BIOLOGY IN HUMAN AFFAIRS 



possess two sets of the units of inheritance, one from the 

 father and one from the mother; and in pure-breeding types 

 each set was assumed to be alike. The reproductive cells, 

 naturally, were supposed to contain only one set of genes, 

 which functioned as if they were wholly uninfluenced by 

 association with the companions forming the other set, 

 during the succession of cell divisions previous to the 

 maturation of the germ cells. This being the case, when a 

 germ cell containing a dominant gene unites with one 

 containing a recessive gene, the body cells of the hybrid 

 will each contain a gene D and a gene K; but when the germ 

 cells of the hybrid are formed, the dominant gene D will 

 pass into one, while the recessive gene R will pass into 

 another. 



The second principle of inheritance discovered through 

 the pea experiments is known as the Law of Independent 

 Assortment. It states that the behavior of each homologous 

 pair of genes in a hybrid, and presumably of each homolo- 

 gous pair of genes in a pure stock also, is independent of 

 the behavior of all other pairs of genes. 



The experiment which led to the enunciation of this law 

 was a cross between a pea plant having round and yellow 

 seeds with one having wrinkled and green seeds. The 

 hybrid individuals were round and yellow, for these were 

 the dominant characteristics. But the progeny of the 

 hybrids were not solely of the grandparental types. 

 Instead, the two contrasting pairs of characters appeared 

 in all of the combinations possible. There were round and 

 yellow peas, round and green peas, wrinkled and yellow 

 peas, and wrinkled and green peas; and the proportions of 

 these types invariably approached the ratio 9:3:3:1. Plainly 

 such a result could only be obtained if the germ cells of 

 the hybrids had received one member of each pair of genes 

 by independent segregation. This would mean, if the two 

 genetic differences be represented by Aa and Bby that the 



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