Let us now consider some of the physical traits of man that follow the same law as brown eye color, traits that are clearly positive, and due to a definite determiner in the germ plasm.
Hair color is due either to a golden-brown pigment that looks black in masses, or else to a red pigment. The lighter tints differ from the darker by the absence of some pigment granules. If neither parent has the capacity of producing a large quantity of pigment granules in the hair, the children cannot have that capacity, that is, two flaxen-haired parents have only flaxen-haired children. But a dark-haired parent may be either simplex or duplex; and so two such parents _may_ produce children with light hair; but not more than one out of four. In general, the hair color of the children tends not to be darker than that of the darker parent. Skin pigment follows a similar rule. It is really one of the surprises of modern studies that skin pigment should be found to follow the ordinary law of heredity; it was commonly thought to blend. The inheritance of skin color is not dependent on race; two blonds never have brunette offspring, but brunettes may have blondes. The extreme case is that of albinos with no pigment in skin, hair, and iris. Two albinos have only albino children, but albinos may come from two pigmented parents.
Similarly, straight-haired parents lack curliness, and two such have only straight-haired children. Also two tall parents have only tall children. _Shortness_ is the trait: tallness is a negative character. Also when both parents lack stoutness (are slender), all children tend to lack it.
We may now consider briefly the inheritance of certain pathological or abnormal states, to see in how far the foregoing principles hold for them also. Sometimes the abnormal condition is positive, due to a new trait; but sometimes, on the contrary, the normal condition is the positive one and the trait is due to a defect.
Deaf-mutism is due to a defect; but the nature of the defect is different in different cases. Deaf-mutism is so varied that frequently two unrelated deaf mutes may have hearing children. But if the deaf-mute parents are cousins, the chances that the deafness is due to the _same_ unit defect are increased and all of the children will probably be deaf.