36 TYPES OF MENDELIAN HEREDITY 
StmizaR Errects 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 F, offspring are inbred the F, 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 two 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 F, 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. 
