|Athro, Limited Biology Genetics Shetland Sheepdog|
|Sheltie Coat Color Inheritance|
Genes are structures that are carried on larger structures called chromosomes. The genes for each characteristic come in pairs, and the two genes together produce a given characteristic. One of the genes of the pair came from the father and the other came from the mother.
Genes can come in different flavors, called alleles. Two genes involved in Shetland Sheepdog (Sheltie) coat color are shown here. The Agouti gene determines if the coat is sable or has black in it. The Agouti gene comes in three flavors - sable, tricolor, and bicolor. Sable is dominant over the other alleles, but not completely so. A Shetland Sheepdog with sable and tricolor alleles will usualy have some black in its coat, but doesn't have the mostly black coat of a dog with two tricolor alleles. Merle is a gene that alters the expression of the Agouti gene. The allele for Merling is dominant over not merling. The presence of a dominant Merle allele dilutes (in a splotchy pattern) the colors produced by the Agouti gene
The Agouti gene has three flavors - sable, tricolor (black with tan points) and bicolor (black). Tricolors have three colors when one counts the white that is usually present in their coats, but this is controlled by a separate gene. (Just to confuse things, in other dogs that controlled lack white in their coats, the tricolor allele produces black and tan dogs and can be called bicolor)
The Merle gene also has two flavors - Merle and not merled.
Genes are on chromosomes. There is one copy of the gene on each chromosome (some genes also come in many copies). Since chromosomes come in pairs, each individual has two copies of each gene. These two copies can be the same flavor (allele) or different flavors.
Genes are used to produce proteins. A gene that comes in two flavors might come in one flavor where the protein takes one form (has one color) and another flavor in which the protein takes another form (and has another color). Thus an individual with one copy of each flavor for a gene could produce a proteins of both colors.
The Agouti gene is a case of incomplete dominance. The sable flavor produces dogs with sable in their coat. The tricolor and bicolor flavors produce dogs with black in their coats. If an individual is heterozygous for sable and tricolor (one of its copies of the Agouti gene is sable, the other is tricolor) the dog has a sable coat (sable being dominant), but there is some extent of black in the fur (the dominance being incomplete, and both gene products presumably are present). These dogs are shaded sables (also, confusingly, refered to as trifactored sables). The extent of black color in the fur of a shaded sable is quite variable - some are dark brown, others are Golden Sable and indistinguishable from a pure for sable (homozygous sable) dog. (As a side note: Pure for sable Shetland Sheepdogs usually have the golden sable phenotype, but can have some amount of dark hair in their coat and have a shaded sable appearance).
Dominance can also be produced by a range of differences between the protein products of alleles - a recessive allele does not mean a defective gene product. Dominant and recessive should be taken only as descriptions of the expression of alleles, and not given any value judgement. In this particular case, merling is dominant, and double merle dogs are defective (often being blind and/or deaf). This is no reflection on the soundness of Blue merles, Sable merles, or Bi-Blues (which carry one copy of the Merle allele). It is simply a result to avoid in breeding. Thus, it is generally a poor idea to breed a dog and a bitch that carry both carry one copy of the Merle allele, as there is a 25% chance that each of their offspring will be a double merle (and be blind and/or deaf).
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