278 



EVOLUTION AND GENETICS 



found to include two kinds of peas. One of these three quarters 

 produced only smooth peas while the remainder produced a three 

 to one ratio like their parents. The wrinkled peas bred true. 



Mendel used a simple algebraic expression of his results. If 

 we let S represent the character of smoothness and s the alterna- 

 tive, wrinkled, then in the original smooth and wrinkled strains 

 only S and s respectively could be handed on to the next genera- 

 tion. In the hybrid there would be a possibility of either character 

 being handed down. The total possibilities to be derived from 

 either hybrid parent may therefore be represented by S+s, and 

 all possible combinations in the second hybrid generation by the 

 following computation : 



S+s 

 S+s 



SS+Ss 

 Ss+ss 



SS+2SS+SS 



Later Punnett suggested a simple checker-board plan commonly 

 called the Punnett square which expresses the same result (Fig. 

 159). 



In the case of height all offspring of the hybrid seeds were tall. 

 Here of course the development of the plant was necessary for 

 the expression of the character. The tall peas proved to be mixed, 

 however, for when self-fertilized they produced three tall offspring 

 to one short. The short plants bred true, one third of the tall 

 bred true, and the remainder again produced the three to one 

 proportion. All of the seven characters behaved in the same way. 

 Tables of Mendel's results have been prepared by several writers. 

 In the one which follows the ratios are computed on the basis of 

 unity for the smaller group. The numbers cited are those actually 

 secured in breeding experiments. 



