CHAPTER IV 



THE DETERMINATION OF DOMINANCE; HETEROZY- 

 GOUS CHARACTERS AND THEIR FIXATION; ATA- 

 VISM OR REVISION 



WE have noticed that when a black 

 guinea-pig of pure race is mated 

 with a white one, only black off- 

 spring are produced; and that when rough- 

 coated guinea-pigs are mated with smooth- 

 coated ones, only rough-coated young are pro- 

 duced; and that when short-haired guinea- 

 pigs are mated with long-haired ones, only 

 short-haired young are produced. The char- 

 acter which in each case is seen in the young 

 we call dominant, that which is unseen we 

 call recessive. Thus black is dominant over 

 white, rough coat over smooth coat, and short 

 coat over long coat. 



A question which has given much concern to 

 students of heredity is this, upon what does 



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