xxxii MUTATION, MENDELISM, ETC. 



all those of the other G. In fertilization therefore G 

 must unite with Y and the fertilized ovum must contain 

 YG. The germ, containing the two different allelo- 

 morphs of such a pair of Mendelian characters, is called 

 a heterozygote. All the peas produced by these hetero- 

 zygotes were superficially indistinguishable from the pure 

 yellow parent. Yellowness of the cotyledon is therefore 

 dominant over greenness. In the development of the 

 germ-cells (gametes) from which the next generation will 

 arise it is obvious that the precursors of the Mendelian 

 characters in question can only be represented by half of 

 what they will become after fertilization. This might 

 be effected by each gamete coming to possess in the course 

 of its development both Y and G of half the normal size ; 

 or it might be brought about by each gamete possessing 

 either Y or G, but never both. Mendel proved that the 

 facts (in characters that follow his principle) are invariably 

 consistent with the latter alternative. For, assuming that 

 the allelomorphs Y and G are thus scattered singly 

 among the gametes produced by each individual, it follows 

 that half the gametes will contain each a single Y, and 

 half each a single G. Hence in the event of self-ferti- 

 lization, a gamete containing Y will on the average meet 

 and unite with one containing Y as often as one with G ; 

 and G similarly will unite with G as often as with Y. 

 A quarter of the individuals of the following generation 

 will therefore be developed from a fertilized germ con- 

 taining Y Y, hence called a homozygote ; another quarter 

 from the other homozygote GG, while half will arise from 

 the heterozygote YG. But yellowness being dominant, 

 the latter individuals will superficially resemble those 

 developed from YY, and the generation will produce 

 approximately three yellow peas to one green. And this 

 is precisely what Mendel obtained by experiment. He 



