jespah@milton.u.washington.edu (Kathleen Hunt) (02/08/91)
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.eduannw@ncsugn.ncsu.edu (Ann Williams) (02/09/91)
Kathleen, I don't dispute you. I refer you to another book by R. Geurts called Hair Colour in the Horse. Geurts writes about different alleles than your If you are interested in the book and can't find it, let me know. Ann Williams