300 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, AA 
and aa, are crossed together. 
The external character or visible appearance of the 
heterozygote Aa, 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 AA. In other cases 
the heterozygote Aa is different in appearance from 
either homozygote AA or aa. Sometimes Aa is inter- 
mediate between AA 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 otf 
a single pair of allelomorphs, we find that all 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 
