HORSE COLOUR GENETICS

Jeanette Gower copyright © 1999.
This article is a summary of the book 'Horse Colour Explained' 1999 Kangaroo Press.
Jeanette is available for lecture tours & clinics.

Coat colour testing is available. See Animal Genetics

The study of genetics enables us to determine how certain characteristics are passed on from parent to offspring. Genes are chemicals which are arranged in a particular pattern in the DNA which make up the chromosomes. Each gene has its own position or locus on a particular chromosome and, since chromosomes come in pairs, so do genes. If these two genes are not the same the horse is said to be heterozygous for that gene, but if they are the same, the horse is said to be homozygous for that gene. The importance of this is in predicting outcomes of matings.

Base Colours

Horses have four basic colours: chestnut, bay, brown and black which are produced by the
following combinations of genes:
   
Extension Series
Table 1
  E_ ee
Agouti
A+_ Bay Chestnut
Series
 at_ Brown Chestnut
  aa Black Chestnut

Patterns and dilutions are produced by the presence of other genes in other positions. These can have the effect of
1. Keeping black intact
2. Changing black to red
3. Changing black to mouse or red to yellow (producing dilutes)
4. Or they produce white hairs.

Dilutions
Dilutions affect the base colours outlined above. There are four types: Cremello locus Ccr which has intermediate effects, the Dun D, Taffy Z (which are simple dominants) and the Champagne Ch. The very rare Champagne colour mimics palomino and buckskin but has a wheaten skin. This is the only colour where pink skin does not produce white hair. Homozygous champagne and dun do not produce a psuedo-albino like the cremello dilution (CcrCcr). Duns and taffies are most commonly seen in Miniature Horses, Australian breeds and the Connemara. (See table 3). The effect of Ccr is limited on black pigment and its effect varies from horse to horse. The single dilutes can never become a colour breed as their genes cannot reach purity or homozygosity. This family includes the palominos and buckskins. For example mating two palominos together can never guarantee a palomino foal.
   
sire: Palomino
Table 2
   C Ccr
dam:
 C Chestnut Palomino
Palomino
 Ccr Palomino Cremello

The yellow dun colour is often confused with buckskin, however duns are characterised by having a face mask, primitive markings and bold dorsal stripe running from mane to tail. Dun on a black base gives blue dun (grulla). Yellow dun mimics buckskin. The taffy gene has limited effect on red pigment, but changes black pigment to chocolate giving the silver dapple on a black base and red taffy on bay. Taffy is associated with an eye disorder called A.S.D. in which homozygotes may, very rarely, be blind. Unusual colours such as the Claybank and Taffy Dun may occur when two or more dilutes co-exist as composites.


Table 3
Action of the dilutes    
Base CC
(full colour)		 Double CcrCcr
(pseudo-albino)		 Single CCcr
(dilute)		 Dun D_ Taffy Z_
--------------------------------------------------------------------------------------------------------
Chestnut Cremello Palomino Red Dun Chestnut taffy
Bay Perlino Buckskin Yellow Dun Red taffy
Brown Perlino Dark Buckskin Mouse Dun Dark taffy
Black Smoky Perlino Black Buckskin Blue Dun Blue taffy (chocolate
& silver dapple)
      (grullo)

Patterns

Patterns as seen in Paints, pintos and roans are all dominant. The pattern genes include the following groups:

Greys which are born a base colour whitening out with age. This includes fleabitten, blood markings, melanoma and pinky syndrome. 100% grey producers can be found.

Roans are born a base colour mixed with white hairs. They do not whiten out with age. The homozygote or purebred roan is not found as it is thought to be lethal at the embryonic stage.

Ticking is similar to roan but the white hairs are not as heavily distributed and are seen mostly in the flank

Tobiano pattern consists of vertical white patches. Homozygosity can be identified by laboratory test. This pattern is not associated with any disorders.

Overos have horizontal patches. Homozygosity causes lethal white foal syndrome (OLWS). All overos are heterozygote carriers for lethal white.

Sabino shows as vertical patches and is an extension of the way in which normal white markings occur in any breed. All white and homozygotes can occur. No disorders are associated with sabino. The extreme of sabino patterning is the 'medicine hat' which is either pure sabino, or a sabino combination.

Splashed White pattern is a rarer horizontal pattern with associated deafness. The horse has both eyes blue. The homozygote is thought to be lethal at embryonic stage.

All the these patterns may exist side by side in one animal as composites, producing anything from the extensively white 'medicine hat' or minimally marked horses. Those with incomplete markings do still carry the pattern gene but have insufficient white to be identified as a pinto. These ‘solids’ still carry the gene and cause ‘crop-outs’ in breeds which do not normally allow these patterns.

Spotted

Spotted patterns are seen in the Appaloosa, Knapstrub, Palouse pony, and Pingauzer. This pattern gene has at least four gene positions or loci involved, the exact mechanism of which is unknown but all are thought to be dominant. Gene ApAp which gives the ‘characteristics’ of mottling, white scelera, and striped hooves. This gene must be present for all other spotted characteristics to appear. Hence some horses with spotting genes may not be coloured but still produce colour even when bred to a horse with minimal characteristics. Males colour up more readily than females. The following patterns may occur singly although more commonly they coexist:

Dark spots - produces the spotting on blanket, near leopard and true leopard.

Varnish - gives a progressive ‘silvering’ of white hairs similar to roan, with dark hairs remaining around bony prominences. When combined with dark spots it may give the ‘false leopard’.

White blanket - also known as snowcap, gives white patching over the rump, or sometimes over most of the body and when homozygous, the ‘few spot’. The few spot produces 100% spotted and is associated with night blindness.

White spots - gives frost on the rump or snowflake over the entire body.

Other modifying genes also exist such as those which produce smuts, dapples, marble markings, and eye colour variations. A separate locus is thought to create the 'total solid' by preventing white markings from occurring at all.

A comprehensive coverage with numerous diagrams and beautiful photos is given in the book