MENDEL’S LAW 301 
allelomorphs is found in almost all cases to be perfectly 
complete. , 
This complete segregation of the two allelomorphs in 
equal numbers of the germ-cells of a heterozygote 
constitutes the first and most important section of 
the generalization known as Mendel’s law. 
The second part of the law refers to the fact that, 
as a general rule, separate pairs of allelomorphs segre- 
gate quite independently of one another. To this rule 
a few exceptions have been recorded in cases where 
apparently distinct pairs of determining factors behave 
in segregation like a single pair of allelomorphs. In 
such cases we regard the members of the distinct pairs 
of allelomorphs as being coupled together, although no 
serious attempt has yet been made to picture the way 
in which this coupling comes about. In other cases 
the coupling is only partial ; in others, again, there is 
repulsion between allelomorphs belonging to distinct 
pairs. 
The fact that in the great majority of cases separate 
pairs of allelomorphs segregate independently of one 
another leads to the possibility of new combinations of 
the parental characters being formed in the germ-cells 
of the cross-bred individuals ; in fact, this must always 
happen when the parent types differ in more than 
one pair of segregable characters. When two similar 
germ-cells, each bearing the same new combination of 
allelomorphs, meet together in fertilization, the result 
is a new zygotic combination which is a pure type in 
respect of the characters concerned, and henceforth 
breeds true. Thus if AB. AB is crossed with ab. ab 
