YELLOW MICE 209 



earlier hypothesis there was supposed to be some 

 obstacle in the way of the fertilization of a yellow- 

 bearing egg by a spermatozoon bearing the same 

 character. In this case, since spermatozoa are in 

 great numerical excess as compared with eggs, it is 

 still possible for every Y-bearing egg to be fertilized 

 by a spermatozoon lacking Y, as well as half the non- 

 yellow eggs by Y-bearing sperms, giving a ratio of 

 3:1 in Fz from yellow by yellow. On the other 

 hand, it is possible that pure yellow zygotes are formed 

 in fertilization, but for some unknown cause are incap- 

 able of development. In this case a third of the yellow 

 progeny would be wanting, and the expected ratio would 

 be 2 : i. Cu6not's figures led to the belief that the first 

 hypothesis was the correct one, but in further experi- 

 ments by Castle and by Miss Durham the ratio was 

 found to approach 2 : i in a majority of cases.* 



We have still to describe a case in which two latent 

 factors, one derived from each parent, give rise, by 

 their simultaneous presence in the zygote produced, 

 to the appearance of an entirely new character. The 

 following example is the first one of the kind to be 

 completely elucidated, and is one of those studied by 

 Messrs. Bateson and Punnett and Miss Saunders. 



The white-flowered variety of sweet-pea known as 

 Emily Henderson was found to exist in two forms, 

 only to be distinguished from one another by the 

 shape of the pollen grains which they produced. In 

 one of the two the shape of the pollen is elliptical 



* [It is now definitely established that homozygous yellow zygotes 

 are formed, but die during embryonic life. Disintegrating embryos 

 have been found in the uterus.] 



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