Testing Summary
The polydactyly (common variant) trait test reliably determines if a dog has one of the following genotypes:
| pd/pd | This dog carries two copies of the LMBR pd Allele which is found in dogs with normal, typical toes and likely no hind dewclaws. However, polydactyly can result from variants in other genes. This dog will pass one copy of pd to 100% of its offspring. Interpretation: Normal (typical) toes (likely no hind dewclaws) |
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| PDCV/pd | This dog carries one copy of the LMBR1 pd allele and one copy of PDCV, the variant responsible for polydactyly in some breeds. However, polydactyly can result from variants in other genes. This dog will pass the PDCV allele to 50% of its offspring and the pd allele to 50% of its offspring, which can produce dogs with extra toes and hind dewclaws. Interpretation: Likely polydactylous with hind dewclaws (typical toes Carrier) |
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| PDCV/PDCV | This dog carries two copies of the LMBR1 PDCV allele, which is associated with polydactyly and the presence of hind dewclaws in some breeds of dog. This dog will pass PDCV to 100% of its offspring and can produce dogs with extra toes and hind dewclaws. Interpretation: Likely polydactylous with hind dewclaws |
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Detailed Summary
In general, dogs have five digits on their front paws and four digits on their hind paws presumably due to evolutionary adaptation in which the first digit of the hindlimb was lost. Polydactyly (also known as canine preaxial polydactyly or PPD) refers to the restoration of the first digit in the hindlimb (known as the hind dewclaw) that was lost during canine evolution. Polydactyly in many breeds such as the Beagle, Belgian Malinois, Cocker Spaniel, Rottweiler, Shetland Sheepdog, Standard Poodle, Standard Schnauzer, Shih Tzu, and Yorkshire Terrier is associated with a Mutation in the conserved intronic region of the LMBR1 gene and is inherited in an Autosomal Dominant manner. The mutation in the LMBR1 intronic region is not thought to impact the expression of LMBR1 but rather impacts a conserved region in the canine genome that is involved in regulating the expression of other genes responsible for limb development.
Testing Tips
The polydactyly trait test evaluates the LMBR1 gene region for the presence of the "PD" associated variant. Dogs that inherit one or two copies of this variant are likely to have polydactyly or hind dewclaws. However, other genetic factors are known to be associated with different forms of polydactyly in other breeds. NOTE: Polydactyly in the Great Pyrenees is NOT caused by Mutation of LMBR1. It is associated with a mutation of the ALX4 gene which cannot be detected by this test.
There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.
References
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Park K, Kang J, Subedi KP, Ha JH, Park C. Canine polydactyl mutations with heterogeneous origin in the conserved intronic sequence of LMBR1. Genetics. 2008 Aug;179(4):2163-72.
[PubMed: 18689889]
