APHA color question - The Horse Forum
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post #1 of 15 Old 09-17-2009, 01:18 PM Thread Starter
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APHA color question

I had a couple questions on paint colors that I was hoping someone could help me with and explain. My first question is how do you know if a horse is homozygous or heterozygous?
How is the coat pattern be determined? Such as, if you bred a tobiano to a overo, what would be the resulting pattern?
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post #2 of 15 Old 09-17-2009, 01:36 PM Thread Starter
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Also wanted to ad, my mare is a sorrel/white tobiano. She threw a solid chestnut foal when bred to a chestnut quarter horse. What does that tell about her as far as throwing color?
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post #3 of 15 Old 09-17-2009, 02:51 PM
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APHA.Com - The Genetic Equation

"When you're young and you fall off a horse, you may break something. When you're my age, you splatter." -- Roy Rogers
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post #4 of 15 Old 09-17-2009, 05:57 PM Thread Starter
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Thanks for the link!
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post #5 of 15 Old 09-17-2009, 06:29 PM
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You can also play with the genetic calculator at Color Calculator

It will show you all the probabilities based on pattern/color.

On the sixth day, God created the Quarter Horse.
On the seventh day, he Painted the good ones.
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post #6 of 15 Old 09-17-2009, 11:24 PM
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It tells me that your mare is heterozygous (homozygous would through tobiano everytime). The coat pattern is determined by what is given by each parent.

Tobiano can be heterozygous which means the horse carries only one Tobi gene (and will pass it 50% of the time), and homozygous which means a horse that carries two tobi genes and will pass one 100% of the time.

Overo is alittle more complicated, partly because there are more then one type of overo, and because some forms (maybe all, I don't know) can hide, and a horse can look solid.

If you breed hetero tobi to overo, you may get a tobi, overo, or a combo of both, or you may get a solid foal.

Hope that helps.
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post #7 of 15 Old 09-18-2009, 05:05 AM
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chestnut to chestnut ALWAYS gives chestnut, so that doesn't tell you anything ;) (other than being likely heterozygous tobi... I say likely, because did the foal have any white? There are proven (tested) tobianos who just have white on their legs! But white legs can also be caused by splash and sabino (two of the three over genes, the other is frame)).
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post #8 of 15 Old 09-18-2009, 08:22 AM Thread Starter
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Would I have to bred to a homozygous tobiano or overo stallion in order to have a 100% chance of a colored foal? If I am understanding correctly, if I bred her to another heterozygous stud there is a 25% chance the result could be a solid foal, correct?
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post #9 of 15 Old 09-18-2009, 06:36 PM
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Yes, you'd have to breed her to a homozygous tobi to guarantee color. To my knowledge there are no homozygous overos (although a horse can be tovero and be homozygous for tobi, that would give you a 100% chance of tobi and a 50% chance for tovero).

However, just remember color should be the last thing to consider when breeding. It is my opinion that there are enough horses out there that you should be able to get the one you want, in the color you want, but conformation, temperment, health, etc., should always come before color.
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post #10 of 15 Old 09-18-2009, 07:36 PM
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I see so many horses listed as solid when in fact they are minmal expression tobianos.

Here is one of my stallions Black----I knew when he was a foal he was tobiano because of the white socks. His breeder said no way he was solid. So after I bought him I had him tested and yep he carries 1 copy of the Tobiano gene

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