Albinism a Recessive.—As an easily understood illustration of this type of case we may take human albinism, a condition which is due to the absence of a pigment-developing determiner. According to Davenport the albinic condition is recessive to normal condition. If albino (aa) is mated with albino (aa) nothing but albino children may be expected. An albino (aa) mated with a normal individual will have normal offspring (Aa), but they will have the capacity for transmitting albinism to their descendants. Thus the normal offspring (Aa) of an albino (aa) and a normal parent (AA) if mated to another normal individual (Aa) who has also had an albino parent will probably transmit actual albinism to one-fourth of his children and the same capacity that he himself has of producing albinos, to one-half of his children, although the latter will appear to the eye to be normal.
Other Recessive Conditions in Man.—If for albinism we substitute certain forms of insanity, hereditary feeble-mindedness (Fig. 28, [p. 118]), or hereditary epilepsy, all of which apparently follow the same law, we can readily understand how unfit such matings are where both strains are affected. Marriage with similarly defective stock will result in the affection appearing in one-fourth of the progeny, and one-half of them, though apparently normal themselves, will have the capacity for transmitting the imperfection. It is in the existence of such hidden factors that the chief danger in the marriage of cousins, or in fact any consanguineous marriage lies.
A few of the various defects which seem to be inherited as recessives when mated with normality are: susceptibility to cancer; chorea (St. Vitus’ dance); true dwarfism (all parts proportionately reduced); Alkaptonuria (urine darkens after passage); alcoholism and criminality, where based on mental deficiency; hereditary hysteria; multiple sclerosis (diffuse degeneration of nervous tissue); Friedreich’s disease (degeneration of upper part of the spinal cord); Merriere’s disease (dizziness and roaring in ears); Thomsen’s disease (lack of muscular tone); hereditary ataxia; possibly the tendency to become hard of hearing with increased age; and possibly, non-resistance to tuberculosis.
Of non-pathological conditions in man which are inherited as recessives, apparently either very great or very small intellectual ability are examples.
Fig. 28
Chart showing descent of feeble-mindedness as a typical recessive (after Goddard). Squares represent males, circles females; DD, homozygous dominant; DR, heterozygous dominant (i. e. normal although a carrier); RR, pure recessive; N, normal; F, feeble-minded; A, alcoholic.
Breeding Out Defects.—Even though recessive defects occur in a stock, there is the possibility of diluting out the imperfection in successive generations if care is taken always to marry into a stock wholly free from it. For example, a normal individual carrying a recessive defect will bear the abnormality in half of his or her germ-cells. This means that when such an individual marries a normal, non-carrier, half of their children will be wholly normal (AA) and half will be carriers; normal but of simplex constitution (Aa). If now this generation, carriers and non-carriers, marry only into normal strains of duplex constitution, then their combined issue will be likewise normal with only one-fourth of them carriers of the imperfections. This means that even if all of this last generation were married to persons having the defect only one out of four would have children showing it although the remaining children would be carriers. On the other hand if mated to normals only one-eight of the next generation would be carriers. Thus by continually marrying into strong strains liability to manifest any recessive defect can be diminished in a few generations until the descendants are no more likely to have defective children than are members of our ordinary population.
The proportion in which the recessive defect would appear in successive generations if all persons in a given generation married only normal individuals who were non-carriers is indicated in the following table where AA indicates a normal individual, Aa one who is normal but a carrier, and aa an individual with the imperfection expressed; to indicate proportions simply after the first generation, four is arbitrarily chosen as the number of children which results from each marriage:
| Matings | Children | |||
| Generation 1 | aa × AA | == | Aa | |
| Generation 2 | Aa × AA | == | 2AA + 2Aa | |
| Generation 3 | AA × AA | == | 4AA | |
| AA × AA | == | 4AA | ||
| Aa × AA | == | 2AA + 2Aa | ||
| Aa × AA | == | 2AA + 2Aa | ||
| 12AA + 4Aa | ||||