276 FOUNDATIONS OF BIOLOGY 



the pure dwarfs (extracted recessives) do the same all the 

 germ cells of one bear S and those of the other, s. The plants 

 are, as we say, HOMOZYGOUS with respect to the characters in 

 question. It is also, clear why the hybrids give rise to 

 hybrids and extracted dominants and recessives an equal 

 proportion of the germ cells bear S and s. The plants are 



HETEROZYGOUS. 



The real difference then between the F 2 hybrids (Ss) and 

 the extracted dominants (SS) is that the former are heterozy- 

 gous and the latter are homozygous. In order to tell which is 

 which, since they are phenotypically the same, it is necessary 

 to breed them. When self-fertilization can be practiced, as 

 in the case of most plants, we get the result directly; that is 

 an individual's progeny are either all dominants or dominants 

 and recessives in 3 : 1 ratio, and thus the garnet ic constitution 

 of the parent is immediately known. However, in the case 

 of animals, where self-fertilization is impossible, the deter- 

 mination can be made by mating the dominants with reces- 

 sives, for a homozygous dominant then will give all dominants 

 while a heterozygous dominant will give half dominants and 

 half recessives. Thus: 



Gametes = D\ /D D x /d 



I \/ I 



Gametes d x x d d x x d 



Possible zygotes = 100% Dd 50% Dd+50% dd 



So far we have considered the inheritance of one pair of 

 alternative characters the resultant of a pair of genes 

 termed ALLELOMORPHS but if the reader has grasped the 

 principles involved, we may pass rapidly over cases where 

 two, three, or more pairs are concerned; that is DIHYBRIDS, 



TRIHYBRIDS, and POLYHYBRIDS. 



2. Dihybrids 

 Mendel found the solution to heredity in dihybrids by 



