Shelties come in three foundational coat colors: sable (a brown ranging from golden to reddish to mahogany with white areas), tricolor (mostly black with white areas and tan points around the muzzle and legs), and bicolor or bi-black (black with white areas). These three colors are determined by the Agouti gene.
The sable allele of the Agouti gene is dominant while the bicolor allele is recessive. Thus any sheltie with a sable allele will be sable, a sheltie with a tricolor allele and no sable allele will be tricolor, and only a sheltie with two bicolor alleles can be bicolor. See the last section of this article for a more detailed description of what this looks like in practice.
Merling Gene
The merling gene dilutes coat and eye color in any sheltie that carries it. A sable sheltie with the merling gene becomes a sable merle (a tan or cream color), a tricolor sheltie becomes a blue merle with tan (a grey or silver color with tan points), and a bi-black becomes a bi-blue merle (a grey or silver color without the tan points). Merles can also have blue eyes or merled eyes–eyes that have blue and brown patches.
The merling gene rarely dilutes the coat uniformly. Most merles retain patches of darker sable color or black in their coat. They also retain the same white areas that are found in shelties without the merling gene.
A sheltie with two merling alleles, which results from having two merled parents, is called a double merle and is mostly white. Double merles usually suffer from serious health problems including deafness and underdeveloped eyes. It is very important to avoid breeding two merles of any color because double merles rarely survive to live healthy lives.
Spotting Gene
The spotting gene determines how much white is present in a sheltie's coat. The Irish spotting allele presents the typical pattern of white on the neck, chest, feet, and tail. The piebald spotting allele, also called white-factoring, presents shelties that look much like shelties with Irish spotting, but white often runs up higher in the rear legs and they can have patches of white, called mismarks, in places where white isn't typically found.
The extreme white piebald allele produces color-headed whites. These are also mostly white, but they do not suffer from the health problems of double merles. Color-headed whites result from breeding two shelties with the piebald spotting allele (two white-factored shelties).
What Colors to Expect in a Litter
Within the Agouti gene, the sable allele is dominant, the tricolor allele is intermediately dominant, and the bicolor allele is recessive. Within the merling gene, the merle allele is dominant and the non-merle allele is recessive.
Each sheltie receives one allele of each gene from each parent, creating a total of two alleles per gene. A dominant allele presents itself in the sheltie's looks, or phenotype. A recessive allele only presents itself in the phenotype if no dominant allele is present–in other words, if the sheltie has two recessive alleles. Otherwise the recessive allele is carried but not visible. So a sheltie that receives a sable gene from either parent must be sable because sable is dominant. A sheltie that receives a tricolor allele and either another tricolor allele or a bicolor allele must be tricolor, because tricolor is recessive to sable but dominant over bicolor. A bicolor sheltie can only have two bicolor alleles, because both tricolor and sable are dominant over bicolor.
Any sheltie that receives the merling allele must be merled, because merled is dominant over non-merled. Therefore it is not possible to be a carrier of the merling gene without presenting it in the coat.
So a sable sheltie will typically produce sable offspring. However, if that sable sheltie received either a bicolor allele or a tricolor allele from one parent, it may produce bicolor or tricolor offspring if bred to another sheltie with a bicolor or tricolor allele. A sable that carries a tricolor allele is called a trifactored sable and one that carries a bicolor allele is called a bifactored sable.
A tricolor sheltie bred to another tricolor may produce only tricolor puppies, but if each parent carries a bicolor allele, it is possible that they will produce bicolor puppies as well. A tricolor sheltie that carries a bicolor allele is called a bifactored tricolor.
A blue merle or sable merle sheltie, assuming it is not a double merle, carries one merled allele and one non-merled allele. Breeding a bi-blue merle to, for instance, a bifactored sable could produce bi-blue merles, sables, or sable merles.
Determining the color makeup of a litter is rarely possible without knowing the colors of the generation before the parents, or the litter's grandparents. This may be the only way to determine what alleles the parents carry. It is not only the color of the parents that matters but the alleles that the parents carry as well. It is difficult to explain how this looks in reality without using multiple examples, so try this sheltie coat color calculator. It demonstrates what offspring parents of any color genotype can produce.
Reference: Susan F. Morris and Paul J. Morris. Sheltie Coat Color Genes. Athro, Limited 12 Dec 1999.
Madeline Salmon - Madeline is a contributing writer for the Science & Nature and Plants & Animals sections of Suite101.com. She is a freelance writer with a ...
Join the Conversation