Shankless, a new mutation on chromosome 2 in the chicken

A mutant condition characterized by absence of shanks is invariably present in chickens that are homokaryotypic for an X-ray-induced pericentric inversion to chromosome 2. The mutation was studied in 260 embryos at 19 days of incubation. Embryos produced from matings of parents with and without the inversion were karyotyped and their skeletons observed and measured. The mutant phenotype, which was seen in all birds homokaryotypic for the inversion, is characterized by malformed and reduced metacarpals, absence of tarsometatarsal shanks, increased tibia length, extra bones in digits two, three, and four, and fusion of proximal phalanges. Heterokaryotypic and homokaryotypic normal embryos had normal phenotypes, but the length of metacarpals was slightly reduced in heterokaryotypes. The primary effect of the mutation is to cause misplacement, malformation, and underdevelopment of metatarsals II, III, and IV. The recessive mutation, located in the proximal region of chromosome 2, has been designated shankless (shi).