36 TYPES OF MENDELIAN HEREDITY 



V 



Similar Effects Produced by Different Factors 



There are many cases in which characters that are 

 superficially alike are the product of different factors. 

 White color that characterizes so many domesticated 

 races of plants and animals is a case in point. There 

 are two pure breeding races of white flowered sweet 

 peas. When crossed, they produce colored flowers. 

 When the Fi offspring are inbred the F2 generation 

 consists of 9 reds to 7 whites. This 9 :7 ratio is a 

 special case of the 9 :3 :3 :1, in which the last three 

 classes are superficially alike. The explanation here 

 is that there are tw^o kinds of recessive whites that 

 have originated independently. On the chromo- 

 some hypothesis one white is due to mutation in one 

 chromosome and the other white to mutation in an- 

 other chromosome. When the races are crossed, 

 each race supplies that chromosome which contains 

 the normal factor of the white of the other race. 

 In the F2 generation any plant that contains at least 

 one of the normal chromosomes of both pairs will 

 not be white. There will be nine such cases. Any 

 plant that contains both of the white-producing chro- 

 mosomes of either pair will be white. There will be 

 seven such cases. 



There are also two pure races of white fowls that, 

 when crossed, give colored birds. Each white 

 behaves as a recessive to color. For instance, the 

 white silky crossed to a white dorking gives colored 

 birds. These inbred give 9 colored to 7 white 

 birds. 



