MENDEL'S LAW 



303 



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 repul- 

 sion 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 



* [Prof. T. H. Morgan and his associates have brought forward 

 remarkable evidence, from their work on the Fruit-fly (Dr oso- 

 phila], that this coupling is due to the determining factors of the 

 coupled allelomorphs being borne by the same chromosome. 

 See pp. 269-271.] 



