24 



PHYSICAL BASIS OF HEREDITY 



green pods, gray versus white-skinned peas, yellow versus 

 green cotyledons (seen through the skin of the seed), 

 and round versus wrinkled seeds (determined by the 

 nature of the cotyledons within the seed coat). 



The 3 : 1, i^o? ratio characteristic for a single pair of 

 characters is the expectation based on the chance meeting 

 of either one of two kinds of eggs with either one of two 

 kinds of pollen grains. In actual numbers this ratio is, 

 of course, not always exactly realized, but only approxi- 

 mately. For the seven pairs of characters that Men- 

 del examined, the F2 ratios were as follows : 



The following collective data for the inheritance of 

 color of the cotyledons of garden peas show that the 

 approximation to a 3 to 1 for the recessive character is 

 very close : 



