FIRST-TIME BREEDERS CAN HAVE FUN LEARNING ABOUT COAT COLOR GENETICS WHILE HEALTH TESTING
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The A locus (ASIP) has 4 alleles that can be reported: ay, aw, at, and a. Unlike the K locus, which has one dominant and one recessive allele, the A locus has an allele hierarchy like the E locus. The ay allele (fawn or sable) is dominant to all other A alleles and aw (agouti or wolf sable) is dominant to at (black and tan) and a (recessive black). The at allele is only dominant to a. However, this dominance hierarchy may not apply in all breeds. For an example using the standard hierarchy above, a dog that is ayaw or ayat will be fawn/sable, and only a dog that is atat or ata will be black and tan. It has been thought that the alleles are not additive and an awat dog and an awaw dog will both be the same degree of wolf sable. However, feedback from many breeders suggests that there may be breed-dependent interaction of the A alleles. There are also several different coat patterns for fawn/sable dogs, which goes beyond the scope of this post. Just like with the K locus, both alleles may be important to record to know what your breeding dog may produce. Next are the B and D loci . Both will lighten pigment, and the alleles at the B and D loci are recessive in their phenotypic expression. This means two copies of the recessive genotype (b or d) are needed to be phenotypically expressed. The B locus (TYRP1) will determine if black pigment in the coat, nose, paw pads, and eyes is lightened to brown pig- ment. Brown, chocolate or liver all refer to the same genotype (bb) and have a breed-specific phenotypic terminology prefer- ence. The genotype at the B locus affects any black hairs regard- less of the allele causing them, and a bb dog’s eyes will likely
become amber or copper. While there are a variety of recessive b alleles (bc, bs, for example) that interact at the B locus to create the brown phenotype, for simplicity, we use bb as the genotype of a brown dog. Additionally, some breeds refer to the brown phenotype as red, but genetically speaking, red is used for phaeomelanin and not eumelanin. The D locus (MLPH) will determine if eumelanin in the coat, nose, paw pads, and eyes is “diluted” to blue or Isabella. For dd dogs, noses can be any shade from light to deep charcoal grey, and their eyes can range from light brown, yellow, yellow-green or grey. What does the term Isabella mean? It is a special term for dilute brown (bbdd) . This is the color often associated with Weimaraners. WHAT EMBARK REPORTS FOR THE S LOCUS: The S locus (MITF) controls where pigment is produced in a dog’s coat and skin. An insertion near the MITF gene turns off pig- ment production in the coat and skin, resulting in white hair and/ or pink skin. Dogs with two copies of the insertion (sp) will likely have breed-dependent white patterning, with a nearly all white, parti or piebald coat. Dogs with one copy of this variant will have more limited white spotting and may be considered flash, parti or piebald. It is important to note that the MITF insertion variant does not explain all white spotting patterns in dogs, and other variants are currently being researched. For example, some dogs may have small amounts of white on their paws, chest, face, or tail regardless of their S Locus genotype. This is referred to as residual white. Dogs of some breeds are fixed for Irish spotting, a pattern of white undersides, often a white neck collar, and sometimes white facial markings. Research into the genetics of Irish spotting is ongoing.
144 | SHOWSIGHT MAGAZINE, APRIL 2021
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