1. PP × PP = PP, or all duplex brown.
2. PP × Pp = PP and Pp, half duplex brown and half simplex brown.
If each parent has brown eyes but in simplex condition, then one-fourth of children will have blue or gray eyes; for example,
| Mating | Gametic couplings | Product | ||
| Pp × Pp | == | == | PP, Pp, pP, and pp, or one-fourth duplex brown, one-half simplex brown, and one-fourth blue or gray. |
If both parents have blue or gray eyes they can not have children with black or brown eyes, since the recessive condition in each parent means total absence of brown pigment in both.
If one pair is duplex brown and the other blue, then all children will have brown eyes but of simplex type.
If one parent has simplex brown eyes (type Pp) and one blue (pp) then one-half of the children will have brown eyes of simplex type and one-half will have blue eyes.
Occasional objections have been raised against the Mendelian interpretation of inheritance in eye-color, but the cases cited in evidence against the theory usually narrow down to those in which the color is so diluted as to render classification uncertain. For example, hazel eyes are sometimes called gray; they belong however to the melanic pigmented type although the brown pigment may be much diluted and occur mainly around the pupil. So-called green eyes are due to yellow pigment on a blue background. In the rare cases where in the same individual one eye is brown and the other blue, the individual should probably be rated as brown-eyed on the supposition that in the one eye the development of brown pigment has in some way been suppressed.
Hair-Color.—The inheritance of hair-color has also been the subject of considerable study and while the conditions are not so simple as in the case of eye-color, there is little doubt that it belongs in the Mendelian category. In human hair, color has as its foundation apparently two pigments, black and red. Absence of one or both or various combinations or dilutions of these seemingly account for the prevailing colors in human hair. In general dark hair is dominant to light, although because of the delay sometimes in the darkening of the hair in children this fact is often obscured. Black is dominant to red. People with glossy black hair, according to Davenport, are probably simplex for black, the glossiness being due usually to recessive red. The expectation would be for some of the children of such a pair to have red hair.
In man occasionally a congenital white lock contrasting strikingly with the remaining normally pigmented hair occurs. It behaves as a simple dominant in heredity.