Somnier. About one case of hereditary six-fingered. Bemerkungen zu einem Fall von vererbter Sechsfingerigkeit. Klinik, t. psych. u. nerv. Krankh. v. R. Sommer. B. V. H. 3. S. 297

The author describes a case of six-fingeredness in the family, in which this abnormality has already been observed in the course of four generations. On this occasion, he cites a whole series of works concerning the question of the inheritance of morphological abnormalities, both in humans and in animals. In one of these works (Gruber. Vortrag über die Vererbung. Deutsche med. Wochenschz. 1909. S. 1995) it is indicated that, as a rule, the number of deformities decreases from generation to generation, if the deformities are not introduced from outside by marriages and are not amplified by kindred marriage (Insucht). Further in the same work, it is indicated that the appearance of such abnormal signs follows the apparently Mendel's laws of inheritance.