Monday, January 19, 2009

A Question on Color Genetics

Instead of finishing my post on beginning long lining, I got caught up in answering a genetics question on Science VS Tradition blog post Probability Genetics are so fascinating I lost track of time as I worked on trying to describe in simple terms my answer to this color question. I though maybe others might be interested in this information as well, so I'm going to post my answer here and get my long line post ready for tomorrow.

The question is "A mare homozygous at the bay loci, EEAA, is crossed to a stallion who is heterozygous at the bay loci, EeAa. What is the probability that their offspring will be bay?"

OK, this is kind of my thing since I love genetics. Which doesn't mean I can answer all kinds of color questions because I'm not good at the patterns except basics and I'm only beginning to get the hang of dilutions but I'm usually good with the basic colors. With that being said lets break this down and see what we get.

The bay agouti A (no bay agouti is represented by a) is dominant. It can only express on black, however, restricting that black color to the points. The bay agouti A remains hidden on a red coat color. The lack of agouti a is recessive so must be present as aa (one from each parent) for a horse with black A to actually be a black horse.

Here we have a mare EEAA being homozygous at the E Locus. Since the E represents black and e is red, she is homozygous not just for the bay agouti AA but also black with EE.

Since the black gene E is always dominant over the red gene e, you must have ee (one from each parent - which in this case you cannot get because the mare does not carry an e) to get a red coat color that would not show the presence of the bay agouti A.

With the mare contributing E it doesn't really matter whether the stallion contributes an E or an e, the resulting foal will carry an E to produce a black coat color. That color, however, will be restricted to the points by the dominant bay agouti. That pretty much guarantees you a bay horse.

Note: The density of the coat color will be affected by the Black E gene versus Red e gene coming from the stallion.

I hope this all makes sense. If not, fire away with your questions.

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  1. I don't know very much about the codes of genetics, but our two barb mares are bay, and we wanted the offspring to be so too (Arwen is of a very rich reddish colour, whearas the second mare Sahara is slightly more golden). Arwen was mated to an identically bay-coloured stallion and produced a perfect filly, identical to her parents. Sahara was mated to a dark chestnut as I had heard that chestnut is recessive to bay, and her newly born filly seems to be taking after her mother.

    Is it true that grey is the most dominant horse colour, followed by bay, black and then chestnut (those would be the colours available here in West Africa). I have made it a point avoiding all grey stallions as potential sires for fear of producing a grey foal, or even worse, a grey-reddish foal.

    I'd love your take on it!

  2. Color genetics are always a little confusing to me, but I sort of get it. I wasn't that good of a student in Science class though.

  3. Esther, Grey is the more dominant color. However, a grey horse can carry a gene that isn't grey so can produce a foal that isn't grey.

    I have 3 gray mares here and each carries another color along with the grey. One of those mares has had 4 foals for me and not one of them is grey.

    Your sequence of dominant horse colors is correct. The only way to get black is to get a horse free of the bay agouti. For chestnut the foal must get the chestnut gene from both parents since it is a recessive trait.

    With the letter notations on genes, the dominant is the large letter and the recessive is the small one, if that helps at all.

    Arlene, I can understand about genetics being tough. I was a good student in science and they can still make my head buzz when all they talk is those letters. I have to sit down and map them to sort it out. It's much easier for me to talk the colors than the letters. I don't know what it is they have to make things look more difficult than they are.

  4. OOooo, genetics is fun! Even though the principles are simple there is SO much to learn! And still so many color modifiers are undefined or misunderstood even still!
    The one that still gets me is the difference of mane/tail color on chestnut horses. We had a red mare that was regularly mis-defined as a bay her mane/tail were so dark liver and another red gelding whose mane/tail were so beautiful only slightly lighter than his body but with many separate lighter and darker hairs all mixed in, the effect was VERY striking.
    Have you discovered ?

  5. Glad my post got you thinking. Not many breeders or owners unfortunately know much about genetics.

    The grey gene G exhibits simple dominance over other genes. Capital letter alleles are dominant and lower case are recessive. To understand this you need to know Mendel's 3 principals of genetics.

    1. Dominance. In a heterozygote one allele may conceal the presence of another. Some alleles control the phenotype even when they are present in a single copy.

    So for example a grey stallion is bred with a chestnut mare and their foal is white because the grey gene G is always dominant.

    2. Segregation. A heterozygote has two different alleles that separate during the formation of gametes. One allele is transmitted faithfully to the next generation, even if it was present with a different allele in a heterozygote.

    Example: A black mare and black stallion have a bay foal.

    3. Independent assortment. The alleles of different genes segregate or assort independently of each other.

    Example: a Heterozygous buckskin has a bay colt. Three dominant genes must be present together to have a buckskin. The three genes occur at the black red and creme dilution loci EE, Ee, AA, Aa and CcrC must be present.

  6. I did a post a while back on color genetics... in an attempt to explain color dilution genes in a laymen way even if it meant that it was not techincally correct (it is just easier to understand that way).

    Check it out! I'd love to hear your thoughts...

  7. Okay, to be honest here, I didn't actually read all of that...I got completely and utterly confuddled. I would LOVE to learn everything about genetics, but I wouldn't have a clue where to start. How did you learn all this?

  8. Jeanette, I think you're right that genetics are fun and the concept simple. I think it's the modifiers and all of those letters that freak people out. And yes, I do know

    Syndey, yes, genetics always gets me thinking. They have come so far since I was in high school which is when they first discovered DNA.

    Adventures, you're right about the dilutions, it's much easier to simplify them and understand. It's went people want to know the probablity (which seems to be a given with breeders lol) that you have to be more specific.

    Gecko, I learned it one step at a time. Once I got comfortable with the basic coat colors....and I mean very comfortable then I've gone on from there.

  9. This color talk is interesting. It makes me think of Legs and Solidare's kid, Rhythm. How Legs is bay, Solidare is grey, and they produced a chestnut.

    And how Legs has produced grey, bay, black and chestnut offspring. With the help of the mares, of course though :).

  10. I have a grey arabian mare with both parents as greys (I do understand both those parents received genes from previous lineage that contained chestnut). Is there any possibility I could produce a black colt by mating my mare to a homozygous black stallion? What are the possibilities?

  11. Kim, Legs has not produced black. The black horse here is from a Legs daughter.

    karen, with two gray parents there is a chance you mare could be homozygous gray. If that is the case she can only produce gray foals. If she has had foals already and any is other than gray then you would know that she is not homozygous.

    If she were homozygous gray there would be no chance she could produce black. If she isn't then you would know she carries a chestnut gene but you won't know if that chestnut gene also has an agouti (which produces bay when a black gene is present) becasue the agouti only expresses on a black gene and doesn't show on a red.

    If she has that agouti you have no chance of a black foal. If she doesn't you have a 50/50 chance of getting black.

    I'm not sure what tests are currently available for color but you might check to see if you can test her to find out what she carries. Then it would be easier to know your odds of getting a black foal.