Colorful Coats

So for this post I'm going to try to explain some of the genetics of sheep color since it's a passion of mine. To start off I'll explain some basic genetics concepts:

Sheep have 27 pairs of chromosomes (compared to humans who have 23 pairs), with each chromosome composed of different genes that code for different proteins. Alleles compose these genes on the chromosomes, but each sheep can only have 2 alleles, since that is all a locus, location of a gene, can allow. There can be more than two different alleles for a given gene but sheep, or any animal, can only have 2 of them, since there's only enough room for 2 on each pair of chromosomes, one allele on each.

A black & tan ewe with two white ewes

Moving onto basic color production, all sheep have a base pigment that their melanocytes will produce unless directed otherwise by the genes. The color of pigment produced depends on the animal's DNA but it will be either a black pigment, eumelanin, or a brown/red pigment, phaeomelanin. The melanocytes receive a hormone called melanocyte-stimulating hormone (MSH) that (you guessed it) stimulates the melanocytes to produce a type of melanin. MSH does this by traveling to the melanocyte and binding to a receptor (MC1R) on the cell and signaling to it produce its base pigment. One might think, based solely on this mechanism, that all sheep should be black. So then why are majority of sheep white if their melanocytes are supposed to produce pigment? 

Now to answer that question I need to start discussing specific genes. In sheep the main gene that determines coat color is the Agouti gene. The purpose of this gene is to code for WHITE display on the animal. This gene codes for the protein called agouti-signaling protein (ASP). This protein binds to the melanocyte, specifically to the MC1R receptor, and alters it so that it will no longer receive MSH. Because the melanocyte is no longer receiving MSH it will be unable to produce dark pigment making that area on the sheep white or tan. So the important thing to remember is that the Agouti genes are responsible for the white parts of the fleece. One allele at the Agouti gene codes for a pure white sheep, denoted Awt. The reason most sheep have full white fleeces is because they have at least one copy of this allele at their Agouti gene. They only require one copy because this white allele (Awt) is dominant to the other 17 alleles that are also possible at this gene. Since each sheep carries two alleles for this gene, a white sheep can carry an allele for white while also carrying another allele coding for a different color pattern besides white, but only the white color pattern will be observable to the shepherd.

Dorothy and one of her lambs napping

So I'll leave you here until the next blog entry where I'll explain about all the alleles that occur at Agouti and other genes that interact with Agouti. I was originally thinking this would be two entries but I'm starting to think I may need three to cover everything I would like to. And to everyone in the Northeast, Enjoy the Snow!