Sheep Color Genetics Primer
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My discussion here is only about color genetics in sheep. It is simplified on purpose to make this complex subject a little more clear to those who, like me, have no prior background in biology to help with terminology and concepts. If geneticists stumble across this page and have serious reservations about what I've written, please feel free to email me, firstname.lastname@example.org. I will do my best to understand your critique and incorporate it here.
Each new lamb has pairs of genes to determine its coloring as well as all other aspects of its physical makeup. A gene is a tiny packet of information which causes something to happen while the animal develops. It can, for example, make pigment production result in either black or brown. Or it can switch off pigment production entirely.
I wrote each animal has "pairs" of genes. One gene in each pair comes from its mother, and one from its father. Likewise, each mother and father has pairs of genes. When mom or dad donates one to baby, the breeder doesn't know which of the pair mom or dad gave unless she takes some time to figure it out.
Each gene pair has a particular place to live in the animal, as well as its own particular job to do. There can be more than one variation of each gene which sits in that particular place. Each variation of a particular gene is called an allele of that gene.
Standard nomenclature for genetics is a capital letter for the gene followed by a superscript or subscript letter or number to indicate which allele of that gene is referenced. However, on this website I will not be using superscript or subscript letters for alleles, as it messes up the spacing of the lines and is harder to read. I will stay with lower-case letters to indicate the alleles.
If a gene is known, but not its specific allele, a question mark may follow the gene. For example: Ag/A? indicates we know the animal carries the Agouti 'graying' allele, but we don't yet know what other Agouti allele it carries.
Interactions of Genes
Some alleles are partially dominant, partially recessive, or equal in strength to other alleles. For example, Ag (graying pattern) is the same in strength as Ab (badgerface pattern). When a lamb is born with the genes Ag and Ab (written Ag/Ab), it will show both graying and badgerface patterns at the same time.
Some alleles influence the 'expression' (the way the sheep appears as a result of the gene's action) of the dominant gene. That is, the color changes slightly as a result of the action of the more recessive on the more dominant gene. Confused? This is an area of color genetics which is probably the most complex, so don't let it bother you!
Finally, there are genes which are fully recessive. These can act only in pairs of the same gene. A solid color black or moorit (brown) Shetland sheep must have the recessive pair Aa/Aa.
Up to now I've only discussed what happens when considering what one gene pair can do. However, sheep may have as many as four color genes which may explain their phenotype (appearance). I won't get into detail about that just yet, but will mention one important aspect of having more than one kind of gene to determine color.
Just as one allele of a gene can dominate, be equal to, be influenced by, or be recessive to, another allele of a gene, so can one gene be influenced by another gene. In fact, one gene pair can sometimes completely cancel the impact of another gene pair!
Most modern domesticated sheep are white. They mostly have Awt/Awt for their Agouti genes, telling their bodies to produce no black/brown pigment. But once in a while a black sheep turns up in an all-white flock, and it's not necessarily the result of a ram jumping the fence. Instead, it's the result of a recessive pair of genes at a different location, called 'extension'. A sheep which has Awt/Awt and Ed/E? will be colored! The Ed/E? influence cancels out the Awt/Awt influence and allows the action of yet another gene, BB/B? to show up on the sheep.
Common Sheep Color Genes
Here's a quick list of the common color genes which impact sheep. Not all these genes may exist in sheep. More research is needed, but I think it helps to understand what each of these genes can do, in case a breeder sees a phenotype that may be explained this way.
Folks familiar with color genetics of other animals will note the absence of the C gene in this explanation. In rabbits, a pair of recessive C genes causes the bunny to be albino--no pigment at all in the fiber or skin or eyes. Albinoism in sheep is rare. It is not usually the cause of white sheep. Even the very whitest sheep will have dark-colored eyes as proof that it isn't albino. So don't worry about figuring out how the C gene works in sheep.