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tary character determiner for its special eye color. And these 
representatives of heredity, you recall, are associated with the 
chromosomes. Chromosomes for the most part live in pairs. In 
the new pea plantlet, as in the new baby, one of the chromosome 
pair is a maternal gift, the other is paternal. Seven paternal and 
seven maternal chromosomes go into the making of a pea plant; 
fourteen canoes, so to speak, full of character-determiner pas- 
sengers—genes or factors, scientists call them. But only one pair 
carry the eye pigment determiners. If the pea plant breeds true 
to white-eye, one seat in each chromosome of a certain pair is oc- 
cupied by a white-eye determiner. If the pea plant is pure for 
brown-eye, the same seats are occupied by brown-eye determiners 
instead. But if the plant child had a parent from each of these 
varieties—say a brown-eye mother and white-eye father—the 
chromosome seats naturally would be occupied in one by a brown- 
eye, and in the other by a white-eye determiner. 
But the seeds of this hybrid would only show brown-eye char- 
acters, although they would have the hereditary determiners for 
both kinds. The brown pigment obscures whatever is respon- 
sible for the white-eye effect, so only brown is apparent. The 
expression of one determiner dominates the other. It has analo- 
gies with the expression—Mr. Brown and wife, rather than with 
Mr. and Mrs. Brown or H. W. and L. T. Brown. 
Through the combinations of chromosomes, operating largely 
under the law of chance during the reproductive period of the 
pea plant, the details of which are too long and too complicated 
to relate here, the grandchild generation with its 3 : 1 ratio is pro- 
duced. And thus we see why chromosomes might be called 
“tyrants.” 
From this brief glimpse into the more intimate phases of a pea 
plant’s life, it is easy to understand how hybrid pea plants have 
both a visible and an invisible hereditary make-up. 
The visible characters are the dominant members of the pair. 
The invisible are those occupying chromosome transportation 
seats, but seemingly inexpressive when their dominant mates are 
present. 
From peas to human beings seems a great leap. But just this 
happened in the history of the study of heredity. The first laws 
