Postaxial polydactyly in three Indian families.

In most of the reported pedigrees on human polydactyly, the manifestation of the trait has been attributed to a single autosomal dominant gene. That this gene is not always penetrant and exhibits a great degree of variable expressibility has been stressed by various authors (listed by Gates, 1946, and McKusick, 1966). By contrast, recessive polydactyly has been described by only a few authors (Snyder, 1929; Bell, 1930; Shevkenek and Thompson, 1933; Oliver, 1940; Salzano, 1961, etc.). Two main phenotypically different varieties of postaxial polydactyly have been recognized (McKusick, 1966). In type A, the extra digit is well formed and articulates with the fifth or an extra metacarpal or metatarsal, while in type B the extra digit is vestigial and is frequently merely a skin tag. It has been suggested that these two phenotypes of postaxial polydactyly may also be genetically different (Gates, 1946; McKusick, 1966). Type A seems to be inherited as a dominant trait in most pedigrees and the gene shows marked penetrance. The genetics of type B is more complicated, and two different inheritance mechanisms have been proposed for it. Gates (1946) considered this type to be representative of recessive polydactyly, but pedigrees of type B studied by Sverdrup (1922), Callan (1942), and Walker (1961) were described as dominant. Sverdrup (1922) reported a large Norwegian kindred and noted the occurrence of both types A and B in the same pedigree. He suggested that type B was produced by a single factor and type A was probably due to an additional modifying factor. Instead, Walker (1961) postulated two dominant genes for type B and suggested that the additional gene controlled the penetrance of the polydactyly gene. This paper deals with the inheritance of postaxial polydactyly type A in three Indian families. Of all possible modes of inheritance, a single dominant gene for this type of polydactyly seems to be the