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From: jespah@milton.u.washington.edu (Kathleen Hunt)
Newsgroups: sci.bio
Subject: Still more about horse color
Message-ID: <16064@milton.u.washington.edu>
Date: 8 Feb 91 10:47:00 GMT
Organization: University of Washington, Seattle
Lines: 87

News flash!  I have just found out that the "dilution" gene is controlled
by a different locus than the chestnut gene.  (And yes, grey is controlled
by yet another locus.)  The genes we're dealing with are:

E	Non-chestnut; allows the horse's "natural" coloration (black, bay,
	brown; determined by other genes) to show.
e	Chestnut.  What this gene does is make the pigment "phaeomelanin",
	a reddish or yellowish pigment, appear all over the horse's body,
	regardless of what the black, bay, & brown genes say.
	(black, bay, & brown horses have the darker "eumelanin".)
A horse with EE or Ee is non-chestnut (usually bay).  An ee horse is chestnut.

Then, at a totally different gene locus, we have:
C	Allows ordinary expression of bay, black, chestnut, etc.
c	The dilution allele.  This gene is incompletely dominant, as follows:
		CC   The horse is bay or chestnut, as determined by the E locus
			and other genes.
		Cc   The horse's coloration is "diluted":  a genetically bay
			horse becomes buckskin, and a genetically chestnut
			horse becomes palomino. (By "genetically" I mean 
			"according to what the E genes say")
		cc   A "cremello" horse; a blue-eyed light cream or white
			horse (not to be confused with the lethal dominant
			"white" allele, which is controlled by yet another
			locus...)

All right.  So this is what we have:

	Bay-------Chestnut         Palomino-----Chestnut
        E?CC  |     eeCC             eeCc    |    eeCC
              |                              |
             Bay sire-----------------Palomino dam 
               EeCC         |             eeCc
                            |
                           foal 
                            ?

                Possible sperm from stallion:
                      EC            eC
               |----------------------------|
Possible    eC | EeCC, bay   |  eeCC, chest.|
eggs from      |----------------------------|
mare:       ec | EeCc, buck. |  eeCc, palo. |
               |----------------------------|

So you have equal chances for bay, buckskin, chestnut, and palomino!

This is all assuming that the sire & dam aren't carrying around other
miscellaneous recessive alleles.  You may have noticed that the E allele
just lets a horse be "not chestnut", which isn't necessarily bay.  There
are other genes that control whether a "not chestnut" horse will turn out
to be black, bay, brown, or whatever.  In the chart above I assumed that
"not chestnut" would be bay.

I have found out that there are at least 16 genes controlling horse coloration.
Here's a quick rundown, just to give the idea:

Locus	General effect
-----------------------
A	Bay vs. black/brown 
B	Black vs. brown 
C	Fully expressed A & B colors vs. diluted A & B colors vs. cremello
D	Dun vs. non-dun
E	Dark colors (bay, black, brown) vs. red colors (chestnuts)
F	In chestnuts:  flaxen mane & tail vs. non-flaxen mane & tail
G	Grey vs. non-grey (Grey will override other genes)
P	Pangare (lightened colors, e.g. a black horse is lightened to 
	seal brown) vs. regular colors
Sty 	Smutty (black hairs mixed into coat) vs. non-smutty (clear color)
Z	"Silver dapple" (changes dark pigment to flaxen) vs. non-silver dapple.
	Only important in a few breeds.
R	Roan vs. non-roan.  A lethal dominant (all RR's die)
T	Tobiano vs. non-tobiano
O	Overo vs. non-overo. 
Sb	Sabino ("medicine hat", or very high white socks with very white 
	face; common in Clydesdales, for instance) vs. non-sabino
W	White (often called "albino" but these horses have dark eyes) vs.
	non-white.  A lethal dominant (all WW's die).
Apl	Non-appaloosa vs. brightly patterned appaloosa (the heterozygote)
	vs. few-spotted appaloosa (the recessive)


Yow!  Pretty complex, huh?
Epistasis & pleiotropy galore.

Kathleen
jespah@milton.u.washington.edu