Horse Colour Genetics

   To begin with, I will assume that you have a grasp of the basic principles of genetics. There is a lot of information available about this on the net, and this page is merely for the discussion of horse genetics. Most of this information comes from 'Horse Colour Explained' by Jeanette Gower, with some information also from 'Horse Genetics' by Ann T Bowling.

The base colours are black, brown, bay and chestnut.

Chestnut

• Chestnut is a red horse of any shade.

• There are six main shades based on the combination of genes for dark, medium or light red, with sootiness or it's absence.

• All shades of chestnut may have a flaxen mane or tail.

• Chestnut is the most recessive colour.

• Non-chestnut parents may have a chestnut foal.

• Chestnut bred to chestnut always gives chestnut.

Agouti series (A) - influences shade, Extension series (E) - causes chestnut

Chestnut = ee + AA, AA^t, Aa or aa

so the recessive form of the extension series (e) causes chestnut (ee) and the shade of chestnut depends upon the combination of Agouti genes:

AA or AA^t or Aa - Red chestnut

A^tA^t or A^ta - Standard chestnut

aa - Liver chestnut

Black

• A black horse is all black, except that white markings may be present.

• Black is the second most recessive colour after chestnut (red).

• The E series controls black (E-) and red (ee).

• Black bred to black gives black or chestnut.

• Homozygous black (EE) can never produce a chestnut foal.

• There are two types of black horse, fading and non-fading.

• The goal of black breeders is the jet black.

• Non-fading black (jet black) is recessive to fading black due to the intensifying gene being effected by the fading gene.

• Fading black bred to fading black and give jet black.

• Jet black bred to jet black gives jet black in 75% of progeny and dark chestnut in 25%.

• It is unknown what gene causes fading black.

Black = aa + EE or Ee

Bay and Brown

• Brown is a black horse that had been genetically altered to have lighter colour in the soft parts.

• Bay is a black horse that has been genetically altered to have a lighter, mostly red body colour and head, leaving black in the legs, mane and tail.

• The genetic alteration is brought about by genes of the Agouti (A) series, which result in bay being dominant over brown, and brown is dominant over black.

Bay = AA or Aa or AA^t + E-

Brown - A^tA^t or A^ta + E-

Cremello

• The three main types of dilution in horses are: cremello, dun and taffy.

• In double dose, the cremello dilution produces the pseudo-albinos; cremello, perlino and smokey perlino.

• Cremello dilution is incompletely dominant with base colours, giving and intermediate effect (e.g. palomino or buckskin).

• The eyes of a pseudo-albino are not pink.

Full colour = CC

Dilute colour = Cc^cr (palomino, buckskin)

Double dilute or pseudo-albino = c^crc^cr (cremello, perlino)

In the pseudo-albino chestnut becomes cremello, bay and brown become perlino and black becomes smokey perlino.

Palomino

• The ideal palomino is gold with white mane and tail.

• Genetic palomino is any shade of gold, sooty or cream, caused by the dilution of chestnut.

• There is no palomino gene, therefore palomino can never become a breed.

• The ideal chestnut for palomino production is the cherry red with flaxen mane and tail.

• The parents should not exhibit smuts, spots, patches , dapples or white or dark hairs.

chestnut + Cc^cr = palomino

Red chestnut = golden palomino

Standard chestnut = seasonal palomino

Liver chestnut = chocolate or sooty palomino

Buckskin

• The cremello dilution acting on bay, brown or black produces a genetic buckskin, which may be light, yellow, gold, sooty or off-black.

• Golden buckskin, long regarded as a favourite, is a coppery gold body with black legs, mane and tail.

bay, brown or black + Cc^cr = buckskin

Black horses carry but do not express the cremello dilution as they have no red pigment to be affected.

Dun

• The four categories of dun are red, yellow, mouse and blue.

• All the many and varied names for different duns are simply specialised terms based on local/regional or breed use.

• Duns have minimal iridescence and always have a prominent dorsal stripe, mask and leg barring, known as primitive markings.

• The dunning gene lightens out the body of the four base colours and leaves primitive markings the same colour as the mane and tail.

• The dunning gene is dominant, so it's presence is visible and affects all base colours.

• Yellow dun (dun mechanism) and buckskin (cremello mechanism) are frequently confused as one and the same.

• Buckskin associations include all duns as well as buckskin under their umbrella.

• There are many breeds of dun.

• The native colour of the Fjord, a specialised breed of dun horse, may be the result of a different allele.

Dun = DD or Dd

bay + dun = yellow dun (standard)

brown + dun = mouse dun (dark)

black + dun = blue dun or grullo

chestnut + dun = red dun (several shades)

Shades of red dun (lightest to darkest); light red dun (peach dun), apricot dun, red dun, copper dun, bronze dun and liver dun (muddy dun).

Taffy

• Taffy is the third dilution, which affects only black pigmentation.

• Bay, brown and black give the red, blue and silver dapple taffy.

• Sliver dapple is a term that should be reserved only for those taffies with large full-bodied dapples and silver mane and tail.

• Taffy is a dominant gene.

• Black is the best colour for breeding the sliver dapple, and deep red bay is the best colour for red taffy.

• Taffy is almost invisible on chestnut horses (red pigment) but if a chestnut is known to carry the taffy gene it should be described as chestnut taffy.

Taffy = ZZ or Zz

bay + taffy = red taffy

brown + taffy = blue (dark) taffy

black + taffy = sliver dapple or chocolate taffy

chestnut + taffy = chestnut taffy

liver chestnut + taffy = chocolate chestnut taffy

Grey

• The grey is a dark-skinned horse that progressively whitens with age.

• Grey is not a colour but a pattern superimposed over other colours.

• The base colour of a greying foal should always be recorded.

• A greying foal should never be described as roan.

• Grey is the result of a dominant gene that may mask all other colours.

• Two greys bred together may produce any colour recognised within the breed.

• Melanoma and pinkie syndrome are characteristic of many lines of grey horses.

• Speckles of the flea-bitten grey reflect the original base colour.

• Flea-bitten grey is a recessive trait not linked to the grey gene.

• Blood markings are recessive and only occur on flea bitten greys.

Grey = GG or Gg

Roan

• Roan is a pattern of white hairs superimposed on the body but not on the head and legs.

• It does not lighten with age, but may show darker pigmentation where there has been injury.

• It is evident in the first change of foal coat if not already evident at birth.

• Roan is named after the base colour it is acting on, e.g. bay roan, black roan.

• Roan is caused by a dominant gene.

• Homozygous roan does not occur.

Roan = RnRn or Rnrn

Tobiano

• Tobiano is a dominant pattern of white patches which appears to spread vertically down from the spine, and upwards from four white legs.

• Face markings are the same as on normal horses.

• Tobiano must have one tobiano parent, even if incompletely marked.

• Bloodmarker analysis is now available to detect homozygous in a potential breeding horse.

Tobiano = TOTO or TOto

Features of the tobiano:

Body white across the spine

Body white appears to descend vertically

Four white legs with irregular edges

Ermine spots with dark hooves

Patches of colour around the chestnuts

Sabino

• Sabino is a dominant pattern of white patches which appear to run vertically up the legs and underbelly/neck, accompanied by large ragged splashes, spots or ticking.

• The head has a large blaze and chinspot.

• Sabino occurs in nearly every breed of horse in minimal form as normal white markings.

• Large white sabino markings, sufficient to qualify as pinto can occur in any breed.

• In breeds which disallow the registration of such horses, the spontaneous occurrence of full-blown pinto markings is known as a crop-out.

• Crop-out sabino occurs quite regularly in some breeds.

• Sabino is frequently confused with overo and roan.

Sabino = SS or Ss + nn   (nn = white markings, N- non-white markings)

Features of the sabino:

A patch (or patches) present on the underside at belly, girth or breast, following the underside of the neck

Ticking

Body white appears to run vertically up the body, terminating in irregular peaks or splashes of diminishing size

Chinspot

White legs with white hooves and at least one white leg with diminishing spear or point edge

Overo

• Overo is a dominant pattern of white patches that appears to spread horizontally along the ribs, neck and body, leaving the top line and legs still coloured.

• The face has large irregular white.

• An overo must have one overo parent even if incompletely marked.

• Incomplete overo in one of the parents is the reason for the apparent spontaneous appearance of overo in the Quarter Horse.

• There is no such thing as a true crop-out.

• A homozygous overo foal is born white and dies soon after birth from lethal defects.

• Homozygous overo is always lethal. It is produced from the mating of two overos.

• All overos carry the lethal white factor.

• For humane reasons, overo should be bred to a normal solid horse.

Overo = Oo

Features of the overo:

Body white tends not to cross the spine except in extensively marked individuals

Legs dark up to the knees unless sabino is present

Body white is irregular, appearing to spread horizontally

Tail full coloured, except in extensively marked individuals

Large irregular white face markings

Splashed White

• Splashed white is a dominant pattern where white appears horizontally across the bottom half of the body and tail.

• The face is bald with blue eyes.

• Deafness is an associated characteristic.

• One parent will be splashed white even in apparent crop-outs.

• Splashed white is thought to be non-viable in homozygous form, but does not result in lethal foals like overo crosses.

Splashed white = Spsp

Features of the splashed white:

Body dark on top half, white on lower half

Bald white head

White or bottom half white tail

Two blue eyes

Composite Broken Colours

• Tobiano, sabino, overo and splashed white are all dominant colours.

• The medicine hat is always sabino, but may occur in combination with tobiano, overo or even splashed white.

• Composites are usually high percentage colour producers.

• Since it is difficult to determine the genotype of composite colour horses they should be bred to a normal solid, especially if overo is suspected.

• Pinto can co-exist with Appaloosa spotting. Such crosses are not recognised by most breeds so usually reflect poor breeding.

White Markings

• White markings are frowned upon in some breeds, and highly desired in others.

• In most breeds, white markings do not go above an artificial line determined by breed regulations. These horses are known as solids.

• White markings are the result of the dominant sabino gene.

• The amount of white is determined by modifying genes which may be fixed by selective breeding to be minimal or extensive, or in certain positions.

• Horses without white at all are produced by the action of another factor, a gene for 'no white' or total solid.

• In some breeds the majority of horses are total solid.

Face Markings

0. No white

1. Star - white above the eye line

2. Strip or stripe - area between the eye line and above the nostrils which is not wider than the bridge of the nose

3. Snip - white below the top of the nostrils

4. Blaze - a combination of all three, sometimes wider than the bridge of the nose.

5. Chinspot - white on lower lip or chin

6. Bald - white which includes both eyes and nostrils

Leg Markings

0. No white

1. Coronet - white marking up to 2.5cm (1") above the coronary band

2. Pastern - white marking to the bottom of the fetlock joint

3. Fetlock - white marking to the top of the fetlock joint

4. Sock - white marking extending to the midpoint of cannon

5. Stocking - white marking extending to the excessive white line

6. High white - white extending beyond the line for excessive white

Total solid horse, no white = NN or Nn + SS, Ss or ss

Sabino horse = nn + SS or Ss

Solid horse, ordinary white = nn + ss

White Horses

• All-white horses with pink skin and dark eyes, as well as medicine hats, have been recorded in most breeds from time to time.

• The pseudo-albino is not a true white horse, rather it is off-white.

• Grey horses usually go white, but they have grey skin.

• White horses my be the result of extensive numbers of modifying genes for appaloosa, pinto or some combination of these.

• Spontaneous white or crop-out may be explained by the action of the sabino gene.

• Rare white horses are thought to be the result of a dominant white gene. No homozygous white horse has been found and is thought to be non-viable.

• Dominant white is most likely explained by sabino-white.

White - Ww

Spotted Horses

• Spotting occurs in many forms in many breeds.

• All horses carrying the spotting gene will show 'characteristics' or mottling.

• Spotted-bred horses without characteristic have lost the spotting factor.

• The spotting factor is dominant and must be present before spotting pattern can be exhibited.

• The patterns are dark spots, white spots, snow cap and varnish, each under the control of a separate dominant gene.

• Some of the patterns mimic one another, making identification difficult.

• The extent of visible spotting is dependant on modifying genes and the number of pattern genes.

• The greater the number of patterns in the parents, the higher percentage colour production.

• All spotting patterns may be safely bred to on another.

• The true few-spot leopard gives colour 100% of the time.

Characteristics of the appaloosa spotting pattern:

White sclera of the eye - present at birth

Mottled skin pigment on the face and/or genetalia - present only in one-third of foals and birth and acquired later

Striped hooves - present at birth

For the time being until I get a better grasp on the genetics of appaloosas I will use the following as a loose guide for my horses:

Appaloosa = ApAp or Apap

non-appaloosa = apap

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