 facts would be explained if it were supposed that 

 the " yellow " sperm could fertilise the " yellow " egg, 

 but that the resulting zygote, the homozygous yellow, 

 was incapable of developing sufficiently far to be 

 born. It was pointed out in support of this that 

 litters from two yellow^s were, on the average, smaller 

 than between )'ellow x non-yellow ; as indeed would 

 be expected if the homozygous yellows perished 



