by Angela Smith BVSc
This article has been collated from material published by the University of California, Davis School of Veterinary Medicine, on their website. The bulk of the material presented has been sourced from an article written by Dr. Ann. T. Bowling, an eminent geneticist.
Coat colour genetics in horses is still a very difficult subject.
Understanding the basics of the underlying genetic principles that determine coat colour in the horse will help to more accurately define your youngster’s adult colour.
Some Basic Terminology of Genetics.
GENES: Are the smallest units of heredity, defining the way every living thing grows, appears and continues to live.
Genes can not be seen, but are defined as specific segments of the DNA [chemical molecule] that constitutes the chromosomes of every cell in the living body. Each cell contains a duplicate set of genes. Each set is derived from the single set contributed at conception by the mother and the father. The gene sets contain similar, but not necessarily identical information. For example, both sets may contain a gene determining hair structure, but one set may contain the instruction for straight hair and the other for curly hair.
The alternative forms of each gene are called ALLELES. If both alleles are identical, then the animal is considered to be HOMOZYGOUS at the gene. If the alleles are dissimilar, then the animal is considered to be HETEROZYGOUS at the gene.
Information about homozygosity or heterzygosity for various genes can be inferred from information about parents and/or progeny [another example is blood and DNA typing] and can be used for predicting the outcome of matings. For most of the alleles of the horse coat colours one cannot tell by looking at an animal whether it is homozygous at each coat colour gene.
Sometimes, information about coat colours of parents may be used as an indication of incorrect parentage or erroneous identification, so some knowledge of the genetic relationships may be useful.
Another characteristic of gene pairs which especially comes into play in the case of heterozygosity, is that only one allele may be visibly expressed. If this is the case, this allele is DOMINANT. The unexpressed one is referred to as the RECESSIVE. These terms are only used to describe the expression of alleles and their relationship to each other, they do not refer to any physical or temperamental characteristics of an animal as such. In any animal the dominant allele of a gene is the one being expressed outwardly.
Therefore one cannot determine by the looking of the animal whether the second allele is a dominant or a recessive one.
If a second recessive allele is present, it is usually masked by the dominant allele, which leads to the expression of “hidden recessive”. In turn, dominant alleles can never be hidden by their related recessive alleles. For simplicity, genes are symbolised by letters. Dominant genes will have capital letters and recessive will have italicised lower case letters.
Gene W [inability to form pigment in skin and hair] and gene G [exclusion of pigment from hair]
Gene W and gene G both have alleles whose action can obscure the action of the other coat colour genes. This means if either the allele W [dominant] or the allele G [dominant] is present in the hereditary material of a horse, it is difficult or impossible to determine its other coat colour genes by superficial examination. A horse with the dominant allele W, is white from birth, sometimes called albino. The W [dominant] allele is only rarely encountered. All non-white horses are ww [homozygous recessive ]. A young horse with the dominant allele G will be born any colour but grey and will gradually become white or white with red or black flecks as an aged animal. Earliest indications of change of hairs will be seen around the eyes. In intermediate stages of the greying process, the horse will have a mixture of white and dark hairs, a most confusing stage for trying to identify colour. Since grey is produced by a dominant gene, at least one parent of a grey horse must be grey. If a grey horse does not have a grey parent, then it should be seriously considered that the purported parentage is incorrect. In contrast to a white [W] horse, a grey horse is born pigmented, will go through lightening stages, but always contains pigment in skin and eyes at all stages of colouration changes. A grey horse will be either GG or Gg, it is not possible to tell by looking at the horse whether it is homozygous for G. All non-grey horses will be gg (homozygous recessive).
Observed Effect of Alleles in Homozygous and Heterozygous Conditions
- W-Inability to form Pigment in the skin and hair
- WW-Lethal
- Ww-Horse lacks pigment in skin, and eyes and appears to be white.
- ww-horse is fully pigmented
- G-Exclusion of pigment from hair
- GG-Horse shows progressive silvering with age to white or flea-bitten, but is born any non-grey colour. Pigment is always present in skin and eyes at all stages of silvering.
- Gg-Same as GG
- gg-Horse does not show progressive silvering with age.
- E-Black Hair Pigment
- EE-Horse has ability to form black pigment in skin and hair. Black pigment in hair may be either in a points pattern or distributed overall.
- Ee-Same as EE
- ee-Horse has black pigment in skin, but hair pigment appears red.
- A-Distribution of Black Pigmented Hair.
- AA-if horse has black hair (E), then that black hair is in points pattern. (A) has no effect on red (ee) pigment.
- Aa-same as AA
- aa-If horse has black hair (E), then that black hair is uniformly distributed over body and points. (A) has no effect on red (ee) pigment.
- C-Pigment dilution.
- CC- horse is fully pigmented.
- CCcr pigment is fully diluted to yellow; black pigment is unaffected.
- CcrCcr- Both red and black pigments are diluted to pale cream. Skin and eye colour are also diluted.
- D-Dun pattern and Pigment dilution.
- DD- horse shows a diluted body colour to pinkish-red, yellow-red, yellow or mouse grey and has a dark points including dorsal stripe, shoulder stripe and leg barring.
- Dd- Same as DD.
- dd- Horse has undiluted coat colour.
- TO-Tobiano Spotting pattern.
- TOTO- horse is characterised by white spotting pattern known as tobiano. Legs are usually white.
- TOto- Same as TOTO.
- toto- No tobiano pattern present.
Gene E: Black hair pigment.
If the horse is neither white or grey, you first have to see if the horse has any black hair. These hairs may be found either in a distinct pattern on the points (legs, mane, tail ), or black hair may be the only hair colour (with the exception of white marking ) over the entire body. Allele e allows black pigment in the skin but not in the hair. The pigment conditioned by the e allele makes the hair appear red.
If an animal has no black pigmented hair, it has the genetic formula ee. Such an animal will be any shade of red ranging from liver chestnut to chestnut to sorrel.
Manes and tails may be lighter (flaxen), darker (not black ), or the same colour as the body. These pigment variations cannot be explained by simple genetic schemes. The shades of red are not consistently defined in anyone’s terminology and therefore can be confusing. When two red horses are bred together, the offspring should also be red. If the offspring has black pigment or is grey or white, then the parentage as given is most likely incorrect.
Gene A: Distribution of black pigmented hair.
This gene controls the distribution pattern of black hair. The allele A in combination with E will confine the black hair to points to produce a bay. Various shades of bay from dark bay or brown through mahogany bay, blood bay to copper bay and light bay exist. Any bay horse will include A and E in its genetic formula as well as ww and gg. The alternative allele a does not restrict the distribution of black hair so that in the presence of E and a the animal is uniformly black. In most breeds of horses, the a allele is rare, so black horses are infrequently seen. Many black horses will sun-fade, especially around the muzzle and the flanks and such animals may be called brown.
Neither “A” nor “a” affects either the pigment or its distribution in red (ee) horses. Thus it is not possible to determine by examination of coat colour which alleles of the A gene a red horse has.
GENETIC FORMULAS and COLOUR DEFINITIONS.
W-colour White
G-colour Grey
E,A,CC,dd,gg,ww,toto-colour Bay
E,aa,CC,dd,gg,ww,toto- colour Black
ee,aa,CC,dd,gg,ww,toto-colour Red
E,A,CCcr, dd,gg,ww,toto-colour Buckskin
ee,CCcr, dd, gg, ww, toto,- colour Palomino
CcrCcr-colour Cremello
E,A, CC, D, gg, ww, toto-colour Buckskin dun
E, aa, CC, D, gg, ww, toto-colour Mouse dun
ee, CC, D, gg, WW, toto-colour -Red dun
E,A, CC, dd, gg, ww, TO-colour Bay tobiano,
ee, CC, D, gg,ww, To -colour- Red dun tobiano.
This article mentions only some of the major points discussed in the article by Dr. Ann Bowling. For more information please refer to the UCDavis website.