Hereditary syndactyly in Angus cattle.

Twenty-five syndactylous Angus cattle, characterized pathologically, were reported from 16 herds in 10 states from 1979 to 1994. Twenty-one (84%) had all 4 legs syndactylous, 3 (12%) had 3 legs syndactylous, and 1 (4%) had 2 legs syndactylous. All syndactylous cattle walked with considerable difficulty. Hooves of aged animals became curled and bent laterally or medially. Affected hooves had the appearance of a truncated cone, the base of which was located at the coronary band. Most hooves were fused completely with no indication of dual anlage. An occasional hoof had a distal notch, and other hooves had a dorsally located groove indicating dual embryonic origin. Lateral dewclaws were enlarged in most cases. Radiographs and dissections of limbs of 19 animals revealed a consistent pattern of fusion in most affected calves. Lesions included 1 or more of the following: disappearance of the large metacarpal and metatarsal intertrochlear notches, horizontal fusion of 1 or more carpals and tarsals, fusion of proximal sesamoids, 1 distal sesamoid, and fusion of paired phalanges. Evidence of a genetic cause consisted of 11 syndactylous calves among 70 offspring of 4 3/4 sib families, 8 preterm syndactylous fetuses among 148 preterm fetuses and 13 calves in progenies of 19 animals tested for possible heterozygosity, and 5 syndactylous calves from matings of an Angus syndactylous bull with 1 Angus and 4 Holstein syndactylous cows. Data were consistent with recessive inheritance at a single locus. Angus cattle with sydactytly had a larger number of affected limbs than did syndactylous Holsteins and their Angus crosses, suggesting existence of 2 recessive alleles. The allele of Holsteins (syH) appeared to influence phenotypic expression in a dominant pattern over the Angus allele (syA). Both syA and syH alleles acted as recessives to the normal SY allele. Phenotypic effects on limb development were most dramatic in calves with the syA/syA genotype.