238 HEREDITY AND ENVIRONMENT 



When a pure recessive is crossed with a 

 mixed dominant-recessive 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 pro- 

 portion 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 combi- 

 nations of these are as follows: 



S germ cells W \/ W 



$ germ cells B ^ 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 

 differ in many characters, and this leads to a 



