color genetics questions - The Horse Forum
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post #1 of 26 Old 05-08-2008, 07:52 PM Thread Starter
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color genetics questions

Okay, so...

Why do I never see horses that are homozygous overo? only tobiano?

What is the punnet square break down for a horse with one overo and one tobiano parent?

APHA told me that Dez could not be homozygous because he has one overo parent and one tobiano, but both are color, so why can't he still be homozygous for "paint"?

I am sure I will have more questions as I get more answers, but I would really appreciate anyone's insight. :)
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post #2 of 26 Old 05-08-2008, 08:16 PM
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I never actually thought of that but now that you bring it up it's true.

I'm actually interested in reading these answers as well :)
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post #3 of 26 Old 05-09-2008, 06:32 AM
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Hmm, ok.

'Paint' is not a gene, so a horse can neither be homozygous or heterozygous for it. Beyond that, 'paint' is a breed, rather than a colour - the more correct term for a horse with white patches of any kind is 'pinto'.

'Pinto' is not controlled by one gene, but is a pattern created by one or more white pattern genes which include:
A horse can be homo or hetero for any of these (although homo for frame results in OLWS, see below)

'overo' is not a term I like to use since it is a 'blanket' word which basically covers all of the white patterns EXCEPT tobiano. It comes from when people didn't recognise that there were all these different patterns and was just used to mean 'white pattern that isn't tobiano'.

HOWEVER, a lot of people now use the work 'overo' when they should say 'frame' or 'frame overo'. It's important to make this distinction because if a horse is homozygous for frame overo, it has a condition known as 'overo lethal white syndrome', which results in a completely white foal and is lethal within days or hours of birth. This is why two horses carrying frame (and they MUST be tested, as they could be hiding it) should NEVER be bred together - you would have a 25% chance of a foal that would have to either be euthanised or die horribly.
This is another reason I don't like the blanket term 'overo' because it's only FRAME overo that is lethal - the distinction is important.
So, in short, the reason you never see a horse described as homozygous 'overo' is because nowadays, 'overo' pretty much equals 'frame overo' and a homozygous frame overo would be, well, dead.

Dez COULD however be homozygous for tobiano BUT only if both his parents carry the gene, and even then it's not a 100% chance he would be homozygous.

Did you test his sire and dam for frame?
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post #4 of 26 Old 05-09-2008, 12:07 PM
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Interesting.. Thanks Kyani!
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post #5 of 26 Old 05-09-2008, 01:03 PM Thread Starter
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I don't have access to test either Dez' sire or dam. His papers say that his sire is Overo (don't know which kind). His dam is tobiano. I send Dez' DNA in because it is required to list a stallion, but I don't know if they tested for anything - just got a thing back stating that he was an eligible sire for APHA. At that point I asked them if he might be homozygous, and they said that he could not be, because he had an overo parent and a tobiano parent.

I am aware that there is not a paint gene :), but I just meant basically is there some other homozygous color combo than tobiano?

I am really trying to get a visual idea of what alleles each color type carries. for instance, what possible alleles could a frame overo carry. Can different apha horses carry genes for two different color types?

I could send DNA in for testing, but all of this time, I was under the impression that there was no way he could be homozygous for tobiano, and did not want to waste $ getting a result I was already aware of.

I am not sure if Dez needs to be tested for OLWS, as he is a tobiano, but from what I am learning, it might be possible to be carrying a frame gene?
Any idea what possible gene combos Dez could be carrying right now? And what does it mean that he is "showing" the tobiano gene.

I don't know if this adds insight, but:
he was bred to a sabino mare and produced a sabino (sorrel/white I think) foal.

he was bred to the chestnut AQHA mare and produced a chestnut and white tobiano foal.

He is sorrel/white tobiano

What pattern traits are recessive? dominant?
what color traits are recessive? dominant?

From what I remember, because Dez is sorrel, that means that he is homozygous for the color sorrel/chestnut, as that is a recessive gene??

I am really curious about all of this. Just when I thought I was understanding "paints", a whole new list of questions popped in my head. :)
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post #6 of 26 Old 05-09-2008, 04:56 PM
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He is homozygous recessive for chestnut, ee is a chestnut, EE or Ee is black (with modifiers and such changing the colors from there).
As for anything other than basic color I'm totally lost.
This site seems to have a lot of info on paint genetics that might help.
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post #7 of 26 Old 05-10-2008, 08:44 AM
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Check out this genetic calculator...
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post #8 of 26 Old 05-12-2008, 02:34 PM
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As far as being homozygous for red, he isn't. He was always bred to something similar to himself to throw the same color. If you bred him to a black tobiano mare, she could throw a red black or bay foal depending on what genes she throws and if she was dominant for black.

With the color, every time you breed two tobianos together, there is a chance for a homozygous baby, TT. If he has ancestors on his sire's side that are tobiano, i believe there is such a thing called the "Barlink Factor" that could mean he was still passed those genes. Don't quote me on that though, since I'm not sure.

If he could be a carrier of the OWLS gene, I would have him tested, that way you can be sure not to breed to a mare that is also a carrier, if that mare is tested. There is a chance that foal could be a lethal white, which would die within a week.

Alot of breeders won't even breed a horse that is a carrier of OWLS, hopeing that if people are more careful on what they breed, the OWLS gene will fade away.

Genetics for my tovero mare who is now sold was TtEeAa, she was a bay tovero and threw a solid chestnut filly with a little white throughout her coat, but I don't think its enough to reg. her as a sabino. My mare had lots of roaning on her neck and on the edges of her pattern, and might be considered a sabino, since she threw it to her foal. IDK lol.
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post #9 of 26 Old 05-12-2008, 02:36 PM
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Oh, I forgot to ask, is he a tobiano or tovero? The toveros carry an overo gene from a parent or grandparent and he might need testing.
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post #10 of 26 Old 05-12-2008, 04:29 PM Thread Starter
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Thanks for the genetics calculator - SO Cool!!!

Dez is not a tovero...he is tobiano. :)
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