Genetics of Horse Coat Color May 14, 2008 Review • Chromosomes come in pairs • So genes are in pairs -- two different versions possible in an individual • Sex cells in each parent contain only one chromosome, one allele • So each parent passes on only one allele of a particular gene - 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 alleles 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 horses! Types of Genes • Gene for black/brown or red/chestnut 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 genotype 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-dominance • Cream color gene = dilution 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 Palomino 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 = Bay 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 = Roan • 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 • Tobiano • Frame overo --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 21 Appaloosa 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 Buckskin 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 Icelandic horse. A genetically black horse that exhibits the typical silver phenotype with a dark body with dapples and a shiny white mane and tail. Photo: Tim Kvick. B. Two Black Silver Rocky Mountain Horses. Photo: Bob Langrish. C. A Brown Silver Morgan horse. 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 Rocky Mountain Horse colt. Photo: Unknown A chestnut Morgan horse that carry the Silver mutation. 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 protein with domains and known mutations. 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. 25.