MENDELIAN LAW OF SEGREGATION 325 



descendant of the green seed coat stock, and its protoplasm is 

 probably different. Mendel made further experiments with his 

 peas to find out what happened in later generations. He made 

 three kinds of crosses: (1) he cross-pollinated a hybrid with the 

 dominant parental type ; (2) he crossed a hybrid with the recessive 

 parental type ; and (3) he crossed a hybrid with a hybrid. 



Consider the third experiment first. When hybrids were self 

 pollinated, their seeds always produced species of both ancestral 

 types. For example, hybrid peas with yellow seed coats produced 

 seeds with both yellow and green coats. The hybrids must have 

 carried greenness even though it did not show. There was a split- 

 ting up, so to speak, of the combined inheritance into its two 

 components, yellowness and greenness in the seed, or into domi- 

 nance and recessiveness. This sorting out of members of a pair of 

 factors is called segregation. This has been found to occur not only 

 in hybrids but also when pure-breeding varieties are crossed. When 

 large numbers of offspring of hybrids are considered and tabulated, 

 the segregation is always in the proportion of one recessive to three 

 dominants, of which one is pure and two are hybrids. 



I P 



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Black dominates red in cattle. Name the three generations shown in the diagram. Note 

 also on the sides of the diagram the result of what the animal breeder calls a back cross ; i.e., 

 hybrid with black and hybrid with red. 



