244 HEREDITY AND ENVIRONMENT 



When a pure recessive is crossed with a 

 mixed dominant-recessive (Fig. 51, d) another 

 typical ratio results. Thus if a wrinkled- 

 seeded variety of pea is crossed with a hybrid 

 between a round- and wrinkled-seeded one, 

 round-seeded and wrinkled-seeded peas are 

 produced in the proportion of 1 : 1. This is 

 due to the fact that the hybrid produces two 

 kinds of germ cells, the pure-bred but one, and 

 the possible combinations of these are as 

 follows : 



$ germ cells W \ , W 



$ germ cells R ' x W 

 Possible combinations 2 RW: 2 WW. 



This ratio of 1 : 1 is approximately the ratio 

 of the two sexes in many animals and plants, 

 and there is good reason to believe that sex is 

 a Mendelian character of this sort, in which 

 one parent is heterozygous for sex and the 

 other homozygous (Fig. 51, d). 



2. Results of Crossings where there is more 

 than one Pair of Contrasting Characters. — It 

 rarely happens that two individuals differ in a 

 single character only; more frequently they 



