Fig. 18.—Squares denote males and circles females, as is usual in the charts compiled by eugenists; black circles or squares denote affected individuals. A1 had all fingers affected in the way shown in Fig. 17; B2 had all but one finger affected; C2 had all but one finger affected; D2 had all fingers affected; D3 has all but forefingers affected. The family here shown is a branch, found by F. N. Duncan, of a very large family first described by Harvey Cushing, in which this abnormality has run for at least seven generations. It is an excellent example of an inherited defect due to a single Mendelian factor.
One of the best attested single characters in human heredity is brachydactyly, "short-fingerness," which results in a reduction in the length of the fingers by the dropping out of one joint. If one lumps together all the cases where any effect of this sort is found, it is evident that normals never transmit it to their posterity, that affected persons always do, and that in a mating between a normal and an affected person, all the offspring will show the abnormality. It is a good example of a unit character.
But its effect is by no means confined to the fingers. It tends to affect the entire skeleton, and in a family where one child is markedly brachydactylous, that child is generally shorter than the others. The factor for brachydactyly evidently produces its primary effect on the bones of the hand, but it also produces a secondary effect