MENDEL'S LAW 91 



sives are always homozygous. For they do not contain the 

 dominant character; otherwise they would show it. 



It will be observed that, in the cross of colored with albino 

 guinea-pigs, color and albinism behave as a pair of alternative 

 units which may meet in fertilization but separate again at 

 the formation of gametes. 



Mendel's law as illustrated 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 segregation 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. 32 and 33) 

 is dominant 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 generation 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 fur (Figs. 36, 

 and 37) is recessive in crosses with normal short hair. All 

 the immediate offspring of such a cross are short haired, but 

 in the next generation long hair reappears in approxuiiately 

 one-fourth of the offspring. 



In cattle, the polled or hornless condition is dominant over 

 the normal horned condition; in man, two jointed fingers 

 and toes are dominant over normal three- join ted ones. 



In each of the cases thus far considered a single unit- 

 character is concerned. Crosses in such cases involve no 

 necessary change in the race, but only the continuance within 

 it of two sharply alternative conditions. But the result is 

 quite different when parents are crossed which differ simul- 

 taneously in two or more independent unit-characters. Cross- 

 ing then becomes an active agency for the production of new 

 varieties. 



In discussing the crosses now to be described, it will be 

 convenient to refer to the various generations in more pre- 



