HEREDITY 



dominant character in crosses. It can scarcely 

 be explained by loss; the only alternative is 

 to consider it an acquisition, unless we choose 

 to consider it a modification of the normal 

 condition. 



Aside from the sorts of variations already 

 discussed, which consisted either in the loss or 

 modification of existing unit-characters or in 

 the gain of new ones, we must also recognize, 

 as a cause of permanent and heritable varia- 

 tion, changes in the potency of unit-characters, 

 i. e. their tendency to dominate in crosses. 



When a gamete containing a particular unit- 

 character unites with a gamete not containing 

 it, the zygote formed will ordinarily show the 

 character in question fully developed. This re- 

 sult following Mendel's terminology we call 

 dominance. But dominance is frequently im- 

 perfect and may even be reversed. The zygote 

 in which a character is doubly represented fre- 

 quently develops the character more fully than 

 the zygote in which it is represented but once. 

 If a black guinea-pig is crossed with a yellow 

 one the offspring are black, but oftentimes of 

 a slightly yellowish shade. Likewise if black 



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