DI-HYBRID CROSSES 185 



So far we have seen that both a pair of structural 

 characters and a pair of colour characters can 

 4 Mendelize,' according to the phrase coined by the 

 Germans that is to say, the germinal representa- 

 tives of such pairs of characters remain perfectly 

 distinct in the hybrid plant, and separate completely 

 at the formation of its gametes, in such a way that 

 an equal number of gametes arises containing either 

 character. 



The members of a pair of characters which behave 

 in this way on crossing are called allelomorphs. When 

 a pair of gametes fuse together in the process of fer- 

 tilization the resulting cell is known as a zygote. A 

 zygote formed by the conjunction of two like gametes 

 is called a homozygote. When the gametes contain 

 opposite members of a pair of allelomorphs the result 

 is called a hetero zygote. The same terms may also be 

 applied to the adult multicellular organisms into which 

 these fertilized egg-cells develop. 



We have still to consider what happens when parents 

 are crossed which differ in more than one pair of allelo- 

 morphs. The actual result is as follows : 



Suppose a smooth yellow type of maize to be crossed 

 with a wrinkled white variety, both smoothness and 

 yellowness being dominant. The grains produced in 

 F! are therefore yellow and smooth. Let the F x plants, 

 arising from the smooth yellow heterozygote grains, 

 be crossed with the wrinkled white parent, which 

 is recessive in respect of both these characters. 

 In this way the true nature of every germ cell 

 produced by the heterozygote will be able to manifest 



