There are multiple mutations involved in mule foot - it is a monogenic recessive with incomplete penetrance from Theriogenology 70 (2008) 535–549
Syndactyly in cattle is also called ‘‘mulefoot’’ and refers to the fusion or non-division of the two functional digits of the bovine foot with synostotic (fusion of normally separate skeletal bones) phalanges. Phalanges are synostotic horizontally and by pairs; the second pair of phalanges is most fully synostotic, followed by the third, and then the first. This abnormality is often subject to a right-left and a front-rear gradient, being most often seen in the front and right feet. Proximal limb structures can also be affected in syndactylous animals, resulting in a reduced number of sesamoid bones. Synostotic phalanges can be detected as early as 37–40 d post-coitum in the bovine embryo.
Bovine syndactyly is an autosomal monogenic recessive trait with incomplete penetrance (79% in Holstein cattle) and variable expression. Crossbreeding experiments between Holstein and Aberdeen-Angus have produced syndactylous progeny, suggesting a common locus responsible for the disorder in both breeds. The bovine syndactyly locus was localized to chromosome 15 in 1996 [59] and is due tomutation in the low-density lipoprotein receptor-related protein 4 gene (LPR4). The occurrence of syndactylous cattle peaked in the 1970s as carrier animals were indirectly selected for superior production of milk and butterfat. The development of test mating and genetic testing greatly reduced the incidence of syndactyly. Unfortunately, syndactylous animals are still occasionally observed in the Holstein population and some carrier animals are still being used for breeding. Eradication of syndactyly will require a precise method to detect the causal genetic
mutation. Unfortunately there is extensive allelic heterogeneity in LPR4, with the six independent mutations described still not accounting for all analyzed cases. The two most frequent mutations are the Holstein-specific exon 33 mutation and the Angus-specific exon 37 mutation. Because not all causal mutations have been detected, genetic testing for the carrier status of single individuals remains difficult. Several labs offer testing for the known genetic mutations [Germany (
http://www.tieraerztliches-institut.uni-goettingen. de/); Italy (
http://www.lgscr.it/eng/index.htm); France (
http://www.labogena.fr/); the Netherlands (
http://www.vhlgenetics.com/vhl/index.html)]. Unfortunately, each laboratory utilizes different technologies and seldom analyze for the same genetic mutations.
The American Angus Association lists the status of several bulls and cows as carrier or free of the mutations responsible for syndactyly. However, this freedom from carrier status is based solely on progeny test information, and could be questioned in the context of the allelic heterogeneity of syndactyly in Holstein cattle.
