Brown Bay

    Horse Coat Colors

    The interaction of several different genes,
    each with multiple alleles, creates a wide array
    of coat colors in horses.

    Two putative loci interact to produce
    the most common horse coat colors.

    • The Agouti Locus
    • The Extension Locus

Black
Seal brown

    The Extension (E) Locus

    The extension locus (E) (a.k.a. "red factor") is involved in melanin manufacture.
    • E (dominant) - brown/black eumelanin
    • e (recessive) - encodes red/gold phaeomelanin
      • Genotype E- horse: black or brown
      • Genotype ee horse: chestnut/sorrel

Chestnut/Sorrel
Red Bay (dark points)

    The Agouti (A) Locus

    The agouti locus (A) is involved in the deposition of melanin in skin and hair.
    • A (dominant): bay pigmentation
      (eumelanin only on "points"-legs, nose, mane, tail)
    • a (recessive) encodes uniform pigmentation
      • Genotype A- horse is bay.
      • Genotype aa horse is uniformly pigmented.

Chestnut/Sorrel (uniform pigmentation)

    Now Some Figuring

    In the absence of any other loci affecting their expression, the E and A loci interact to produce these common horse colors:
    • EE A- will produce brown bay
    • Ee A- will produce red bay
    • ee -- will produce chestnut/sorrel
    • E- aa will produce seal brown or black

Horse Coat Colors: Modifier Genes

Genes that affect the degree to which another locus is expressed
are called modifiers.

Two modifiers in horses cause pigment hypomelanism (dilution):

buckskin:

palomino:

smoky black:

    The Cream Allele (CCr)

    The cream "gene" is a mutant allele of the SLC45A2 gene, which encodes membrane-associated transporter protein (MATP).

    The cream gene exhibits incomplete dominance and a distinct dosage effect.

    • C CCr genotype
      • dilutes phaeomelanin to yellow or gold
        • buckskin (dilute bay)
        • palomino (dilute chestnut)
        • smoky black (dilute black)
      • affects mane and tail more than body
      • does not significantly dilute eumelanin
      • has little effect on eye color

    • CCr CCr genotype
      • dilutes both phaeomelanin and eumelanin
      • cremello (very dilute chestnut)
      • perlino (very dilute bay)
      • smoky cream (very dilute black)
      • dilutes iris pigmentation to blue

      The very pale horses are not albino,
      as they can produce melanin.
      There is no known albino allele for any locus in horses.

cremello:

perlino:

smoky cream:

gold champagne:

sable champagne:

    The Champagne Allele

    The champagne allele is a dominant mutation of the SLC36A1 gene, which encodes a proton-dependent, small amino acid transporter.

    Multiple transcript variants can be produced
    by alternative exon splicing.

    The allele affects multiple traits:

    • hazel eyes
    • freckled skin
    • black (eumelanin) is diluted to chocolate
    • red (phaeomelanin) is diluted to gold
      • chestnut --> "gold champagne"
      • bay --> "amber champagne"
      • seal brown --> "sable champagne"
      • black --> "classic champagne"

      The Champagne gene is found only in certain American horse breeds.

amber champagne:

characteristic hazel eye:


by Flamestorm11, who has lots of great illustrations of horse coat color genetics

    Multiple Loci, Multiple Alleles

    The interactions of
    • agouti gene
    • extension gene
    • cream gene
    • champagne gene
    • S (piebalding) gene
    • other dilution genes: dun, pearl
    • and others
      ...creates a rainbow of horses.