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Arabian Horse Color Genetics Chart


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#1 Pete Hiatt

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Posted 04 April 2005 - 11:41 PM

Equine Genetics of Color
Peter B. Hiatt

There are two colors in Arabian horses, black and red. The other colors are results of modifier genes. The subject can get quite involved. Luckily there is an accurate way to simplify the means of determining the probability of different colors. The chart to be presented assumes genetic codes that are not actual but ARE an accurate way of calculating color probability.

Please note that there are some horse colors that do not exist in Arabians. These include dun, grulla, buckskin, and palamino, for example. Now some people sell “palamino” Arabians and they are actually registered in the palamino registry. But these Arabians are actually chestnuts that approximate the palamino color. Their base color is red however, not yellow. There are about 100 roans in the AHRA registry but this is rare and requires a photograph to be sent to the registry. A horse can have considerable roaning without actually being a roan. Two definitions of importance are “phenotype” = physical characteristics, and “genotype” = the actual genetic makeup of the animal.

Equine Color Genetics Chart

GG---- is gray (homozygous) and can produce ONLY gray but passes on other color genes
Gg---- is gray with a 50% chance of producing gray when bred to a horse of color (any color)
gg---- is NOT gray. It may be bay, chestnut, or black. It can produce gray only when bred to a gray.
ggAAEE is bay, and can produce bay, but cannot produce black or chestnut
ggAaEE is bay, and can produce bay or black, but cannot produce chestnut
ggAAEe is bay, and can produce bay or chestnut, but cannot produce black
ggAaEe is bay, and can produce bay, black, or chestnut
ggaaEE is black, and can produce bay or black, but cannot produce chestnut
ggaaEe is black, and can produce black, bay, or chestnut
ggaaee is chestnut, and can produce chestnut, black, or bay
ggAAee is chestnut and can produce chestnut and bay but cannot produce black
ggAaee is chestnut and can produce chestnut black and bay



This chart assumes three gene pairs (Gg), (Aa), and (Ee). Each parent gives one gene of the embryonic gene pair. The resulting embryo’s gene pairs are a result of statistical chance. This allows us to be able to calculate the probability of the resulting color and what the resultant foal can eventually produce itself.

As per the chart, the order of color dominance is as follows. Gray is most dominant with the large dominant G ALWAYS resulting in gray. Only one dominant G is needed for gray color. Oddly enough, the next most dominant set of genes is a PAIR of recessive chestnut genes (ee). A pair of these genes always results in chestnut unless a G gene is present. A horse will NOT be chestnut unless there are a PAIR of these genes present. The next order of dominance is the bay gene (A). Only one dominant (A) gene is necessary to produce bay if NEITHER dominant G (gray) gene or pair of recessive (ee) genes are present. Finally, the black recessive genes (aa) will result in black unless either gray (G), or recessive chestnut (ee), or bay (A) is present. As a result, the black gene can be present but stay hidden for many generations. We also see that only one bay or gray gene need be present to produce those colors, but both recessive chestnut or black genes must be present to produce those colors.

Here is an example how the genes split up to produce a foal. Let us assume that we have a stallion GgAaEe. This is a gray stallion. He is bred to a mare that is ggAaEe. This is a bay mare. Consider one color set of genes at a time. We are crossing the gray genes first Gg of the stallion to the gg of the mare. There are four possibilities. The first stallion gene is G which crossed with the two mare gray genes gives us Gg and Gg. Crossing the second stallion recessive gray gene with the two mares genes gives us gg and gg. From this we see that the foal has a 50-50 chance of being either gray or colored. Doing the same type of cross with next most dominant gene set (chestnut) gives us EE, Ee, eE, and ee. Only 1 chance in 4 of these is a chestnut. So far, there is a 50% chance of gray and a 25% chance of the REMAINING probabilities that the foal will be chestnut so 25% times the remaining 50% gives us 12.5% or 1 chance in 8 that the foal will be chestnut. Next most dominant is bay. The possibilities are AA, Aa, Aa, and aa. Since we need only one Dominant bay gene for bay, there is a 3 out of 4 or a 75% chance of the REMAINING probability of being bay. This is 50% (gray) minus 12.5% (chestnut) or 62.5% chance of more dominant colors so the remaining possibility is 37.5% (3 of 8) So 37.5% times 75% equals 9 of 32 or about 28% chance of producing bay. The remaining possibility is the chance of black or 100% minus 50% minus 12.5% minus 28% =9.5% chance of producing black…or about 1 in 10 chances. Yeah, a bit of math but fun.
A few examples will be the easiest way to understand how to use the chart. A black horse can have only one unknown color gene. A bay or chestnut has two unknown genes. A gray has five unknown genes. However, if the birth color is known, that simplifies matters. It is also probable that a gray’s flea bite colors explain their bay/chestnut nature but these are often small and hard to distinguish between bay and chestnut. A gray born bay, for example, would be (G?A?E?). That reduces the unknown to 3. A gray born chestnut would be (G???ee). That still leaves 3 unknowns. Please be assured that the identification process and probability calculations are easier than first appears.
Your horse’s registration papers include 4 generations of ancestral colors. The registry’s computer disc is very helpful.
We will now examine examples of how to determine your horse’s exact color code.
Easiest first! If you have a black with a chestnut parent or if that black has ever thrown a chestnut foal, the color code is ggaaEe. Please note that the term “homozygous gray” means that the horse can only throw gray (GG). However, the term “homozygous black” means that the horse can only throw black IF BRED TO ANOTHER BLACK. Two blacks can produce a chestnut (and a 25% chance at that!) if both their codes are ggaaEe. A homozygous black is ggaaEE. You can see that he has no recessive chestnut gene to offer offspring so no chestnut offspring is possible.
If a bay has a chestnut parent or if the bay ever produces chestnut offspring, it is ggA?Ee. Similarly, if it has a black parent or ever produces black, it is ggAaE?.
Chestnut gene codes are found similarly. A chestnut starts out as gg??ee.If it has a black parent or ever produces black, it is gg?aee. If a chestnut ever produces a bay when bred to a black, it is ggA?ee.
Now some folks always breed chestnuts to black to produce blacks. However, you can see that if the chestnut in question does not carry a black gene, the offspring will NEVER be black. The best you can get is an offspring that carries one black gene and then the grandget may produce black.
There is a very rare occurrence of a special black “gene” attached to the chestnut genes that produce black when bred to chestnuts. There is only one Arabian I have heard of that has this gene set up, but naturally, it is not an asil horse.
Grays are harder, but if a gray ever produces color, it is Gg. If it has a black parent or ever produces black, it is aa or Aa. If it has a chestnut parent or ever produces chestnut, it is Ee or ee. The birth color is very important, if it is known. A gray born chestnut has ee, and one born bay is Aa or AA. So you see that the codes can be determined with enough knowledge.
Now my stallion, Rahima, is a bay produced from two grays. So his code is ggA?E?. He has produced chestnut so he is ggA?Ee. He has also produced black so his total code is known as ggAaEe. Armed with this information, we can see that if we breed him to a homozygous black ggaaEE, we can not get either chestnut or gray. The only possible color combinations are Aa, Aa, aa, and aa. So the foal will have a 50-50 chance of being either bay or black. If bay, it will probably be a dark bay. If we breed him to a chestnut who has produced both bay and black (ggAaee) then there is a zero chance of gray (gg only), a 50% chance of chestnut (Ee, Ee, ee, and ee). Next dominant is bay and this possibility is (Aa, Aa, aa, and aa). So this is a 50% chance of the remaining 50% possibilities so .5 x .5=.25 or 25% chance of bay. The remaining probability is black at also 25%.
I have a bay mare so she is ggA?E?. She has a sister who is a chestnut so both parents carried the recessive chestnut gene. That gives my mare a good chance of being Ee. One grandparent was black so she has a 50% chance of having a black gene. She has very dark shoulder and other points so I think she does carry the black gene. I cannot be sure yet, but I think she is ggAaEe. If I cross them, the possibilities are 0% gray (gg only), 25% chestnut, 9/16 bay (about 56%), and 19% black. With practice, you can do it in your head.
I hope this has been of interest to you. As an engineer, I can get a bit carried away with the math. What is really important is the phenotype and not color, but when you are waiting 11 months it makes the winter pass easier with something to keep you busy.

#2 Azure Arabians

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Posted 05 April 2005 - 12:05 AM

Hi Pete,
I have a bay stallion with 15 colts on the ground, He has produced 5 blacks and the rest mohogany and black bays. For now I am assuming he is homozygous black. So is my thinking correct in assuming if he is bred to a black...thats 50% chance of a black, 50% bay, as he does or doesnt not throw a bay gene, then with a bay mare you are adding in another chance of a bay gene, so 75% bay, 25% black and with a chestnut, they can also carry a bay gene that is not expressed correct? So its a 75% bay 25% black again assuming she has a bay gene, if not becomes 50% 50% black and bay. I figure on an average he should have 25% black, 75% bay
Is my reasoning correct?

#3 Pete Hiatt

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Posted 05 April 2005 - 12:21 AM

He is probably ggAaEE since he has not thrown a chestnut. Bred to a black (ggaaEE or ggaaEe) , you are right, it is 50-50 bay or black.

Now bred to a bay (ggAaEE, ggAaEe, or ggAAEe, ggAAEE), it is 75% chance of bay, 25% chance of black IFIFIF the bay mare carries the black gene. If she does not carry the black gene, it is 100% chance of Bay, but a 50% chance of the foal carrying a black gene like daddy.

Now bred to a chestnut, you will get either bay or black. Again, it depends what the chestnut genes are. They can be:
ggAAee 100% chance bay
ggAaee 50-50
ggaaee 75% chance black, 25% chance bay

Again, I describe this as having a black gene only to more easily describe the probabilities.

There is a VERY rare alelle sometimes on the chestnut gene which allows more blacks to be born, but there is only one known Arabian with this, and that is not an asil horse. That shows you how rare it is.

#4 Azure Arabians

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Posted 05 April 2005 - 12:30 AM

have a bay mare so she is ggA?E?. She has a sister who is a chestnut so both parents carried the recessive chestnut gene. That gives my mare a good chance of being Ee. One grandparent was black so she has a 50% chance of having a black gene. She has very dark shoulder and other points so I think she does carry the black gene

I would say yes absolutely youre mare has a "black" gene she has to if she is bay, because bay is a black base coat modified by bay. a bay has to be at LEAST Ee or it wouldnt be bay. So she is for sure Ee or EE...if she didnt carry a "black" gene she would be chestnut. her parents were both Ee Ee if they had a chestnut, (both passed on ee in that senario abd assuming they were both bay) but its is plausable that both though their E E and you have a homozygous black mare with a bay modifier, now if one of the parents was a chestnut you would know for sure you have Ee.
I dont seewhere it has anything to really do with the grand parents being black.

Ee black coat base
EE black coat base
ee read coat base.

is my thinking faulty?
shauna

#5 Azure Arabians

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Posted 05 April 2005 - 12:39 AM

However, the term “homozygous black” means that the horse can only throw black IF BRED TO ANOTHER BLACK. Two blacks can produce a chestnut (and a 25% chance at that!) if both their codes are ggaaEe. A homozygous black is ggaaEE. You can see that he has no recessive chestnut gene to offer offspring so no chestnut offspring is possible.

Do you mean that a homozygous black can produce nothing but black when bred to another black? this is just worded confusing

not trying to nit pick I am just trying to see if I have all this right in my brain.

Technically homzygous means only that they cannot produce a chestnut.

#6 Azure Arabians

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Posted 05 April 2005 - 12:58 AM

Now bred to a bay (ggAaEE, ggAaEe, or ggAAEe, ggAAEE), it is 75% chance of bay, 25% chance of black IFIFIF the bay mare carries the black gene. If she does not carry the black gene, it is 100% chance of Bay, but a 50% chance of the foal carrying a black gene like daddy.

they both carry black genes, there is not ififif black or they wouldnt be bay. The 100% chance senerio of bays doesnt depend on the black gene, it depenes on if the other bay is homozygous bay, then yes you would get 100% bays as bay modifies a black coat and if a horse is homozygous bay its going to through bay no matter what. two bays considering neither one is homozygous

foal would be
aaEe this horse is black (both horses pass small aa no bay )
AaEe bay mom throughs A to make bay
AAEe Bay both through A to make homozygous bay
Aa Ee bay dad throughs A to may bay



all will have black base coat because no chance of ee and actually could be EE (50% chance ) senarios also but that doesnt really matter because all will be black the bay will modify it or not.

btw not trying to pick a fight Ijust thought I had it right in my head now I am confused :blink:

shauna

#7 Pete Hiatt

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Posted 05 April 2005 - 02:09 AM

Shauna, you ARE confused. You are comparing actual color genetic information with my chart which is a simplification of actual. Here is the answers:

First of all, the term homozygous means different things with different colors which is a poor definition and leads to misunderstandings. Homozygous gray is GG and means WHATEVER that horse is bred to results in a gray foal. Homozygous black ggaaEE means simply that when that black is bred TO ANOTHER BLACK, that the result is a black. This just means that the horse in question does not have a recessive chestnut gene.

"if she did not carry a black gene, she would have been chestnut." This is an untrue statement. Chestnut is dominant over the other colors (gray is not a color) IF both genes are recessive (ee). If they are not both recessive, the horse will be either black or bay (AA, Aa, aa). The two have nothing else to do with each other.

"has to have a black gene if bay" No! You can be AA without any black gene and be bay.

Forget modifyers and basecoats. They mean nothing here.

Post #6, no, they do not both carry the black gene. Remember forget base coats and modifyer genes.

#8 heathert

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Posted 05 April 2005 - 01:44 PM

I am very lame today, I read thru this 2 x's and still don't get it :blink:

can you show me with these examples ? PLESASE :D

#1 Chestnut Stallion bred to fleabitten grey mare ( flea bites are chestnut)
Stallions sire & dam are both Bay
Mares sire is black, dam is grey
resulting foal ?

#2 Black Stallion with both parents black bred to bay mare with bay sire and grey dam
resulting foal ?

Thanks, I have always wanted to learn this !


Grover say, "Hey Elmo, I wanna play..."

#9 Pete Hiatt

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Posted 05 April 2005 - 04:38 PM

[quote=heathert,Apr 5 2005, 10:44 AM]
I am very lame today, I read thru this 2 x's and still don't get it :blink:

can you show me with these examples ? PLESASE :D

#1 Chestnut Stallion bred to fleabitten grey mare ( flea bites are chestnut)
Stallions sire & dam are both Bay
Mares sire is black, dam is grey
resulting foal ?
#1 Stallion: ggA?ee
#1 Mare: Gg?aee

Birth color and flea bites are important info but there are things missing to give final genetic code. Here goes, the foal would have 50% chance of gray, 50% chance of chestnut. Foal has at least 50% chance of carrying the black gene.

Knowing what the sire and dam has produced before when bred to what can fill in the unknown blanks.

The genetics experts say liver chestnut is not related to the black gene, but all the livers I know of due carry the black gene.

#2 Black Stallion with both parents black bred to bay mare with bay sire and grey dam
resulting foal ?

#2 Stallion: ggaaE?
#2 Mare ggA?E?

Again, there are some unknowns. Foal colors and dam's parent's colors would be helpful, but here goes:

Foal: 0% chance gray. If BOTH parents carry a recessive chestnut gene then 25% chance of chestnut. The chances are only one does which would mean 0% chance of chestnut. The black gene is rare so chances are the mare is AA. That would mean you can only get a bay foal but it would carry a black gene. The foal would likely be ggAaE?.

Remember this is a simplified version of actual genetic code but absolutely accurate for statistical purposes.

#10 heathert

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Posted 05 April 2005 - 04:44 PM

Thank you! I think I get it ;)


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#11 Pete Hiatt

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Posted 05 April 2005 - 04:49 PM

Remember to forget all actual genetics you know and think SIMPLE. (I am very good at thinking SIMPLE ;) ;) ;) )

#12 Samrdr1

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Posted 05 April 2005 - 06:33 PM

math moron here. Not proud to admit it but will.

the stallion that I bred to is ggaaEe and the mare is ggA?Ee I know that they can produce chestnut because I am looking out the window at one, but what are the other probablities? I find this very interesting and would like to know.
Thank you

Sue N

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#13 Pete Hiatt

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Posted 05 April 2005 - 07:10 PM

QUOTE(Samrdr1 @ Apr 5 2005, 03:33 PM)
math moron here. Not proud to admit it but will.

the stallion that I bred to is ggaaEe and the mare is ggA?Ee  I know that they can produce chestnut because I am looking out the window at one, but what are the other probablities? I find this very interesting and would like to know.
Thank you

Sue N
View Post



0% Gray
25% chance chestnut
75% chance Bay unless the unknown is a black then: 37.5%bay, 37.5% black
the foal will carry the black gene

#14 Azure Arabians

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Posted 06 April 2005 - 12:11 AM

I dont understand by what you mean if the unknown is a black,
isnt A stand for bey?? so the question is if this horses is homo or heterozygous bey?
what do you mean by the unknown being black? and carrying the black gene?

okay, I think i figured it out, your chart calls aa a black recessive gene, to me the way I learned it, aa would be a horse that is recessive bay gene, meaning it doesnt have the dominant bay modifier. confusing because you are calling the bay combinations "black" where to me its the E combination that determines black.
and "A" determines bay...sort of a backwards logic saying a missing dominant bay increases chances of black correct?

but when I see your senario above as missing part of the bay gene not part of a black gene, I see it and think the unknown horse is a bay...because it only takes one large "A" to bay, and you are looking to see if there is a little a which means he is hetero or anothe big A wich means hes homo and can only have bays. (but ofcours a bay doesnt show up on chestnuts)

Heres another thing....
"
if she did not carry a black gene, she would have been chestnut." This is an untrue statement. Chestnut is dominant over the other colors (gray is not a color) IF both genes are recessive (ee). If they are not both recessive, the horse will be either black or bay (AA, Aa, aa). The two have nothing else to do with each other.

this really confuses me because chestnut and black are differnt FORMS of the SAME gene, it is ONE gene that detemines black or chesnut, depending on the combination. Dominant form is black, recessive form is chestnut.

the first line I mean if she didnt have a black gene her BASE coat would be chestnut. but i am looking for a big E your looking afor a little "a" for that correct?

so thats where I am totally confused but I do see your logic now. although it backwards from how I learned!

shauna

#15 RousseauRidge

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Posted 06 April 2005 - 12:38 AM

Vetgen color prediction

VetGen's new ChromaGene™ Coat Color DNA test for Equine Breeds
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#16 Pete Hiatt

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Posted 06 April 2005 - 01:22 AM

Yes, Shauna, you are getting it. My chart is NOT actual genetics but a simplification of actual equine genetics so that one can easier calculate statistics. It is all explained in the verbage with the chart.

Now in reality, there are only two genetic colors in Arabians, red (chestnut) and black. Gray is the absence of color and bay is due to modifier genes reaction on black. Then alleles come into play and who wants to deal with all that.

Remember to THINK SIMPLE ;) The chart gives all the color combination possibilites.

Everyone can use this chart once they get used to it, although as an engineer I may think it is easier than it actually may be.

By the way, I have never heard anyone use the term "heter or homozygous bay". I suppose homozygous bay may mean that the horse can only produce a bay when bred to another bay. That would mean a code of ggAAEE. That would also produce a bay when bred to a chestnut or black. It would not always work for a gray since gray is dominant.

#17 Azure Arabians

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Posted 06 April 2005 - 02:36 AM

He pete,

Thats why I am confused, A usually stands for the agouti gene which is the bay modifier. Well genetically its homozyous bay if it has two large As. Doesnt mean anything more than it cant have black because it can only throw a large A which causes black to turn bay. it COULD even be a chestnut! as it could carry the bay gene but bay doesnt effect the chestnut gene... will explain below...



heres how I have learne it

grey is dominnat over everthing
bay over black, black over red
one gene produces the base coat the -E alleles Dominant form Ee causes black
recessive form ee chesnut.(red) Chestnut is called a recessive gene because its only expressed in the recessive form. Dominant meaning it only needs one large E to be expressed.
So black is dominant over red.

The gene that contrrols the distubution pattern of black pigment is called the agouti gene "A" the dominant allel Aa(bay gene) when combined with a dominant Ee (black) limits the black hair to points. it changes black to bay. So there for all bay horses have dominant Aa and Ee in their make up. and could be AA meaning they they only give the dominant form to their offspring creating bay out of a black coat (can never have a black)
if a horse has the recessive agouti aa then black will not be modified. so its either black or red base coat but not bay. Because the chestnut ee has no black hair it the horsecan have Aa but it is NOT expressed




so I learned to look at the E see what combinations make black and chestnuts...figure % and then chances of bay...figure % but you have to remember if the bay gene is with a ee its not expressed. and of course than gray.

so you can see why I was soo darn confused...
heehhe
shauna

#18 Pete Hiatt

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Posted 06 April 2005 - 11:55 AM

More fun to think simple, isn't it? wink.gif




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