Page:Popular Science Monthly Volume 85.djvu/441

Rh crossed with varieties having white glumes the ratio of 3 white to 1 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 1 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 generation the monohybrid ratio of 3 red: 1 white, but three strains gave the dihybrid ratio of 15 : 1 and two gave the trihybrid ratio of 63 : 1. 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 three factors; notable departures from expected ratios may thus be explained.

But the most serious objections which can be presented against the universality of the Mendelian doctrine are found in phenomena of "blending" inheritance. In some instances contrasting characters of parents appear to blend in offspring and even in the F2 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 color of the mulatto, which is intermediate between the white parent and the black one, and even in the F2 and in subsequent generations mulattoes do not usually, if ever, produce pure white or pure black children, though the children of mulattoes show considerable variation in color. Here there is an apparent failure of the Mendelian principle of segregation.

But white skin is not really white nor is black skin ever perfectly black. Davenport has shown that there is a mixture of black, yellow and red pigment in both white and black skins, though the amount of each of these pigments varies greatly in negroes and whites. A white person may have a skin color composed of black (b) 8 per cent., yellow (y) 9 per cent., red (r) 50 per cent., and absence of pigment or white (w) 33 per cent. On the other hand a very black negro may have b 68 per cent., y 2 per cent., r 26 per cent., w 4 per cent. The nine children of two mulattoes, the father having 13 per cent, of black and the mother 45 per cent., ranged all the way from 46 per cent, to 6 per cent, of black—the latter so far as skin color is concerned being virtually white. On the other hand, where both parents have about the same degree of pigmentation the children are more nearly uniform in color; thus seven children of two mulattoes, the father having 36 per cent, and the mother 30 per cent, of black, ranged only from 27 per cent, of 39 per cent, of black.

Such variations in color in the F2 and in subsequent generations is