266 CONCLUDING CHAPTER 



display. The demonstration that there exist definite 

 and separable unit characters of this kind is the first 

 great debt that science owes to Mendel. 



Up to the present our certain knowledge of the 

 Mendelian behaviour of unit characters has been con- 

 fined to cases of cross-breeding. In the simplest case 

 which we have to consider, two homozygote forms, A A 

 and aa, are crossed together. 



The external character or visible appearance of the 

 heterozygote A a, produced in this manner, differs in 

 different cases. In the commonest case A represents 

 the dominant allelomorph, and in this case the appear- 

 ance of the heterozygote Aa is practically indistinguish- 

 able from that of the homozygote A A. In other cases 

 the heterozygote A a is different in appearance from 

 either homozygote AA or aa. Sometimes Aa is inter- 

 mediate between A A and aa, in other cases it is to all 

 appearances totally distinct from either. 



So much for the external appearance of homozygote 

 and heterozygote forms. In the production of the 

 gametes, or germ-cells, we arrive once more at the 

 simplest possible form of hereditary constitution, for 

 we believe each feature in the body to be represented 

 in the germ-cells by a single determining factor only. 

 Still confining our attention to the representatives of 

 a single pair of allelomorphs, we find that the germ-cells 

 of a homozygote contain only A or only a, as the case 

 may be. But in the case of the germ-cells derived 

 from a heterozygote, A and a are represented in an 

 equal number of the gametes produced by the same 

 individual. And the separation between the two 



