HEREDITY 



uals are always homozygous, as W W for ex- 

 ample. For they do not contain the dominant 

 character, otherwise they would show it. 



It will be observed that in the cross of black 

 with white guinea-pigs black and white behave 

 as units distinct and indestructible, which may 

 meet in fertilization but separate again at the 

 formation of gametes. Mendel's law as illus- 

 trated in this cross includes three principles: 

 (1) The existence of unit-characters, (2) domi- 

 nance, in cases where the parents differ in a 

 unit-character, and (3) segregation .of the units 

 contributed by the respective parents, this seg- 

 regation being found among the gametes formed 

 by the offspring. 



The principles of dominance and segregation 

 apply to the inheritance of many characteristics 

 in animals and plants. Thus in guinea-pigs a 

 rough or rosetted coat (Figs. 23 and 24) is domi- 

 nant over the ordinary smooth coat. If a pure 

 rough individual is crossed with a smooth one, 

 all the offspring are rough ; but in the next gen- 

 eration smooth coat reappears in one fourth of 

 the offspring, as a rule. Again, in guinea-pigs 

 and rabbits a long or angora condition of the 



38 



