Heredity of some human physical characteristics

Abstract

It has already been shown 1 that dark brown eye color is a Mendelian dominant to gray and gray to blue. It now appears that the form of the hair also follows Mendel's law. Thus, if the three grades of straight, wavy and curly be recognized then straight hair is recessive to curly. But “wavy” is peculiar in that it appears to be a hybrid or heterozygous condition indicating the presence of both straight and curly germ cells. It follows from the foregoing facts that two blue eyed, straight haired parents will have only children like themselves; but two brown eyed, curly haired parents may have a variety of these characters in their families. Again, the records collected by Mrs. Davenport and myself show that two flaxen haired parents have flaxen haired children and probably only such; two parents with light brown hair have children with hair of that color and lighter, but not darker; while children of two parents each with dark brown or black hair produce children with all of the varieties of hair color. Also two golden haired parents have only golden haired children and none with red hair. Consequently two blue eyed, flaxen or golden and straight haired parents will have only children like themselves. And the reason is that the germ cells as well as the somatic cells of such parents lack the dark and curly characters.

In the foregoing cases of heredity the more advanced condition is dominant over the less advanced. This is often, if not usually, true and this rule enables us to predict the outcome of untried matings between parents having dissimilar characteristics. The more highly developed condition dominates over the less highly developed; and in the extreme case the presence of a character dominates over its entire absence.