Eye Defects.—Congenital cataract is another not uncommon defect in man which is transmitted as a dominant (Fig. 25, [p. 112]) with occasional irregularities. It is a condition of opacity of the lens of the eye which produces partial or total blindness. In a paper on Hereditary Blindness and Its Prevention, Clarence Loeb (1909) mentions 304 families of which pedigrees have been published. Of the 1,012 children in these families 589, or 58 per cent., were affected. It is obvious that this is near the expected percentage in the case of a dominant trait where matings of affected with normal individuals prevailed. An unfortunate circumstance about this malady from the eugenic standpoint is the fact that it is frequently of the presenile form which comes on late in life so that it is usually impossible to predict whether an individual of marriageable age is immune or will later become affected.
Fig. 22
Radiograph (Courtesy of Dr. W. B. Helm) showing polydactyly in a child’s hand. For genealogy of this see Fig. 23, [p. 110].
Fig. 23
Chart showing a history of polydactylism through five generations in the B—— family. The individual whose hand is pictured in Fig. 22, [p. 109], is of the fifth generation. Squares represent males, circles females.
Another defect of the eye following the course of a dominant in heredity is a pigmentary degeneration of the retina known as retinitis pigmentosa. Atrophy of the optic nerve is also involved and the final result is blindness. Still another example frequently cited is that of hereditary night blindness (hemeralopia), a disease in which the affected person can not see by any but the brightest light. In most affected families the final outcome is usually total blindness. One of the most remarkable pedigrees of defects in man ever collected is one of this disease published by Nettleship. He succeeded in tracing the defect through nine generations, back to the seventeenth century. The genealogy includes 2,116 persons. The character behaves as a single dominant in males, but frequently, though not always, females may be carriers of the defect in transmissible form though not exhibiting it themselves. That is, males in which the condition is simplex (Aa) develop the defect but females of similar simplex constitution (Aa) frequently do not. It follows, therefore, that normal males of such strains will have normal offspring but normal females may have affected children.