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Re: RC: Surprise!...colors

In a message dated 3/24/00 11:49:44 AM Pacific Standard Time, writes:

<< The only horse I can think of off the top of my head that was black from 
 grey's was Hillany Mistanny, but I would like to learn more about the whole 
 color-genetics thing!!! >>

OK, first of all, you are dealing with three different color genes.  The 
first one is for black pigment vs. no black pigment, and in genetic shorthand 
is called E.  To get black pigment, you have to have the big E allele 
(dominant version of the gene), and if you get two little e's, you get no 
black pigment, period.  The second gene here is called the agouti gene (A) 
and it causes black pigment to be segregated on the points and to the mane 
and tail.  If you have no big A allele for agouti (which is dominant), then 
you have two little a's, so if the E allele for black is present, you can 
have a black.  Most Arabs DO have the agouti gene, so that is why black is so 
rare--those who have the black pigment gene also have the agouti gene and are 
therefore bay.  Now, we'll throw the third gene in the soup, which is 
grey--it is dominant (hence is a big G), and no matter WHAT other color genes 
are present, the offspring will turn grey if he has a big G.  To not be grey, 
he has to have two little g's.  

So--to your example of how to get a black from two grey parents.  First of 
all, you've already stated that both had a non-grey parent, so you KNOW that 
both are heterozygous for the grey gene.  (They can also be heterozygous from 
two grey parents, provided one of them was heterozygous, but you don't "know" 
that from looking at either the horse or the pedigree.)  In genetic 
shorthand, both parents are Gg for the grey gene.

You also know that at least one parent had to be at least heterozygous for 
the agouti gene, since one parent had a black parent.  This would be 
represented as Aa in genetic shorthand.  If your foal is truly black, then 
obviously the other parent was at least heterozygous for the same gene.  
Without knowing the parent's actual birth colors, you don't know if they were 
bay-turning-grey, chestnut-turning-grey, or black-turning-grey.

OK, on to the E gene.  Obviously, at least one parent had to pass on the big 
E allele for black pigment.

So--your grey parent that had a chestnut parent could be any of the following 
combos:  GgEeAa, GgeeAa, or Ggeeaa.  The parent with a black parent could be 
any of the following:  GgEEAa, GgEeAa, GgEEaa, or GgEeaa.  

To get a black foal, both parents had to pass on their small g's and their 
small a's, and at least one parent had to pass on a big E (or else the foal 
would have been chestnut).  So the two possible combos for your foal, if it 
is a true black, are:  ggEeaa, or ggEEaa.

It IS unusual to get this combo because of the frequency of the big A allele 
for agouti, but certainly not impossible--and the small a allele is even more 
common from the QH side than from the Arab side.


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