Phenomena of Inheritance 109 



the parental types reappear in the F 2 generation as in the cross 

 between red and white "four o'clocks." Sometimes departures 

 from the 3 to i ratio are caused by the fact that two or more fac- 

 tors of the same sort are involved in the production of a single 

 character. Nilsson-Ehle found that when oats with black glumes 

 were crossed with varieties having white glumes the ratio of 3 

 white to i black was usually found in the second generation ; but 

 one variety of black oats when crossed with white gave in the 

 second generation approximately 15 blacks to i white which is 

 the dihybrid ratio. From this and other evidence he concludes 

 that in this variety of oats two hereditarily separable factors are 

 involved in the production of black. In crosses between red- 

 grained and white-grained wheat he usually got in the second gen- 

 eration the monohybrid ratio of 3 red to i white, but three strains 

 gave the dihybrid ratio of 15 to i and two gave the trihybrid ratio 

 of 63 to i and in subsequent generations each of these strains 

 continued to give the same ratios. Consequently he concludes 

 that while the red color of wheat grains is usually due to one 

 factor for red, it may in some cases be due to two or even to 

 three factors; notable departures from expected ratios may thus 

 be explained. Other departures from regular Mendelian ratios 

 are caused by the early death of certain gametes or zygotes due 

 to lethal factors, as explained on page 105. 



Blending of Color in Mulatto. Perhaps the most serious objec- 

 tions which can be presented against the universality of the Men- 

 delian doctrine are found in phenomena of "blending" inheritance. 

 In some instances contrasting characters of parents appear to blend 

 in offspring and even in the F 2 and in subsequent generations the 

 descendants remain more or less intermediate between the parents. 

 One of the best known illustrations of this is found in the skin col- 

 or of the mulatto which is intermediate between the white parent 

 and the black one, and even in the F 2 and in subsequent generations 

 mulattoes do not usually produce pure white or pure black chil- 

 dren, though the children of mulattoes show considerable variation 



