or 50 percent abnormal, according to circumstances, and such a marriage is unfit; but two parents who, though derived from brachydactyl strains, altogether lack the inhibitor of growth will have only normal children, for normality implies entire absence of the determiner that stops the growth of the fingers. Such a union is entirely fit.
The rule that the abnormal condition is [italics]induced[end italics] by something, so that normal parents never produce abnormal offspring, holds for many abnormal conditions such as presenile cataract; the congenital thickening of the skin known as keratosis; xanthoma, in which the skin acquires yellow patches; hypotrichosis congenital familiaris, or early absence of hair, and other abnormal conditions of skin and hair whose inheritance has been analyzed by Gossage. Probably the same is true of diabetes insipidus and stationary night blindness, according to Nettleship. In all these cases the intermarriage of normal descendants, even of abnormal ancestry, is entirely fit; but abnormals will reproduce their peculiar condition.
Fit and Unfit Matings
In another class of cases the abnormal or diseased condition is due to the [italics]absence[end italics] of a characteristic or quality. Thus albinism is due to the absence of pigment and two albino parents have only albino children. Normal offspring of an albino and a pigmented parent may transmit the albinic condition; and the marriage of a pigmented male of an albinic strain with the pigmented female of another albinic strain or with a pigmented cousin is unfit. In the case of the degenerative disease of the retina known as retinitis pigmentosa normals may carry the disease so that two normal cousins from retinitis stock may have offspring with retinitis. In fact, a large percentage of all cases of retinitis come from consanguineous marriages. Surely such marriages are highly unfit.
Deafmutism is due to a great variety of causes; any one of a variety of defects may produce it. But in different individuals of the same family the chance is large that it is due to the same defect. This defect is frequently recessive, hidden in the normal children. Two such normal children who are cousins and from deaf-