Help understanding homozygous vs heterozygous - Page 2 - The Horse Forum
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post #11 of 20 Old 12-15-2013, 10:34 PM
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Homozygous just means two of the same gene, which can be two dominant genes (example AA) OR two recessive genes (aa). Homozygous means one dominant and one recessive.
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post #12 of 20 Old 12-15-2013, 10:41 PM Thread Starter
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Quote:
Originally Posted by Tryst View Post
Homozygous just means two of the same gene, which can be two dominant genes (example AA) OR two recessive genes (aa). Homozygous means one dominant and one recessive.
ok, I understand that now, thank you.

The buckskin mare in the first photos is a maden. We were taking guesses as to what colors we might get. then debating the pinto factor & discussing homoz vs hetero & a simple conversation got very confusing quickly. I knew the color experts on here would understand this far more than I ever could

Cowgirl up!
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post #13 of 20 Old 12-15-2013, 11:07 PM
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I know next to nothing about paint genetics, but I understand base coats and modifying genes fairly well.

There are only two base coats possible; homozygous black (EE), heterozygous black (Ee), or sorrel/chesnut (ee). You can only have a homozygous sorrel, because the gene for sorrel (e) is recessive. Therefore any copy of the dominant gene (E) will make the horse black.

So assuming that stallion is black, he could be either homozygous black (EE) or heterozygous black (Ee). If he's homozygous, he won't produce a sorrel based foal, period. (so no red duns, no palominos, no red roans etc.)
I'll attempt to do a punnet square here, but I can't guarantee that it'll turn out lol!

Let's say he's homozygous black and the mare is sorrel; (in parenthesis are the possible offspring
------
Mare- e - e

Stud- E (Ee) (Ee)

E (Ee) (Ee)

So this stud bred with this mare will produce a heterozygous black foal.

Here's a heterozygous black stud with a homozygous sorrel mare;
---------
Mare- e - e

Stud- E (Ee) (Ee)

e (ee) (ee)

There is a 50% chance of a red based foal and 50% chance of a heterozygous black based foal.

Did that make any sense? Haha, it's nearly impossible to show you without a picture or drawing, but I tried!
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post #14 of 20 Old 12-15-2013, 11:08 PM
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It took out all my spaces :(
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post #15 of 20 Old 12-15-2013, 11:14 PM
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Do you know the mare's parents colors?

As far as pinto the only way to tell 100% is to test him. Those foal's don't tell us much since they got the color from the dam.
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post #16 of 20 Old 12-16-2013, 12:07 AM
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Here's a pretty good document to read about genes;

http://www.google.com/url?q=http://w...PAfOujYiDfv6lg
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post #17 of 20 Old 12-16-2013, 12:32 AM
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Quote:
Originally Posted by Tryst View Post
Homozygous just means two of the same gene, which can be two dominant genes (example AA) OR two recessive genes (aa). Homozygous means one dominant and one recessive.
Tryst made a typo HETEROZYGOUS means one dominant one recessive
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post #18 of 20 Old 12-16-2013, 01:18 AM
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Color genetics are more confusing when discussing homozygous vs heterozygous so lets use the pattern gene instead.

First off is the very basic that each foal inherits 2 sets of genes, one set from the sire and one set from the dam. Keep in mind that sire and dam each have 2 sets of their own so are capable of passing along a gene from either set.

Tobiano is a dominant gene but if the mare or sire is heterozygous for tobiano then approximately half the time you'll get a tobiano foal and half of the time a solid foal if the other parent is solid. If both parents are heterozygous for the tobiano gene then the percentage of a tobiano foal raises to like 75%. (I'm doing this from memory so may not be totally accurate in my percentages but just trying to give you the jist of how it works.) And that 75% breaks down into something like 50% heterozygous and 25% homozygous for the tobi gene.

If the dam or sire is homozygous for the tobiano gene this means they inherited a tobi gene from their sire and a tobi gene from their dam. They will produce color 99.9% of the time and recent studies have shown that the .1% is still tobiano just not showing any white but will still throw the tobiano pattern onto their own offspring 50% of the time just like any other heterozygous tobi.
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post #19 of 20 Old 12-16-2013, 05:24 AM
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I just want to clarify something quickly.

Homozygous means there are only the same genes present, to translate it literally. So this means that there are two the same, AA, aa, EE, ee, ToTo, tt, OO, oo and so on.

Heterozygous means there are differing genes present. This does not necessarily mean that they are one dominant and one recessive - remember that several different loci (the places pairs of genes are found) have multiple dominant mutations. So looking at the agouti loci for example, there are several ways the horse can be homozygous (AA, aa, AtAt, A+A+) and more ways to be heterozygous (Aa, AAt, AA+, aAT, aA+, AtA+). Some of these combinations are two differing dominant genes.

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post #20 of 20 Old 12-16-2013, 08:17 AM
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Without knowing if the sire and dam are homozygous or heterozygous and assuming the sire IS black and the dam IS buckskin here are the chances (about, without decimals)

35% buckskin
35% bay
11% smokey black
11% black
3% palomino
3% sorrel

That's considering that the parents can be either of these;
Sire- Eeaacc or EEaacc only
Dam- EeAaCc, EEAaCc, EeAACc or EeAACc

(A-agouti[bay]gene C-cream gene E- black)

If the mare is buckskin (assuming she isn't a dunskin [buckskin + dun] or anything) she has to have a dominant black gene (Ee or EE) the agouti gene (Aa or AA) and only one copy of the cream (Cc) (two copies would make her a double dilute/perlino)

And assuming the sire is honestly black he has to have a dominant gene for black (Ee or EE) no agouti (aa) and no cream (cc)

The two foal pictures you posted don't help much as far as figuring if the sire is homozygous black (EE) or heterozygous black (Ee) because neither of the foals look to be red based. A black foal is obviously black based and a bay foal is black based with at least one copy of the agouti gene (EeAa, EEAa, EeAA, or EEAA). And of course he could be brown (A+ I believe) but I didn't really account for that.

If you could tell us more information, such as other foal this sire has produced, the color of the sire's parents or the color of the dam's parents it would be very helpful In narrowing down the possible genotypes of the mare and stallion.

Anyone feel free to correct me, I'm new to genetics!
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