HEREDITY 



two sorts equally numerous, yellow and black. 

 From this result alone it is impossible to say 

 which is the dominant character, but breeding 

 tests of the offspring show that yellow is the 

 dominant character. For the black offspring 

 bred together produce only black offspring, 

 but the yellows bred together produce both 

 yellow offspring and black ones. The curious 

 feature of the case is that when yellows are 

 bred with each other no pure yellows, that is, 

 homozygous ones, are obtained. Hundreds of 

 yellow individuals have been tested, but the 

 invariable result has been that they are found 

 to be heterozygous; that is, they transmit yel- 

 low in half their gametes, but some other color 

 in the remaining gametes — it may be black, 

 or it may be brown, or it may be gray. The 

 black, brown, or gray animals obtained by 

 mating yellow with yellow mice never produce 

 yellow offspring if mated with each other. 

 This shows that they are genuine recessives 

 and do not contain the yellow character, which 

 is dominant. 



Now ordinary heterozygous dominants, when 

 mated with each other, produce three domi- 



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