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  1. #1
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    Default Agouti question: "black" and "homozygous for black points" -- impossible, right?

    Just reading a stallion ad for a black stallion who is homozygous for black points (their words, not mine).

    I don't think that's possible. Am I correct?
    Arrange whatever pieces come your way. - Virginia Woolf

    Did you know that if you say the word "GULLIBLE" really softly, it sounds like "ORANGES"?



  2. #2
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    It is. You can get a genotype test that will check for certain genes. If the stallion's genotype is BB ( in this case the B's stand for having black points) then he is homozygous. It doesn't matter what his coat Colour is, he will pass on the trait for having black points.
    if having black points is the dominant trait in horses, then no matter what his babies will have black points (incomplete dominance and codominance is not possible for black points) if black points are recessive and say the mare is heterozygous for black points, then 3 out of 4 of the babies will have black points.



  3. #3
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    You are correct JoZ.

    He is black (maybe homozygous) so therefore he CANNOT be homozygous for "black points". black points would indicate agouti (bays, buckskins, etc). A black horse has NO agouti gene, or he'd be a bay. So this horse can't pass on the agouti gene, because he doesn't have it.

    perhaps they mean he's homozygous for black, and won't produce chestnut offspring.



  4. #4
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    Really? Ive never heard that, tapperjockey, but I'm no expert so you are probably right. I know that what I said is correct for buckskins, but I was thinking that every horse has the agouti gene and that when it is dominant the horse will have black points and when it is homozygous recessive the horse will not. I was thinking that maybe it just did not show up on black horses because they are, well, black



  5. #5
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    You might want to ask them for clarification. Is the stallion homozygous for black AND for Agouti (EE, AA)? That would mean foals would always be bay, unless the dam has a modifier to pass on, such as gray.

    Agouti is dominant, so even if foal is Aa, foal will be bay. A stallion can be homozygous for black and for agouti. Homozygous for black means he can't have a red baby. If he wasn't homozygous for black, he could still be homozygous for agouti, but agouti doesn't modify red in any way so it wouldn't matter on a red foal.

    I love color questions, and bore my family to tears with all the iterations. Haha!!



  6. #6
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    Oh, and he could be one of those that is actually a bay but is really dark and looks black. Only way for them to know that would be if they had his mane tested, maybe? Some black horses can fade in the sun and look bay, and some bays can be really dark and look black. That's why I rely on mane testing to know for sure.



  7. #7
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    I agree with you, rancho del marr! I love to try and find out what the babies of two horses would look like! I always jump at Color questions, just because they are so fun to try and answer.

    Anyways, back to the topic. He is probably homozygous dominant for both, or at least is a really dark bay.



  8. #8
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    If the DNA tests as E?AA, then he isn't black, despite what he might look like, and he is homozygous for "black points".

    Yes, every horse has the Agouti gene, it's a matter of whether each copy is turned on or off, and if on, which form (bay, brown, wild bay).

    Regardless of being Ee or EE, if the horse is A?, he will not be black, genetically, but as there are some seal browns that are very, very dark, he may look for all intents and purposes, black.

    If he is "homozygous dominant for both", then he's not black. If he looks black, then he's brown instead of bay.
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  9. #9
    JoZ is online now Schoolmaster Premium Member
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    skippy, what is this homozygous dominant/homozygous recessive of which you speak? Homozygous means two copies of whatever gene one is speaking of, which by extension means that the fetus definitely gets one of those genes. Are you talking about whether the gene itself is dominant or recessive? (And yes, this is getting a bit off-track I know, but hey it's my thread so...!)

    Tobiano and agouti and I think dun factor (not sure) will all appear in the phenotype of a foal with only one copy of the gene from one parent. I think several types of overo/sabino/whatever it's being called now do NOT necessarily show up even if one or two genes for it are present in the genotype. And now we've exhausted my knowledge of THAT type of thing!

    A black horse cannot have one OR two copies of agouti, getting back to my original question. If it had agouti, it wouldn't be black.
    Arrange whatever pieces come your way. - Virginia Woolf

    Did you know that if you say the word "GULLIBLE" really softly, it sounds like "ORANGES"?



  10. #10
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    They can have one or two copies of agouti. It's like gray. It's dominant so only one copy is needed for it to modify the horse's color. However, if the horse is homozygous, it will always be passed on to offspring.



  11. #11
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    Homozygos dominant is when you are homozygous for the dominant gene. Homozygous recessive is when you are homozygous for the recessive gene. Ditto for heterozygous.



  12. #12
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    Quote Originally Posted by JoZ View Post
    skippy, what is this homozygous dominant/homozygous recessive of which you speak? Homozygous means two copies of whatever gene one is speaking of, which by extension means that the fetus definitely gets one of those genes. Are you talking about whether the gene itself is dominant or recessive? (And yes, this is getting a bit off-track I know, but hey it's my thread so...!)
    Homozygous simply means "the same". So, if both copies of 1 gene are the same, they are homozygous for that.

    The Extension gene is shown as E (on, black) or e (off, red). A red-based horse, such as a chestnut or palomino, is "homozygous recessive" for Extension, meaning ee. It's generally not referred to that way though. In this case, the horse doesn't have to be homozygous dominant (EE) to have the effect of being a black-based horse - EE or Ee does the same thing9888888888

    aa for Agouti would be homozygous recessive. As with Extension, the horse doesn't have to be AA to be homozygous dominant, BUT, since Agouti only affects black pigment, being Aa or AA has no phenotype change in the ee horse, only the E? horse

    Tobiano and agouti and I think dun factor (not sure) will all appear in the phenotype of a foal with only one copy of the gene from one parent.
    Yes, all of those do/often do appear the same whether hetero or homozygous. The "often" comes from the Tobiano, which often presents with cat tracks/paw prints. That's not a reliable indicator, as they can be present on the Tt horse, and absent on the TT horse. But it's much more common for the TT horse to have them and the Tt horse to not have them

    I think several types of overo/sabino/whatever it's being called now do NOT necessarily show up even if one or two genes for it are present in the genotype. And now we've exhausted my knowledge of THAT type of thing!
    Overo is just "not Tobiano"

    The heterozygous presence of the Overo patterns can be either nonexistent or minimally existent in the phenotype

    However, homozygous Sabino1 is a 95%+ white horse. Homozygous Splash horses have obvious white markings, mostly quite loud but with the new tests I've seen some SW1/SW1 horses I'd have never guessed were homozygous for Splash - yes, obvious white, but pretty restricted to legs and face, as opposed to the typical body white the homozygous Splash horse has.

    Homozygous Frame is an all/mostly white foal (who then dies).

    Dominant White is largely believed to be embryonic lethal in the homozygous form, so you don't see live DWDW horses. But, the hetero form, just like that of the other overo patterns, can be pretty minimal. The difference here is that the hetero DW horse can be very, very loud, including being all white.

    A black horse cannot have one OR two copies of agouti, getting back to my original question. If it had agouti, it wouldn't be black.
    Exactly
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  13. #13
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    Quote Originally Posted by Rancho del Marr View Post
    They can have one or two copies of agouti. It's like gray. It's dominant so only one copy is needed for it to modify the horse's color. However, if the horse is homozygous, it will always be passed on to offspring.
    Yes, but I think she was talking about a black horse - he's black because he's aa. If he were Aa he couldn't be black, though he might look it
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  14. #14
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    Oh... Gotcha!



  15. #15
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    Oohhhhhh ok I get it. Still.. maybe hes not black, just a super, super dark bay. Or he could be dark, dark bay and they dyed his coat.



  16. #16
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    I have only seen people advertise "homozygous for black points" when the horse tests as AA, so it would be interesting to know what they really mean
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  17. #17
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    JB- how do you pronounce these: homozygous & heterozygous? Lol you don't wanna know how i do lol!!! TIA!



  18. #18
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    I know how I abbreviate it in my mind!

    zy-gus, following the homo- and hetero- which are pretty self-explanatory
    ______________________________
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  19. #19
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    Quote Originally Posted by skippy60 View Post
    Really? Ive never heard that, tapperjockey, but I'm no expert so you are probably right. I know that what I said is correct for buckskins, but I was thinking that every horse has the agouti gene and that when it is dominant the horse will have black points and when it is homozygous recessive the horse will not. I was thinking that maybe it just did not show up on black horses because they are, well, black
    That is incorrect. If a horse is recessive (heterozygous) for agouti, they would look just like a horse that is dominant (homozygous) for agouti. The only way to tell is by testing to see if the horse is or isn't (unless one parent is black because then it's absolutely heterozygous).

    A black horse does not carry agouti. Agouti (A) is the gene you referred to as BB. it's actually called A). AA and Aa on a black based horse would both turn the horse to a shade of bay.

    The only difference for buckskins, is that you add 1 cream gene (Crcr) to the mix. Genetically a buckskin is a bay horse w/ one cream gene. If there was no agouti, it would be a smoky black.



  20. #20
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    I think saying "If a horse is recessive (heterozygous) for agouti, they would look just like a horse that is dominant (homozygous) for agouti. " is very, very confusing.

    "recessive for Agouti" really should be aa - nothing to modify black, recessive.

    Likewise, "dominant for Agouti" should really be A? - at least one form of Agouti that would dominate the modification of black pigment.

    Agouti in the A, At, or A+ form is a dominant form, and does not have to be homozygous to affect the black pigment. It's still dominant in a red horse, it's just there's nothing to dominate
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