Inheritance of Polydactylism in the Fowl

OLYDACTYLISM, where studied in vertebrates, has usually been shown P to be a heritable characteristic, although in some instances the mode of inheritance has been somewhat involved.In the case of the fowl, the usual number of toes is four although five toes constitute a breed characteristic in some instances.The four-toed condition of the fowl is the result of the loss of the fifth digit from the typical pentadactyl foot of higher vertebrates.It has been shown by various workers (KUAFMANN-WOLF 1908; HARMAN and ALSOP 1938) that the additional toe in five-toed breeds does not constitute a restoration of the missing fifth digit but is the result of the development of a new toe on the opposite side of the foot.The work of BOND (1920, 1926) and PUNNETT and PEASE (1929) has indicated that the five-toed condition behaves genetically as a dominant to four toes.Certain irregularities in the expressions of the character including heterodactyly and the occasional lack of penetrance have been noted.A considerable body of data on polydactylism has been accumulated by the writer in connection with other genetic studies on the fowl.Polydactyly, as here studied, cannot be referred to any particular breed, since it was present in crossbred stocks used in various genetic studies, but the original stocks were Silkie Bantam and Houdan.It was the purpose of this study to obtain further information on genetics of polydactylism in the fowl. INIIERITANCE O F FIVE TOESSeveral workers (BOND 1926; PUNNETT and PEASE 1929) have studied the inheritance of the five-toed condition as found in some standard breeds of poultry.The conclusions were that polydactylism of this type behaved as an autosomal dominant, with some of the individuals carrying the gene failing to show extra toes.HUTCHINSON (1931) suggests that the occasional normal individual resulting from a mating involving homozygous dominant polydactylous parents may be due to genetic modifiers influencing dominance.The numbers of birds included in previous studies have been somewhat small, and therefore additional data are presented by the writer.The large numbers recorded for heterozygous polydactyls backcrossed to normals are from linkage tests.Certain facts are evident from the data in table I. First the earlier evidence of the dominant behavior of polydactylism in confirmed.It is also noted that the ratios show a deficiency of polydactyls.The greatest deficiency is found where the segregating polydactyls are all heterozygotes (backcrosses to normals).The shortage in these crosses is greater than in the Fz generation, indicating the possibility that the failure of expression of the character is greater Contribution No. 155, Department of Poultry Husbandry.