Colorful Coats Part 2

So now I'm going to discuss some more on the different alleles that can occur at Agouti. As I briefly mentioned in the previous blog post there are some 18 different options for alleles at the Agouti locus. A sheep can have any 2 of them including two copies of the same type of allele. However because of the way that sheep breeds were developed not all of these alleles are found in each breed. For example the Romney breed only has 6 alleles of the 18 that are common throughout the breed. Whereas the Romadale/CVM and the Navajo-Churro has around 9 alleles that are frequent through the breed.

Nightshade, our AaAa ram

As I mentioned in the last post the first of the these different alleles is the one that you see in most sheep. The White allele (Awt) is dominant to everything else and masks any of the other alleles so all you see is a white fleeced sheep. Now seeing that there are 17 other possible alleles, and explaining each of them would be too long for most people to read, I'm going to limit my explanation to the most common of these alleles. The next allele is known as the Badgerface (Ab). This gene exhibits as a half white and half black sheep. The white is along most of the dorsal part of the sheep. The exception is bars above the eye and from the chin working down into the brisket with the underbelly and inside legs exhibiting pigment. The next allele would be Black & Tan (At). This sheep is essentially the negative of the Badgerface pattern. With the belly and underside being white with white bars above the eyes while the rest of the sheep shows pigment.

The next three genes are darker genes with more than 50% of the wool on the sheep being pigmented. The first would be Light Blue (Albl). This is a sheep that has a black belly extending up to the rump as well as the nose and nose bridge with the rest of the animal having white fleece. In order for there to be a Light Blue there must be a Blue(Abl). The Blue allele shows black on all the inner and under areas with the shoulder tops, neck, and nose bridge with the rest of the animal appearing white. The last allele I am going to explain is the Self(Aa) allele. This results in an animal with absolutely no white pattern on it. It is a solid black animal that exhibits pigment in all of its skin and fiber. 

Frost, our AtAa ewe getting sheared.

Now these alleles work together or mask one another at times depending on the strength hierarchy of these alleles. As I mentioned earlier White is the most dominant and masks any other allele it is paired with. So some genotypes of a white sheep could be AwtAwt or even AwtAa but you would never know what that second allele is because the white allele masks it, unless you progeny test (topic for another day). For the rest of the alleles they work in a co-dominance fashion. So you could tell the difference between a AblAlbl and a AblAa due to the two alleles working together, just maybe not equally, but enough for the shepherd to determine the two alleles based on the color pattern. I may need to elaborate on this more later but I'll leave it here for now.

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!

From Breeding to Lambing: A Summary

So this time of year is the normal lambing season for most shepherds. What that really translates to is long restless nights and exhausting hours spend waiting out in the barn for ewes to lamb.

A little visitor to the barn the past few days.

In order for the ewes to have lambs they need to be pregnant! This requires the ewes to be bred about 146 days prior to when lambing should start. This means that the rams were placed in with the ewes back in August or September through October or November in order for them to lamb this spring. Generally this is plenty of time for all the ewes to conceive and recognize the pregnancy.

Two lambs born just this morning.

The first tell-tale sign that ewes are pregnant is the development of the mammary tissue in the udder. This begins approximately a month before the ewes are due to lamb. This is the most utilized tool to determine if the ewe is pregnant. However ultrasound can be used to see if the ewes are pregnant as soon as 30 days after breeding, but most ewes are checked around 45 days. The way the ultrasound machine detects a ewe is pregnant is by utilizing sonar, or sound waves, to 'see' the tissue being examined. The device works by transmitting, receiving, and then interpreting those sound waves into an image viewed on the machine screen, by the CPU within the device. The sound waves are directed towards the uterine horns to determine if there are fetuses in there. The different tissues such as bone, muscle, fluid, and fat appear as different shades on the monitor, which allows the technician to determine if there are fetuses within the uterine horns.

For each lambing ewes are supposed to have 2 lambs since they have 2 teats on their udder. Generally ewes have no trouble raising twin lambs on their own, and some ewes can even manage triplets just as well. However some breeds are known for having multiple births such as the Finnsheep or Polypays. Most shepherds will allow their lambs to nurse from the ewes for about 60 days which coincides with the ewes starting to dry off (stop producing milk).

A day old romney ewe lamb in the feed tub.

Sorry for the shortness of the post, but its Super Bowl Sunday. To make up for the brevity there are 3 pictures instead of the usual one.