no Heredity and Environment 



germ cells carry only single factors and not double ones the cross 

 between negro and white would have only one set of these fac- 

 tors for black color, as shown by the formula AB x ab = ABab ; 

 hence the color of the F 1 generation is intermediate between that 

 of the two parents. In the F 2 generation there should be a variety 

 of colors ranging all the way from white to black (Fig 34), though 

 pure white (ab ab) or pure black AB AB) would be expected in 

 only i out of 16 of the offspring. As a matter of fact it is known 

 that the children of mulattoes vary considerably in color, and in 

 some cases a child may be darker or lighter than either parent, 

 which would indicate that segregation does actually occur. It is 

 very probable that this classical case of "blending" inheritance is 

 really Mendelian inheritance in which two or more factors for 

 skin color are involved. 



Blending of Size. Similar "blending" inheritance is found in 

 certain other cases where the parents differ in form or size. Thus 

 Castle found that when long-eared rabbits were crossed with 

 short-eared ones the offspring have ears of intermediate length, 

 and in all subsequent generations the ear length remained inter- 

 mediate between that of the parents. He found the same thing 

 true of length and breadth of the skull (Fig. 35) and of the 

 size of other portions of the skeleton, and he concluded that 

 such quantitative characters are not inherited in Mendelian 

 fashion. 



More recently MacDowell, working on the inheritance of size 

 in rabbits, concludes that this character as well as other quan- 

 titative differences between parents which appear to blend in the 

 offspring, such as Castle's case of ear length in rabbits, is not due 

 to a single factor, as in the case of Mendel's tall and dwarf peas, 

 but to several factors. Consequently in the formation of the germ 

 cells there is not a clean segregation of all the factors for tallness 

 or large size or long ears in half the germ cells and their total 

 absence in the other half of those cells, but some of these factors 

 go into certain cells and others into others, as in the case of dihy- 



