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Home , Black horse, Chestnut (horse color), Equine coat color, Icelandic horse, Palomino, Roan (horse)

Genetics of Coat Color

May 14, 2008

Review

• come in pairs

• So are in pairs -- two different versions possible in an individual

• Sex cells in each parent contain only one , one

• So each parent passes on only one allele of a particular - If parent has two of the same allele = - If parent has one of each allele =

1 What happens when there are two or more genes that affect the same trait?

Equine coat color genetics

• Inheritance of horse coat color follows the same genetics rules just discussed

• “Simple” dominant and recessive are present

• But coat color comes from many genes that modify and interact with each other

2 There are basically only two colors in the coats of !

Types of Genes

• Gene for black/brown or red/ color E locus = pigmented

• Genes that determine if there will be any pigment at all W locus = NO pigment in skin or hair

• Separate genes that modify/interact with the E locus G locus = Pigment gradually greys with age A locus = Distributes pigment to points C and D locus =Dilutes pigment TO locus = Spotted

3 E locus

• Controls the type of pigments made in the hair and skin

 Black pigment = E _

o Black pigment in skin and in hair due to eumelanin o dominant

 Reddish pigment = e e

o Black pigment in skin but hair is red due to phaeomelanin o recessive

E locus

black/brown red/chestnut pigment

4 E locus

black/brown red/chestnut pigment

What about horses that have NO color?

5 W locus

W = NO pigment (dominant)

w = Able to make pigment (recessive)

W W = lethal W w = white w w = pigment present

W w = white • Skin is pinkish • Hair white • No color anywhere

All non-white horses are ww

What is the genotype for the W locus in these horses?

6 G locus

G = “Greying” locus

G = pigment is gradually lost to grey color with age

g = pigment stays throughout life (no greying)

GG = greys with age Gg = greys with age gg = forms normal pigment

G_ phenotype

• Skin is pigmented • Hair white, gray, mottled • Greying occurs with age • Horse is gray to silver

7 What is the genotype for the G locus in these horses (adding to E and W loci) ?

What is the genotype for the G locus in this horse if you had not seen it as a foal?

8 G = grey g = non-grey chestnut

Assume both Grey X Non-Grey are e e

Sire F1 Generation Offspring G G

g

Dam

g

Sire is Gg F2 Generation Offspring G g

Dam is G Gg g

9 Sire is Gg F2 Generation Offspring G g GG Gg Dam is G Grey Grey Gg Gg gg g Grey Chestnut

Phenotypes of offspring are: 75% Greying Ratio is 1:2:1 genotype 25% Non-Grey 3:1 phenotype

Genotypes of offspring are: 25% Homozygous Grey (GG) 50% Heterozygous Grey (Gg) 25% Homozygous Non-Grey (gg)

X

Sire

Offspring G g

g Dam

g

10 C locus = example of semi-

• Cream color gene =

• Lightens hairs with red pigment

• But dominance is not “complete” C = Normal color

CCr = dilutes/lightens color

Homozygote = CC Gives full color

Homozygote =CCrCCr Full dilution to cream color (cremello)

11 Heterozygote = C CCr Partial dilution to pale color

CCr CCr together affects any pigment.

The CCr allele alone only affects red pigment.

12 What about the G, W, and E loci in these horses?

Palomino X Both parents are also e e

Sire Palomino X Palomino C CCr

C Dam

CCr

13 Sire Palomino X Palomino C CCr CC CCCr C Dam Chestnut Palomino CCCr CCrCCr CCr Palomino Cream

25% Chestnut (non-Cream) (CC) Ratio is 1:2:1 genotype 50% Palomino(CCCr) 25% Cream (Pseudo-Albino) (CCrCCr) 1:2:1 phenotype

A locus = Distribution of Black Pigmented Hair

A acts on E by restricting where black hairs are

So black hairs must be present to begin with (genotype can’t be e e )

A = restricts the black pigment to points

a = does not restrict the black pigment

14 E E or E e = any black hairs anywhere

e e = no black, only red hairs

15 A locus = Distribution of Black Pigmented Hair

A acts on the E locus by restricting where black hairs are

So black hairs must be present to begin with (so can’t be e e)

A = restricts the black pigment to points a = does not restrict the black pigment

E _ ; A A or A a = restricts black hair to points =

16 E e ; a a = black hair throughout (Black, Brown)

Genotype of a Bay horse

17 A a, E e X A a, E e

Possible Allele Combinations A E a E A e a e AA EE Aa EE AA Ee Aa Ee A E Bay Bay Bay Bay Aa EE aa EE Aa Ee aa Ee a E Bay Black Bay Black AA Ee Aa Ee AA ee Aa ee A e Bay Bay Chestnut Chestnut Aa Ee aa Ee Aa ee aa ee a e Bay Black Chestnut Chestnut

Phenotype Ratio is 9 Bays: 3 Blacks: 4 Chestnuts

R locus =

• Thought originally to cause death when homozygous

• May be dominant or recessive

R = roan pattern =white hairs scattered throughout RR = lethal? Rr = roan r = non-roan rr = chestnut (with e e)

18 Roan X Roan

Sire Roan X Roan R r

R Dam

r

Ratio Results:

Pinto genes

•

• Frame

--Sabino

--Splash

19 TO locus = Tobiano

• White patterning

• Produces ovals or round patterns of white and color extending down neck and chest

TO _ = white patterns

to to = solid

O locus = Overo or Frame Overo

• Also white patterning

• Color forms a frame around white patterning

• Dominant

O o = white patterns

o o = solid

20 Lethal white - carried in the overo Paint lethal homozygous foal dies soon after birth

The overo patterns

Sabino spotting Combination Paint patterns

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TABLE 1

Genetic Formulas and Color Definitions

Genetic Formula Color W White G Gray E, A, CC, dd, gg, ww, toto Bay E, aa, CC, dd, gg, ww, toto Black ee, aa, CC, dd, gg, ww, toto Red E, A, CCcr, dd, gg, ww, toto ee, CCcr, dd, gg, ww, toto Palomino CcrCcr Cremello E, A, CC, D, gg, ww, toto Buckskin dun E, aa, CC, D, gg, ww, toto Mouse dun ee, CC, D, gg, ww, toto Red dun E, A, CC, dd, gg, ww, TO Bay tobiano ee, CC, D, gg, ww, TO Red dun tobiano

22 Example of Silver dilution allele: Genetics to molecular identification to function

Phenotypic description of Silver colored horses. A. A Black Silver . A genetically that exhibits the typical silver phenotype with a dark body with dapples and a shiny white and . Photo: Tim Kvick. B. Two Black Silver Rocky Mountain Horses. Photo: Bob Langrish. C. A Brown Silver . A genetically brown horse that shows the silver phenotype with the mane and tail diluted from black to white and the lower legs diluted from black to dark greyish. Photo: Laura Behning. D. The legs of a Brown Silver horse. The lower legs are diluted from black to greyish. Photo: Laura Behning.

23 Phenotypic description of Silver colored foals. A. A Silver colored Icelandic horse foal. Silver foals are generally very pale on the body with white mane and tail. Photo: Elsa Storgärds. B. A striped hoof of a Silver colored Icelandic horse foal. Photo: Tim Kvick. C. White eyelashes of a Silver colored colt. Photo: Unknown

A chestnut Morgan horse that carry the Silver . This particular individual (Amanda's Suzie Q) indicate that the Silver mutation in horses has little or no effect on pheomelanin (as mane does not seem to be diluted). Photo: Anthony Domire JR.쇓

24 A schematic picture of the PMEL17 with domains and known . The transmembrane (TM) protein PMEL17 has previously been shown to regulate hypopigmented phenotypes in mouse, chicken, dog, and zebrafish. The location of known mutations associated with hypopigmentation in these species are indicated. R740C in chicken (Dun) is at the same location as the R618C in the horse (Silver).

Brunberg et al. BMC Genetics 2006 7:46 doi:10.1186/1471-2156-7-46

Amino acid alignment of the end of the transmembrane region and beginning of the cytoplasmic region of the PMEL17. Amino acid alignment of the end of the transmembrane region and beginning of the cytoplasmic region of the PMEL17. The site of the silver horse mutation is highlighted. Sequence identities are indicated by dashes and insertion/deletion differences are indicated by dots.

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