|Athro, Limited Biology Genetics Sheltie Coats|
|Sheltie Coat Color Genes|
The Sheltie (Shetland Sheepdog) is a breed of small herding dogs. The breed originated in the Shetland Islands. Originally, Shelties were bred by crofters as to keep sheep out of their vegetable gardens and to warn of intruders. During the development of the modern sheltie breed, known crosses were made with Collies. Thus the coat colors and markings of the Sheltie and the Collie are similar. Here we describe what is known about Sheltie coat color genes, and present an interactive coat color calculator that lets you choose the parent's coat colors and see what colors of offspring you could get.
|Golden Sable||Shaded Sable|
|Bi-blue||Sable merle||Double merle|
These are the main coat color
phenotypes observed in the Sheltie. These coat colors are controled by a set of genes. The genetics of coat color in dogs in general is fairly complex. There are genes that determine basic body color, and others that determine the pattern of the markings, and others that are simply not well understood. There is still disagreement over some of the better known genes. A variety of different genes controlling coat colors and coat patterns have been identified. However, for the most part, we don't know what the proteins produced by these genes are, or the molecular mechanisms by which they produce coat color and color patterns. Several of the genes are probably for melanin and related pigment proteins, others are probably regulatory genes that incidentally alter coat color patterns, others affect the distribution of pigment within coat hairs. These genes are known only as
loci or gene series, with variable
alleles, not as sequenced genes with known mutations.
The variation of coat colors seen in the Sheltie is produced by a number of genes. The interaction of two genes produces the variation in basic coat colors recognized in the breed and shown above. There are several other genes that produce variations on these colors in Shelties, and more genes that produce coat colors and patterns in other dog breeds, but don't vary much in Shelties.
Nine named gene loci for dog coat colors are commonly encountered in the literature. This basic scheme for loci, genes, and alleles was established by Little in 1957 for coat color genes in dogs in general. Here, we largely follow this scheme, though we list only the alleles (flavors of the genes) thought to exist in Shelties.Agouti, Merle, and Spotting Genes
Three genes are probably the most important for controlling coat color in Shelties: The Agouti gene produces sable or black coats. The Merle gene dilutes the base color, usually in a spotty merle pattern. The Spotting gene produces patches of white on the coat, as in the typical pattern of white bib, neck, feet, and tail tip.Agouti gene:
This is the principal coat color gene in the Sheltie, producing sable or black coats. Either two or three alleles of this gene are present in Shelties.ay - sable coat color
Each individual has two copies of each gene, thus, in the absence of merling: ay-ay pure for sable shelties usually have golden sable coats. ay-at and ay-ab Shelties usualy have sable coats with some extent of black and are referred to as shaded sables (and also, confusingly, as trifactored sables). at-at and at-ab shelties are Tricolored. ab-ab Shelties are Bicolored.
The alleles of this gene exhibit a variable incomplete dominance. Pure for sable (ay-ay) Shelties are generally golden sable in color, while Trifactored (ay-ab and ay-at) Shelties are usually sable with some extent of darker fur in their coats (shaded sable). This is incomplete dominance (one copy of the recessive allele produces some effect).
These genes do not follow a simple incomplete dominance. The phenotypes of pure for sable, trifactored sable, and bifactored sable Shelties can be very difficult to distinguish, as the amount of dark fur in the coat is is highly variable. There are pure for sable Shelties with a shaded sable phenotype, and trifactored sable Shelties with a golden sable phenotype. Bifactored and trifactored sable Shelties also produce an indistinguishable range of phenotypes. In general, a sable Sheltie with a trifactor or bifactor tends to have substantial amounts of dark hair in the coat, while a pure for sable Sheltie tends to have a pure golden sable coat. However, this is not always the case. Some trifactored Shetland Sheepdogs are very light in color and closely resemble a pure for sable Sheltie, while others have a distinct darker color. The reason for this variability is unknown.
It is possible that in the Sheltie, tan points are produced by a separate gene, and only a recessive black ab and dominant sable ayalleles of the Agouti gene are present. We suspect that this is the case, and are working to test this hypothesis.Merling gene:
Merle is a pattern gene that dilutes dark coat colors, usually in a splotchy pattern.M - merle
Homozygous Dominant MM (Double Merle) is defective, often with deafness and perhaps blindness (including the possibility of small improperly formed eyes (Willis, 1989 p.229)). Double merle doubly dilutes dark coat colors, and double merle Shelties have almost white coats with patches of faint color. Note that there are other genes that can produce white coats that are not the merle gene and don't come with its defects. Blue merles, Bi-Blues, and Sable Merles are heterozygous (Mm). However, they do not have these defects even though they have a copy of the M allele.
Deafness in double merles appears to result from the absence of melanocytes in the ear. The hearing apparatus fails to develop properly if melanocytes are absent from part of the lining of part of the apparatus of the inner ear (Strain, 1996). These problems with eye and ear development are suggestive of a problem with nerual crest cell migration.
The vast majority of coat color variation in the Sheltie can be explained by these two genes (Agouti and Merle). Our Sheltie coat color calculator explores the actions of these genes on Sheltie coat color. A third major gene, the spotting gene, controls the extent of white in the coat.Spotting gene:
In general today, Shelties lack the dominant S allele (Self color), and thus have some extent of white patches in their coat. Alleles present in Shelties are:si - irish spotting, the typical sheltie and collie white bib, toes, muzzle, and tail tip.
Alleles of the spotting gene appear to have incomplete dominance, so dogs with a genotype of si, sp, and sw may all have different extents of white in their coat.
There have been instances of Shelties without white markings. Early shelties were black and tan with no white. These probably bore the S (Self color) allele of this gene, but strong selection (i.e. recent breed standards [see Riddle, 1991]) has called for Shelties with white in their coats, and evidently largely eliminated the S (Self Color) allele from the breed.
Shetland Sheepdogs were classically considered to be
Homozygous tt, with no ticking (Willis 1989 p.99), but a ticking pattern of is often present on Shelties' legs. We agree with Sue Ann Bowling(Bowling, Pers. Comm. 1999) that ticking is definitely present in Shelties. It is possible that this could be a ticking gene from spaniel crosses early in the history of Shelties.
It is also possible that instead of there being bicolor and tricolor alleles of the agouti gene in Shelties, there is a separate gene that places tan points on a black coat. Breeders are divided in opinion over this. We suspect that tan points are actually produced by a separate gene, but use three alleles of agouti here for simplicity.
It has been proposed that there is a gene that produces a muddy brown on sable coats and tan points.
The overall picture of the genes that produce coat colors in dogs is fairly well known (see: Little, 1957; Willis, 1989). There are several other genes that tend to have uniform states in the Shetland Sheepdog. We note the alleles believed to be present in Shelties, but don't list all the alleles of each gene here. Several genes do have alleles that vary within the breed as a whole but are discouraged by breed standards.Black gene: (may be the gene for eumelanin)
Shelties are classically considered Homozygous BB (Willis, 1989 p.99). However, Bowling (Bowling, Pers. Comm. 1999) indicates that the b allele is present in the breed. The current US breed standard (Coen, 1986 p.2) requires a black nose, so there is strong selective pressure against the expression of the b allele.Albino gene:
Shelties are Homozygous CC
True albinos, lacking any skin pigments, are very rare in dogs. Albinism is generally attributed to a failure of the enzyme that breaks down tyrosine to produce melanin.Pigment Extenstion-Restriction gene:
Shetland Sheepdogs are generally considered to be Homozygous EEDilution gene:
Shetland Sheepdogs are generally considered to be Homozygous DD. This is certainly true for dogs in the show ring, though Bowling (pers. comm., 1999) believes that the d allele is also present in the breed
Allele: g - no greying (the other allele G, produced a distinctive gradual change in coat color with age from black to blue or blue grey as in Kerry Blues).
Shetland sheepdogs are classically considered to be Homozygous gg
Sue Ann Bowling (Pers. Comm. 1999) has seen evidence for a premature greying, that she suspects could be an allele of the Greying gene.
Sheltie Coat Color Genes
|Sheltie Coat Color Genes|
|Sheltie Coat Color Calculator|
|Sheltie Coat Color Test Crosses|
|Mendelian Genetics||Punnett Squares (using Sheltie examples)|
|Working out Punnett Square Examples|
|What "Genetic" Means|