From: tracy@scoraz.resp-sci.arizona.edu (Tracy Scheinkman)
Newsgroups: rec.equestrian
Subject: re: Horse color genetics program
Date: 22 Jan 93 20:26:09 GMT
Distribution: usa
Organization: University of Arizona, Tucson, AZ

In Article 11154 (10 more) in rec.equestrian:
From: sbishop@desire.wright.edu

>Not that hard if you use the 'square' method.  Just make a big square,
>list possible variations of the mare vertically, possible variations
>of the sire horizontally.  Works great, takes a few minutes and the
>margin of error is small.
>
>	Bay stallion x Grey mare
>
>Stallion out of bay mare and chestnut stallion so genotype is almost
>certainly Bc.
>Grey mare out of chestnut mare and grey stallion, known to have produced
>greys, chestnuts and bays, depending on what she is bred to.  Genotype
>almost certainly Ggcc
>
>	Stallion	       	Bc     bc
>                            _____________
>	M		Gc |  GBcc  |  Gbcc |
>	a                  _______________
>	r               gc |  cBcc  |  gbcc |
>	e                  _________________
>
>I think that's right, with this  combination you get 50% grey, 25% Bay,
>25% chestnut.
>
>Tracy, do I have it right?

	You sure do.  The square method is the method I use when trying
to figure out genetic possibilities.  Just a few corrections though --
make sure you do not run gene locuses together.  For example in the
case given above -- the Grey locus (G,g) the Bay locus (B,b) and the 
chestnut locus (C,c) are separate locuses (I'm using Sue's symbols, the
Sponenberg book uses G for grey, A for bay, and e for chestnut).
The horse examples given here have B?ccgg in the stallion's case for
genotype and ???cGg for the mare.  If the mare was chestnut when she
was born before turning grey then her genotype would be ??ccGg, if she was 
bay before turning grey then her genotype would be B?c?Gg because the color 
you see on a baby when born is the color it would have been as an adult if
the greying gene was not present.  Grey is a gradual progressive color
change.  The above genotypes obviously require the use of an 8x8 square 
in order to predict the percentages correctly as all possibilities must 
be equally represented.  The need for this is easily seen in the case of 
two horses who are totally heterozygous (meaning they have one of each
gene possible at each locus):

	Stallion BbCcGg   x  Mare BbCcGg

	Genotype headers for columns and rows would be:

		BCG BCg BcG Bcg bCG bCg bcG bcg
	BCG
	BCg
	BcG
	Bcg
	bCG
	bCg
	bcG
	bcg

	Keep in mind that the letter symbols I am using here are Sue's
symbols, not the common ones used in the Sponenberg book.  There 
would be 64 squares and while many of them would have duplicate
genotypes figuring out which genotypes represent which color and adding
up the number of squares with each color will give you a good estimate
of the chances for each color.  When one or more gene is unknown things
become more complicated still.
	The difficulty in physically drawing out the squares and filling
each block is mountainous when dealing with more than 3 locuses and in
those cases a computer program would be useful.  Fortunately most questions
involve no more than 3 locuses, and some of those involve horses that are
known to be homozygous in one or more locuses, for example a chestnut
is homozygous for chestnut and for non-grey, though we could tell nothing
about the bay locus from just looking at the horse.

				Tracy and everybody
Newsgroups: rec.equestrian