Horse Coat Colour Genetics

Introduction

Horse coat colours are determined by three levels of genetics:
[NB: white spotting genes will not be discussed further here as they do not occur in RMH.]

1. Basic Colours – controlled by Extension (E) and Agouti (A).

2. Dilution/Modifier Genes – lighten or alter the base colour.

3. White Spotting Genes – create patterns (not permitted by RMH breed standard).
   

1. Base coat colours that occur in RMH

Dominant genes will express in the phenotype with only one copy of the allele. Recessive genes have to be homozygous (two copies) for their trait to be expressed, e.g., e/e for chestnut.

• Extension (E locus) – recessive, controls black pigment production.

  • e/e → Chestnut (no black pigment)

• Agouti (A locus) – dominant, controls distribution of black pigment.

  • A/- with E/- → Bay (brown body, black mane, tail, legs)

  • a/a with E/- → Black (full black coat)

2. Dilution and Modifier Genes that occur in RMH

Dilution genes lighten the base colours and modifier genes modify the expression of the genetic phenotypes. Most are dominant; meaning only one copy of the allelle is needed for the trait to express in the phenotype; there is no difference between hetero- or homozygous status. Incomplete dominance, the phenotype expression varies depending whether the horse is hetero- (one copy) or homozygous (two copies), e.g., cream.

• Cream (Cr) – Incomplete dominance, cumulative dosage effect. Single copy lightens the red colour of the skin, hair and eyes and two copies lightens both the red and black colour.

  • Chestnut → Palomino (Cr/-) or Cremello (Cr/Cr)

  • Bay → Buckskin (Cr/-) or Perlino (Cr/Cr)

  • Black → Smokey Black (Cr/-) or Smokey Cream (Cr/Cr)

• Champagne (Ch) – Dominant (Ch/-) or (Ch/Ch), dilutes hair pigment from black to brown and red to gold, rare

  • Chestnut → Gold Champagne

  • Bay → Amber Champagne

  • Black → Classic Champagne

• Dun (D) – Dominant (D/-) or (D/D), dilutes body colour leaving the points, adds primitive markings (dorsal stripe, leg barring)

  • Chestnut → Red Dun

  • Bay → Bay Dun

  • Black → Grulla

• Roan (Rn) – Dominant (Rn/-) or (Rn/Rn), mixes white hairs into body coat (not head/legs)

  • Chestnut → Red Roan

  • Bay → Bay Roan

  • Black → Blue Roan

• Silver (Z) – Dominant (Z/-) or (Z/Z), dilutes black pigment

  • Chestnut → Carrier (no visible effect)

  • Bay → Bay Silver, mane and tail flaxen colour though this can darken with age

  • Black → Black Silver, mane and tail flaxen colour though this can darken with age

• Gray (G) – · Dominant (G/-) or (G/G), horses born dark eventually lose pigment to become nearly all white

  • Any base colour → progressively whitens with age
    

Horse Colour Modifications (with Genotypes) Quick Reference Chart