Page:Popular Science Monthly Volume 85.djvu/442

438 exactly what one would expect in a Mendelian character in which more than one factor is involved, as, for example, in the case of the color of the sweet peas shown in Fig. 59. Davenport, who has made an extended study of this case concludes that "there are two double factors (AA BB) for black pigmentation in the full-blooded negro of the west coast of Africa and these are separably inheritable." These factors are lacking in white persons (this being indicated by aa bb). Since the germ cells carry only single factors and not double ones, the cross between negro and white would have only one set of these factors for black color, as shown by the formula $$AB \times ab = ABab$$; hence the color of the F1 generation is intermediate between that of the two parents. In the F2 generation there should be a variety of colors ranging all the way from white to black, though pure white or pure black would be expected in only a small proportion of the offspring. As a matter of fact it is known that the children of mulattoes vary considerably in

 the skulls of two parents are shown in 1 and 3, of their intermediate offspring in 2. (From Castle.)

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 several factors for skin color are involved.

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