Equine coat color genetics

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For less technical information on horse colors generally, see Horse coat color

There are currently two theories of equine coat color genetics: Dr. Ann Bowling's and Dr. Phillip Sponenberg's. These theories overlap, and have more similarities than differences. This article is a combination of the two gene theories, presented in Dr. Bowling’s format.

Contents

[edit] Background

A basic understanding of heredity is helpful for this subject. For a quick lesson in genetics and heredity, see the article on alleles or this tutorial on inheritance.

Regarding references to melanin (pigment or coloring): There are two chemically distinct types of melanin, the red–yellow phaeomelanin and the brown–black eumelanin. All coloration genes in mammals affect either the production or final effect of these two chemicals.

The terms Alleles and Modifiers are used interchangeably in this article.

Socks, Star, Strip, and Snip genetics are not yet fully understood but are probably influenced by multiple alleles.

[edit] Alleles and effects

Alleles Effect of combined pairs of alleles
W W
w		 WW: Lethal. Results in incomplete colon and the inability to defecate. Probably from a gene near W or O but not influencing the color directly.)
Ww: Horse typically lacks pigment in skin, hair and eyes and appears to be white.
ww: Horse is fully pigmented.
G G
g		 GG or Gg: Horse shows progressive silvering with age to white or flea-bitten, but is born a non-gray color. Pigment is always present in skin and eyes at all stages of silvering. Gray horses range from white to dark gray depending on the proportion of white hairs in the coat, horses' coats gray in a manner similar to graying in human hair.
gg: Horse does not gray with aging.
E E
e		 EE or Ee: Horse forms black pigment in skin and hair. Black pigment in hair may be either in a points pattern or uniformly distributed.
ee: Horse has black pigment in skin, but red pigment in hair.

Horse will not appear red if white, gray, roaning, or dilution genes are present.
E+ also noted as Ed. E+
e+		 Contrary to popular belief, this does not actually occur on the same locus as the E and e alleles.

E+E+or E+e+: Dominant black, non-fading – Overrides the A or bay allele resulting in an all black horse.
e+e+: no effect. Unproven theory.[citation needed]
A A
a		 Agouti-Bay: Acts on "E" to restrict eumelanin, or black pigment, to points, allowing red coat color to show on body. Has no effect on "e" or red.
AA or Aa horse has black hair (E not E+), then that black hair shows only in points pattern (usually mane, tail, legs, sometimes tips of ears).
aa: If horse has E allele, then horse will be uniformly black.
C C
c		 Dilution factor cream gene: Acts three possible ways

1) cc: No dilution factor, horse is fully pigmented.
2) Cc: Single dilution factor, results in Palomino, Buckskin or Smoky Black. Red pigment is diluted to gold with cream to white mane and tail; black pigment is not visibly altered on buckskin or black horses, though genetic testing can reveal "smoky black" coloration.

3) CC Cremello or Perlino: Double dilution factor. Red pigment is diluted to a pale cream (cremello). Black pigment is diluted to a reddish shade. Skin and eye color are also diluted, and blue eyes are common with double diluted creams.
D D
d		 DD or Dd: Dun horse shows a diluted body color to pinkish-red, yellow-red, yellow or mouse gray and has dark points including dorsal stripe, shoulder stripe and leg barring.
dd: Horse has undiluted coat color.
TO TO
to		 TOTO or Toto: Produces regular and distinct ovals or round patterns of white and color that extend down over the neck and chest, giving the appearance of a shield.
toto: No tobiano pattern present.
O Also noted as Fr or FrO O
o		 OO or Oo: Overo or Frame Overo pattern - Color forms a frame around white patterning. Overo as an allele is different from overo as a registration since the registration also includes the splash and sabino alleles. Homozygous associated with lethal white syndrome.
oo: No overo pattern present.
Ch Ch
ch		 Champagne: A rare dilution gene that creates pumpkin-colored freckled skin, amber or blue eyes, and gives a bronze cast to hair. The skin surrounding the eye must be pink with freckles. A champagne horse is born with a darker foal coat that lightens with maturity.
ChCh or Chch: Champagne dilution evident (See Genetic Formulas Chart below.)
chch: No champagne dilution
F F
f		 Effects visible on red or chestnut colors only.
FF or Ff: Red points on ee horses.
ff: Flaxen points on ee horses. Unproven theory.[citation needed]
P P
p		 Pangare (Mealy): The effects of this allele appear along the underside of the horse, in the "soft" parts, or on the muzzle, behind the elbows, in the flanks, on the buttock, above or around the eyes, and along the belly.
PP or Pp: Mealy or Pangare dilution of pigment.
pp: no mealy looking lightening of pigment.
Rn Rn
rn		 RnRn or Rnrn: roan pattern of white hair mixed in with base color. There is no lethal roan question.
rnrn: No roan pattern.
Rb Rb
rb		 Rabicano: Partial roaning with ‘coon’ tail markings. Thought to be a dominant gene by Sponenberg.
RbRb or Rbrb: Rabicano markings.
rbrb: No rabicano traits.
Sb Sb
sb		 Sabino - Assorted white or roan-like markings.
Sabino may be polygenic (a gene-complex rather than a single gene pair) or caused by several different genes. Recognized by abundant white on the legs, belly spots or body spots that are can be flecked and roaned, chin spots, or white on the face extending past the eyes. Sabino is registered as overo by some registries, but does not entail the overo or frame overo allele. No risk of lethal white, though some "Fully expressed" sabinos may be completely white in coat color.
SbSb or Sbsb: Sabino markings.
sbsb: No sabino marks.
SB1:Only Sabino gene currently detected by DNA testing, however does not appear to be the gene that creates sabino coloring in Arabians or Clydesdales.
Spl Spl
spl		 Splash, Splashed White - Looks like reverse tobiano with white moving from the bottom of the horse towards the top. The horse's head will look as if dunked in bucket of white paint. White coloring appears to move boldly and smoothly from bottom up. Commonly has blue eyes.
SplSpl or Splspl: Splashed markings. Splash is registered as overo but does not entail the overo or frame overo allele and is not associated with lethal white syndrome.
splspl: No splashed markings.

Alternate theory: Incomplete dominant. SplSpl:classic Splash markings.Splspl:socks, face markings, may be small in the "normal" marking range or into the pinto range with or without blue eyes. splspl causes no white at all.
Sty Sty

sty

 Sooty
StySty or Stysty: Black mixed into body coat.
stysty: No black mixed into coat.
Z Z

z

 ZZ or Zz: Silver dapple - Dilutes eumelanin or black pigment. Converts black to brown with white mane and tail or results in silver coloring.
zz: No silver dapple.
Lp Lp
lp		 Appaloosa spotting gene. The effects of this allele are minimallized or suppressed in horses carrying E or E+[citation needed]

[edit] Genetic formulas and color definitions

Partial list of Genetic Formulas and Color Definitions
Chart based on Dr. Bowling’s Genetic Formulas
W White
G Gray
E, A, CC, dd, gg, ww, toto Bay or Brown
E, aa, CC, dd, gg, ww, toto Black
ee, aa, CC, dd, gg, ww, toto Red: Sorrel or Chestnut
E, A, CCcr, dd, gg, ww, toto Buckskin
ee, CCcr, dd, gg, ww, toto Palomino
ee, CcrCcr Cremello
E, CcrCcr Perlino
E, A, CC, D, gg, ww, toto Yellow or "buckskin" Dun
E, aa, CC, D, gg, ww, toto Mouse Dun, Blue Dun or Grulla
E,A,CC,D Dun, Zebra Dun, Bay Dun, Classic Dun
ee, CC, D, gg, ww, toto Red Dun
gg, E, aa, CC, dd, RN Black Roan (Blue Roan)
gg, E, A, CC, dd, RN Bay Roan
gg, ee, CC, dd, RN Red Roan
gg, E, A, CCcr, dd, RN Buckskin Roan (Red Roan) 
gg, ee, CCcr, dd, RN Palomino Roan (Red Roan)
gg, E, A, CC, D, RN Buckskin Roan (Red Roan)
gg, E, aa, CC, D, RN Mouse Dun Roan, Grulla Roan, or Blue Roan&
gg,ee, CC, D, RN  Red Dun Roan (Red Roan)
gg, E, A, CC, dd, gg, ww, TO Bay Tobiano
gg, ee, CC, D, gg, ww, TO Red Dun Tobiano
gg, Ch, ee, aa, CC, dd, gg, ww, toto Gold - looks like Palomino
gg, Ch, E, A, CC, dd, gg, ww, toto Amber
gg, Ch, E, aa, CC, dd, gg, ww, toto Champagne
gg, Ch, ee, CcrCcr &Ivory&
gg, E, aa, CC, dd, TO, Sb Black Sabino-Tobiano (Tovero without lethal white.)
gg, E, aa, CC, dd, TO, O Black Tobiano-Overo (Tovero carrying lethal white.) 
ee, CCcr, dd, gg, ww, TO Palomino Tobiano Paint
gg, ee, aa, CC, dd, TO, Spl Red Tobiano-Splash (Tovero without lethal white.)

[edit] Lethal roan question

Hintz, H. F. and VanVleck, L. D., published 1979. "Lethal Dominant Roan in Horses". Journal of Heredity, 70:145-146. This study, based on percentages of roan foals thrown by roan parents, popularized the idea of Lethal Roan Syndrome. The study did not include direct genetic analysis and assumed fetus absorption preventing birth. Similar studies suggested that roan-to-roan breedings were lethal because the resulting embryos were absorbed. This was thought to be true because homozygous roans, horses that carried two alleles for the roan characteristic, were rare.

A recent genetic study of roans by Dr. Bowling [1] refuted Hintz and VanVleck’s inferences. Using genetic analysis, Dr. Bowling found several homozygous roans and no evidence of a Lethal Roan Syndrome.[citation needed]

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[edit] Sources and external links

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