At embryonic day 13 polysyndactyly homozygotes lack the expected initiaion of digit formation and at embryonic day 14 the margins of the footplates are ragged, the forefoot has one central large outgrowth instead of proper digit formation, and the apical ectodermal ridge is abnormal. There is subcutaneous edema of the limbs and trunk and homozgotes die perinatally. Heterozygotes weigh slightly less than wildtype littermates at birth and at 20 days of age are, on average, 10% lighter than sibling controls. All four feet are syndactylous, the first digit is short and broad and there is an extra digit between digits 3 and 4. Both fore- and hindfeet are oedematous in the newborn. The claws are reduced and pointed and these mice do not climb well.
The polysyndactyly mutation was identified in or prior to 1966 at Harwell Laboratory in the progeny of a neutron irradiated (C3H/HeH x 101/H)F1 male that was mated to a T Stock female, which is a linkage testing stock homozygous for non-agouti, brown, chinchilla, pink-eyed dilution, dilute, short-ear, and piebald. This mutation was imported into The Jackson Laboratory in or before 1967 and was backcrossed onto C57BL/6By by Don Bailey and reached generation N3 in 1968, N6 in 1970, and N22 in 1976. Prior to 1978 Eva Eicher crossed this congenic to B6By.Cg-Ve/By and maintained the double mutant congenic by backcross to C57BL/6JEi. In 1983 the velvet and polysyndactyly mutations were separately backcrossed to C57BL/6JEi and in 1984 embryos were generated for cyropreservation from C57BL/6J females and males heterozygous for the polysyndactyly mutation at generation N3 to C57BL/6J. Thus, this strain has a mixture of predominantly C57BL/6J, with smaller contributions from C57BL/6JEi and C57BL/6By in addition to possible traces of the genomes of origin.