Hair-Shape.—Again, straight and curly hair seem to be distinct inheritable characters. Curly is incompletely dominant to straight, the simplex condition yielding wavy hair.
Not to enter into details of the matings, statistics gathered by Mr. and Mrs. Davenport show that, two flaxen-haired parents have flaxen-haired children; two golden-haired parents have only golden-haired children; two parents with light brown hair have children with hair of that color or lighter, but never darker; two parents each with dark brown or black hair may have children with all the varieties of hair-color. Summing together a series of recessives Davenport points out that two blue-eyed, flaxen or golden and straight-haired parents will have only children like themselves.
Fig. 21
Diagram showing descent of brachydactyly through five generations; black symbols indicate affected individuals; ♂, male; ♀, female (after Farabee).
Irregularities.—If a dominant trait or defect depends on more than a single factor, as is sometimes the case, or if it is modified by sex or other conditions, as is true of certain characters, some of which, such as color-blindness, have already been examined, then we shall find some apparently non-affected individuals having affected offspring. Certain diseases, for example, are generally transmitted by affected members of the family to their children in the expected Mendelian ratio for a dominant, yet an occasional skip of a generation may appear in which an apparently perfectly normal individual transmits to his children what, except for the omission in his own case, appears to be an ordinary dominant character. This occasional lapse in the appearance of a character when theoretically it should appear is doubtless due in some instances to the fact that what is really inherited is a tendency, and although this is present in the apparently normal individual, for some reason the condition itself has not appeared. This might especially be true in the case of a disease which does not manifest itself until late in life. In other cases there are undoubtedly complicating accessory conditions which modify the behavior of the trait somewhat.
OTHER CASES OF DOMINANCE IN MAN
Among other normal characters in man, as far as available evidence goes, dark skin is dominant to light skin; normally pigmented condition to albino; and nervous temperament to phlegmatic.
Digital Malformations.—An interesting and easily followed defect is a condition known as brachydactylism, in which the digits are shortened because of the absence or rudimentary condition of one segment. The fingers, therefore, appear to be only two-jointed like the thumb. Several families showing this defect have been charted and it appears to behave as a typical dominant. In looking over such a chart (Fig. 21, [p. 106]) one is struck by the fact that only half of the children from most of the matings show the defect, but when we recall that the affected parent, after the first generation, probably carried the condition in only the simplex form and married a normal individual, such a result is just what would be expected (see formula 2).
Polydactylism (Figs. 22, 23, [pp. 109], [110]) is a condition in which there are extra digits on hands or feet. The character, with apparently slight exceptions in a few records, behaves as a typical dominant. Among other digital defects which are inherited as a dominant is a condition known as syndactylism (Fig. 24, [p. 111]), in which two or more digits are fused side by side. For an example of syndactyly which seems to be in the class of sex-linked characters, see Fig. 15, [p. 65].