DI-HYBRID CROSSES 185 
So far we have seen that both a pair of structural 
characters and a pair of colour characters can 
‘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 heterozygote. 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, 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 
