PHENOMENA OF INHERITANCE 251 



(b) The principle of dominance. — When 

 contrasting unit characters are present in the 

 parents they do not as a rule blend in the 

 offspring, but one is dominant and usually ap- 

 pears fuUy developed, while the other is reces- 

 sive and temporarily drops out of sight. 



(c) The principle of segregation. — Every 

 individual germ cell is "pure" with respect to 

 any given unit character, even though it come 

 from an "impure" or hybrid parent. In the 

 germ cells of hybrids there is a separation of 

 the determiners of contrasting characters so 

 that different Idnds of germ cells are pro- 

 duced, each of which is pure with regard to 

 any given unit character. This is the principle 

 of segregation of unit characters, or of the 

 "purity" of the germ cells. Every sexually 

 produced individual is a double being, double 

 in every cell, one-half having been derived 

 from the male and the other half from the fe- 

 male sex cell. This double being, or zygote, 

 again becomes single in the formation of the 

 germ cells only once more to become double 

 when the germ cells unite in fertilization. 



