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This is about as close to the full genetics as things get right now for breeders online.  Dr. Sponenberg has a book, Equine Color Genetics, which goes into greater detail.  His theories appear more extensive but less clearly explained than Dr. Bowling’s.  This is a combination of the two gene theories presented in Dr. Bowling’s clearer format.

 To begin with, you need a basic understanding of heredity.  If you need a quick lesson in genetics and heredity, Wikipedia’s article on alleles or this tutorial on inheritance can help you.

 Note: for 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.
Note 2: The terms Alleles and Modifiers are used interchangeably in this report.
Note 3: Socks, Star, Strip, and Snip genetics are not yet fully understood but probably multiple allele influenced.

 

Alleles

Effect of Combined Pairs of Alleles

W

W

w

 

The existence of a specific W gene is questioned by some.  Lethal White as a genetic syndrome is accepted universally.

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 slivering 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 colors range from white to dark gray as they do in human hair.

gg: Horse does not gray with aging.

E 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.

Dominant Black E+ horse colorE+E+, E+e, or E+E Dominant black,  non-fading –Overrides the A or bay allele resulting in all black horse

 

 

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.

A A

A+

a

 Agouti Bay A allele Horse color

Agouti-Bay: Acts on "E" to restrict eumelanin, or black pigment, to points.  Has no effect on "e" or red.

AA or Aa: If horse has black hair (E not E+), then that black hair shows only in points pattern.

 

A+A+ or A+a black hair is diluted to dark brown/light black with some lightening around soft parts including joints and belly.
A+A results in a combination of these two effects.
aa: If horse has E allele, then horse will be uniformly black.
C

 

C

 

Ccr  Also noted as ‘c’

 
CCcr Allele Pairing Palomino Color
CCcr Palomino		  

CC: Horse is fully pigmented.

CCcr: Red pigment is diluted to yellow; black pigment is unaffected

 

CcrCcr:

 

CcrCcr

Recessive CcrCcr Cremello Horse Color

 CcrCcr: Both red and black pigments are diluted to pale cream. Skin and eye color are also diluted.
D D

d
Dominate D Dun dorsal striping  

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.

 

 Dominant D Dun Primitive Leg Markings

 

TO TO

to

 Dominant TO Tobiano Spotting Pattern

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

 Dominant O Frame Overo Markings

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

 Dominant Ch Champagne Dilution Coloring in Horses

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  

P P

p

 Dominate P Panagre dilution allele in horses

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

 Dominant Rn Roaning allele in horses no lethal roan

RnRn or Rnrn:  roan pattern of white hair mixed in with base color. There is no lethal roan. (*See footnote.) 

rnrn:  no roan pattern

Rb Rb

rb

 Dominate Rb Rabicano marking in horses

Rabicano:  Partial roaning with ‘coon’ tail markings. Thought to be a dominate gene by Sponenberg.

RbRb or Rbrb: rabicano markings 

rbrb: no rabicano traits

Sb Sb

sb

 Dominate Sb Sabiano Paint Markings in Horses. Only Paint marking in Arabians.

Sabino Feathery Misty of Chincoteague like white markings.

Only paint marking pattern found in full blooded Arabians. Sabino patterning may be caused by single allele as shown here.  However, some horses with sabino patterning do not test positive for the allele.  Sabino is probably polygenic in these cases.  The pattern causes abundant white on the legs, and then creeps up as belly spots and body spots that are can be flecked and roaned. Sabino is registered as overo but does not entail the overo or frame overo allele. No risk of lethal white.

SbSb or Sbsb: sabino markings

 sbsb: no sabino marks

Spl Spl

spl

Splash, Splashed White

Looks like reverse tobiano with white moving from the bottom of the horse towards the top. The horses head will look as if dunked in bucket of white paint.  White coloring appears to move boldly and smoothly from bottom up.

SplSpl or Splspl:  Splashed markings. Spash is registered as overo but has does not entail the overo or frame overo allele and is not associated with lethal white syndrome.  

 splspl:  no splashed markings

Sty Sty

sty

 Dominant Sty Allele Sooty markings in horses

Sooty

 StySty or Stysty:  black mixed into body coat.

 stysty:  no black mixed into coat

Z also noted as S Z

z

 ZZ or Zz:   Silver dapple - dilutes eumelanin or black pigment. Converts black to dappled brown with white mane and tail or results in silver coloring.

 zz:  no silver dapple

Lp Lp

lp

 LpOp Appaloosa Spotting Allele in Horses

Appaloosa Leopard spotting gene. Produces coat spotting patterns, mottling over otherwise dark skin, striped hooves and white sclera around the eye.

 

 

Chart based on Dr. Bowling’s Genetic Formulas

Partial list of Genetic Formulas and Color Definitions

Genetic Formula

Color   

W

White

G

Gray (Insert Anisa picture)

E, A, CC, dd, gg, ww, toto

Bay or Brown

E, aa, CC, dd, gg, ww, toto

Black (Insert Sarah pic.)

ee, aa, CC, dd, gg, ww, toto

Red: Sorrel or Chestnut (Insert Ofi picture)

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

Buckskin Dun

E, aa, CC, D, gg, ww, toto

Mouse 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 (Insert Lancer picture)

gg, ee, CC, dd, RN

Red Roan (Insert Merryberry pic)

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.) (Insert Magic pic.)

gg, E, aa, CC, dd, TO, O

Black Tobiano-Overo (Tovero carrying lethal white.)

ee, CCcr, dd, gg, ww, TO

Palomino Tobiano Paint (Insert Rain pic.)

gg, ee, aa, CC, dd, TO, Spl

Red Tobiano-Splash (Tovero without lethal white.)

*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.

 A recent genetic study of roans by Dr. Bowling (See http://www.hancockhorses.com/article-roanQHNews.pdf) refuted Hintz and VanVleck’s inferences.  Using genetic analysis, Dr. Bowling found several homozygous roans and no evidence of a Lethal Roan Syndrome.

 

Sources:

http://www.mustangs4us.com/

http://www.vgl.ucdavis.edu/~lvmillon/
Ann T. Bowling, PhD.
Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616-8744

 http://members.aol.com/MFTHorses/sponenbg.htmh
D. Phillip Sponenberg, DVM, PhD.
Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA  24061

 http://www.grullablue.com

 http://www.hancockhorses.com/article-roan.html

 http://www.hancockhorses.com/article-roanQHNews.pdf

 http://www.equinecolor.com/
Provides clear explanations—a good source for the beginner. Does contain info on ‘possible lethal roan’.

 http://www.ichregistry.com/identification.htm

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