#colour genetics

Most people are familiar with the following anatomy and so I will not define it (if you do not understand one of these, then please ask. I will not shame anyone for not knowing.)

Familiar anatomy: ears, neck, back, head, shoulder, elbow, thigh, tail

Terminology you will need to know to understand this post:

Chrome: the white markings on a horse (note: neither grey, leopard complex nor roan counts as chrome.)

Croup: the rump of the horse.

Dock: the living part of the tail - i.e. the part not made of long hair.

Fetlock: the joint on all four legs right above the hoof. Analogous to the first knuckle of the middle finger/toe.

Foal: a horse under 1-year of age.

Hock: the 'middle' joint of the hindlegs. Analogous to the ankle.

Hoof: the foot of the horse, made of bone and soft tissue (internal) with a keratin (fingernail) outer layer.

Knee: the 'middle' joint of the front leg, analogous to the wrist

Muzzle: the mouth and nose of the horse

Points: the legs, ears, mane and tail of the horse

Sheath: the protective skin over the penis.

Stifle: the area in front of the hind-end and behind the rib-cage.

Topline: the top of the horse's body - i.e. the top of the neck, withers, back and croup.

Section 2:

Genetics terminology:

A forward slash (/) is used to separate the genes on the same locus from each other. A space ( ) is used to separate loci. A hyphen (-) is used to indicated that any gene on that locus may be present (e.g. E/- means that the horse can be E/e, E/E or E/e^a.)

Autosomal dominant (referred to as 'dominant' in this post): a gene that requires only one copy to express fully.

Autosomal recessive (referred to as 'recessive' in this post): two copies of the gene needed to express.

Base colour: one of three colours (chestnut, black, bay) that the other genes are considered to act on. I.e. a palomino horse has a chestnut base coat because the colour is chestnut with a dilution gene. A black dun is a black base. A bay horse has a bay base.

Eumelanin: black pigment.

Incomplete dominant: only one copy of the gene needed to express but horses with two copies will have a different phenotype.

Locus (plural: loci): the location on the chromosome where the gene is found. There can be, at most, two copies of a gene on one locus (i.e. sunshine (sun) and pearl (prl) are on the same locus, thus you can have a horse that is sun/sun OR prl/prl OR sun/prl but you cannot have a horse that is sun/sun prl/prl.)

Melanin: the pigment of plant or animal structures (e.g. hair, skin).

Phaeomelanin: red pigment.

X-linked or semi-dominant: a gene that is only on the X chromosome. A typical female requires two copies to express whereas a typical male needs (and can only have) one copy. (Note: horses with chromosomal mutations may express these differently, i.e. X females can only have one copy and only need one to express and XXY males would need two copies for it to express.)

Section 3:

Terms you may be interested in knowing:

Cannon bone: the lower-leg bone above the fetlocks in both the front- and hind-legs.

Chestnut (anatomy): a vestigial toe found just above the knees and below the hocks in the form of a callosity.

Colt: a male horse under 4-years.

Ergot: a vestigial toe found just underneath the fetlocks in the form of a callosity.

Feathering: the longer hairs on the lower legs. Most horses have no feathering; some horses, like Clydesdales, have a lot of feathering. Horses originating from colder climates tend to have feathering whereas hotter climate horses tend to not. Increases risk of chronic progressive lymphedema.

Filly: a female horse under 4-years.

Gelding: a neutered male horse.

Mare: a female horse that is 4-years or older.

Pony: for showing purposes; a horse that is under 14.2hh (147cm, 58 inches). In practise, proportions also affect whether a horse is a horse or a pony (for example, Arabians, Fjords, Icelandics and Miniatures are always horses, no matter their height.)

Stallion: an intact male horse that is 4-years or older.

Section 5:

Now, onto the colours:

It should be noted that a horse's fur thins/is not present in some places and the skin is visible - notably, around the muzzle and eyes, the udders (if applicable), genital area, around the anus and on the underside of the dock. The coat is thinner in summer and thus more skin is visible compared to winter. It also varies by breed - some very hairy horses, like the Romani Cob, even have 'beards' and 'mustaches'.

Agouti (A)

Subsection (bay (very old, predate domestication), seal, wild bay)

Bay:

Bay is one of the three base colours and is the most common horse colour. The origin of bay predates domestication. A typical bay has a reddish-brown coat with black points and dark skin and eyes. The colour of the body can range from near black to blood red to a light tan.

Bay requires at least one extension (E) gene and at least one agouti (A) gene to express. E/E A/a (homozygous E, heterozygous A) tend to be dark bay whereas E/e A/A (heterozygous E, homozygous A) tend to be light bay.

Extremely old, predates domestication.

Dark bay:

The body is mostly black with some brown around the muzzle, eyes, chest, behind the elbow, flank and thigh. The brown can be so minimal that the horse appears black.

Light bay:

Like bay, but the black on the legs does not extend past the knee/hock.

Brindle

Rare

BR1:

Very rare.

X-linked gene, semi-dominant. Heterozygous females have a brindled coat with abnormal hair growth pattern - whether the brindle pattern is caused predominantly by pigmentation or by texture varies depending on the horse. Homozygous females and males with BR1 have a sparse mane and tail and no noticeable brindle pattern. In winter, they often have a 'moth eaten' appearance.

Chimaera:

INCREDIBLY RARE. As of 08/03/24, there are only two known chimaera horses.

This is caused when two eggs or fraternal twin embryos fuse into one, thus the resulting horse has the genes of two different horses. A chimaera might not necessarily be visually obvious.

Skewed dun:

A dun or nd1 horse where their primitive markings have formed an unusual pattern, which can appear brindle. (See: Dun)

Skewed grey:

The horse does not grey in the usual manner, this can cause a brindled appearance or markings like 'bloody shoulder'. The horses coat will eventually grey out completely and the pattern will not be noticeable. (See: Grey)

Extension (E)

Black:

An extremely old gene that predates domestication. Dominant. Extension causes the coat to be completely black. The sun can bleach the coat to a brownish colour; some horses are more prone to sun bleaching than others, this may be affected by nd1.

Chestnut:

A very old colour which likely emerged around the same time as the horse was domesticated. Recessive. The coat of a chestnut is typically a reddish-brown colour but can range from appearing black to a light tan/cream colour. The mane and tail are often roughly the same colour as the body but can range in shade from being white to very dark. A chestnut with a dark coat is often referred to as a 'liver chestnut'.

There are two genes that cause chestnut, the more common one, e, and the incredibly rare one, e^a (occurs in Black Forest horses, Knapstruppers and Canadian breeds). The two genes have an identical effect on the coat.

Champagne (Ch)

An uncommon colour, the gene is dominant and dilutes the coat. Black hairs are diluted to a taupe-like colour whereas red hair are diluted to anything ranging from cream to gold to light tan. Champagne horses often have a metallic sheen that makes them difficult to accurately catch their colour in photographs. The skin of an adult champagne horse is lavender-pink mottled with darker skin and the eyes are hazel or amber. As a foal, they are born with pink skin and blue eyes.

Red champagne might be referred to as gold champagne. Black champagne may be called classic champagne. Bay champagne may be called amber champagne. 'Seal brown' champagne (see: OUTDATED) may be referred to as sable champagne.

Chrome

Refers to any white markings on a horse. See: KIT, overo, rabicano, splash white, section 6, white markings

Cream

The following colours are on the same locus, thus, a horse can only have two total copies of any of the following four genes - i.e. a horse can be Cr/Cr OR prl/prl OR sno/sun but it cannot be Cr/Cr AND prl/prl AND sno/sun.

Cream (Cr):

An old gene that appeared after domestication. Incomplete dominant. When heterozygous, it only dilutes red hairs - the shade is typically a cream, tan or gold colour but may range from off-white to black and the skin and eyes are dark (although the eyes may be a lighter brown compared to non-cream.)

When homozygous, it dilutes both black and red - the homozygous creams are considered to be phenotypically indistinguishable from each other, the coat can appear anywhere from white to a light taupe colour, the skin is pink (occasionally with some mottling) and the eyes are a washed out, pale blue.

Some homozygous cream foals may be born with a patched coat that fades after the first shed. Thus far, this has only been observed in black based foals.

Heterozygous cream on red is called palomino.

Het. cream on bay is buckskin.

Het. cream on black is smoky black.

Homozygous cream on red is called cremello.

Homo. cream on bay is perlino.

Homo. cream on black is smoky cream.

Pearl (prl):

A somewhat old gene that appeared after cream, it is on the same locus as cream. It is recessive, when homozygous, it dilutes the hairs, the skin is pink and the eyes are dark. A cream-pearl horse is indistinguishable from a homozygous cream horse.

Snowdrop (sno):

A very new and rare, recessive gene, discovered in 2019 in Vanners, it looks similar to double cream.

Sunshine (sun):

A very new and rare, recessive gene, discovered in 2019 in one Tenessee Walking Horse x Standardbred. The horse was Cr/sun and looked like a double cream. It is unknown what a homo. sun would look like.

Dun

Dun (D):

Dun is a dominant gene that dilutes the coat (the mane and tail are not diluted) and causes the horse to have a dorsal stripe - the dorsal stripe is visible in the tail of the horse. The horse will often have other primitive markings (see below for a list of primitive markings.)

Bay dun is considered to be the wild-type colour of a horse (i.e. the original colour of horses) and is usually a cream, tan, gold or desaturated brown colour with black points.

Red dun is often a pinkish colour.

Black dun, also called grulla, is a greyish to greyish-brown colour.

nd1:

Also an extremely old gene, likely predating domestication. It has a similar effect to dun but will not dilute the horse as much. The dorsal stripe does NOT extend into the tail. A horse that is D/nd1 will appear dun.

nd2:

Another old gene, likely predating domestication but only appearing after nd1. The coat is not diluted in any way and the horse has no primitive markings. This gene is recessive.

Primitive markings:

Barring: non-diluted stripes that appear on the back, legs, thighs or ears of the horse.

Cobwebbing: the horse has 'webs' of non-diluted colour on the face between the eyes.

Darker ear tips: the ear tips are not diluted. May be as much as half the ear to as little as just the very tips. Common.

Darker legs: the legs are not diluted. Common.

Dorsal stripe: a stripe running along the back of the horse. Present in all duns.

Lacing: rare. Non-dilute colour in a lace pattern on the body.

Lighter back of legs: the back of the legs is lighter. Usually around the fetlocks but may extend up to the knee/hock and down to the heel.

Mask: the face of the horse is not diluted.

Eden White and HPS5:

Unfortunately, these genes are reported by Etalon, which has a history of 'discovering' new genes that are already known. Take they're existence with a grain of salt.

Recently reported group of white spotting, little is known about it but it is thought to cause white markings in draught and stock breeds, although it has been found in all breed groups. (Article) (<- i recommend reading the article because whoever wrote the intro missed their true calling as a fantasy-romance writer.)

Three Eden White variants have been found: Eden White 1, Eden White 2 and Eden White 3. It causes similar markings to splash white.

Flaxen

Flaxen refers to the mane and tail being lighter than the body of the horse. Most causes of flaxen are unknown. One known gene that only affects black hairs is silver (see: Silver).

Gulastra's plume

This is where a bay horse has silver hairs in the tail with little to no effect on the mane and sometimes causes a few white hairs at the fetlocks. The black points are often reduced. The cause is unknown.

Grey

Two types slow grey and fast grey

Grey is a common, dominant gene that causes a horse's coat to gradually turn white (referred to as 'greying out'.) The horse is born their base colour (and are often more saturated in colour than non-grey foals). The coat lightens and the hairs gradually turn white, this often happens in a dapple pattern (referred to as 'dapple grey'), the skin and eyes remain their base colour. The horse may eventually gain 'fleabites' - flecks of their original base colour. The rate at which an individual greys out varies. Greys are more prone to melanomas, with homozygous greys being affected worse than heterozygous.

It has recently been suggested that there might be two grey genes - the more common 'fast' grey gene (where the horse greys rapidly and the hairs turn completely white) and the 'slow' grey gene (where the horse greys slowly and the hairs do not turn completely white.) There is a possibility that slow grey does not cause the horse to be predisposed to melanomas.

KIT

The following genes (roan to dominant white) are all found on the same locus, thus, you can only have a total of any two of the genes at one time with the except of W20/W20 W22/n

Roan - refers to white hairs intermingled in the coat to varying effect. This is caused by various genes. Some roans have a drastically different summer coat compared to their winter coat, with the winter coat appearing non-roan. The undercoat is white. Some roan genes may be homozygous lethal.

Classic Roan (Rn):

Dominant, not homozygous lethal. The horse has a roaned coat, the face (excluding the cheeks), mane, tail and legs do not have any roaning. There is a distinctive, upside down v on the legs where the roaning stops. This colour is NOT heat sensitive.

Corn marks - when the skin of a roan horse is even slightly damaged, the hair at the site of the damage grows back the back colour, without any roaning. This can be caused even by the smallest cuts and abrasions that a horse gets from typical play, scratching, etc.

Reverse dapple - occasionally, the white hairs of a roan may be more concentrated in spots on a horse, causing a dappled pattern.

Sabino:

Horse with roaned chrome markings - NOT caused by classic roan or similar genes. For sabino1, see dominant white.

Tobiano (To):

A dominant gene that causes smooth, rounded white markings that typically occur on the legs and topline of the horse and NOT on the head (any head markings are caused by other genes.) Tobiano will often express over the hip of a horse. Eyes are dark and the skin under white markings is bright pink. Technically not found on the KIT locus but is close enough that it disrupts KIT genes anyway.

Dominant white:

(Dominant white 0-35)

Dominant whites are characterised by dark eyes, pink skin (under white markings) and roaned white markings that often increase with age. The following descriptions are based on a select few reference horses. Most of these genes are currently quite rare and, in the future, it may be found that the appearance is more variable than what is described here.

SB1 (W0), sabino1: incomplete dominant, white markings start from the underside and face, spreading upwards, with some white markings on the belly. Horses can range from very minimally white to appearing completely white.

W1 - W4: similar in appearance in that the horse is born white or near-white that fades to white with age. Eyes are dark and skin is pink.

W1: possibly embryonic lethal when homozygous, found in Freiberger horses. The origin horse is Cigale, born in 1957.

W2: found in Thoroughbreds, origin horse is KY Colonel (1946).

W3: Arabian, R Khasper (1996), possibly homozygous lethal. Some descendants have blue eyes but this is thought to be inherited separately from W3.

W4: Camarillo White, Sultan (1912).

W5: Thoroughbred, Punchilingui (1984), possibly embryonic lethal when homozygous. Appearance varies greatly from completely white to almost no chrome.

W6: Thoroughbred, Marumatsu Live (2004). Appearance is heavily roaned all over the body with some solid markings. Skin is mostly pink, eyes are dark. Mane and tail have some white hairs but to a lesser extent than the body.

W7: Thoroughbred, Turf Club (2005), horse is mostly white, with some splotches of base colour on the face and topline and flecks on the neck. Mane and tail appear to be the base colour.

W8: Icelandic, Þokkadis vom Rosenhof (unknown year), 'marbled' appearance.

W9: Holsteiner, unknown horse (2006). Mostly white with a small, base spot on the flank. Dark eyes, pink skin.

W10: American Quarter Horse, GQ Santana (2004), possibly homozygous lethal. Similar in appearance to a greyed-out horse with moderate chrome. Eyes are dark and skin is mottled.

W11: South German Draught, Schimmel (1997), fully white coat, dark eyes, pink skin.

W12: Thoroughbred, unnamed (2010), extinct colour. Colt died at 5 weeks of age, uncertain if W12 was related to its death (cardiovascular shock and ventricular arrhythmia). Splotches of white on base coat, dark eyes, pink skin under markings.

W13: first seen in a Quarter Horse - Paso Fino crossbreed, now in multiple horse and pony breeds, including those unrelated to QH, name of original horse unknown, year unknown. Appears completely white. Probably homozygous lethal.

W14: Thoroughbred, Shirayukihime (1996), appearance is usually white but may have spots of colour.

W15: Arabian, Khartoon Khlassic (1996). Mostly base colour with chrome on the underside and face and roaning.

W16: Oldenburger, Celene (2003). Near white.

W17: Japanese Draught, Hakubahime (2010). Completely white.

W18: Swiss Warmblood, Colorina von Hoff (unknown year). Moderate chome with roaning. Hooves mostly dark.

W19: part-Arabian, Fantasia Vu (1990). Heterozygous has splotches of chrome, homozygous W19 is white (may have base speckles on the chest). Sterile when homozygous.

W20: a very common and ancient colour found in many breeds with no known origin. Causes minimal white markings on the face and legs. Tends to cause other white markings to express more extensively.

W21 (ýruskjóttur): Icelandic, Ellert frá Baldurshaga (2013). White underside.

W22: Thoroughbred, Not Quite White (1989). All W22 have W20, W22/n W20/n is written as W22, W20/W20 W22 is written as W22W20. Probably homozygous lethal. Similar to splash.

W23: Arabian, Boomori Simply Stunning (1989), possibly extinct. Completely white.

W24: Standardbred, Via Lattea (2014). Completely white.

W25: Thoroughbred, Laughyoumay (2001). Appearance may be either solid white, 'faded' or both - reference picture did not clarify which horses in the picture were W25.

W26: Thoroughbred, Marbrowell (1997). All have at least one copy of W20. Splash-like markings with roaning.

W27: originated in Thoroughbred, introduced to 'show jumpers' and Paints, Milady Fair (1960). White mottled with base.

W28: German Riding Pony, Phantasia (2003), moderate chrome with roaning.

W29: unassigned

W30: Berber, Aghilasse (unknown year). Can be completely white or white with base ears, tail and spots on the croup (latter colour may fade to solid white as an adult.)

W31: American Quarter Horse, Cookin Merada (2004). Variable amounts of white. Can be a moderate amount of white splotches or almost completely white.

W32: Paint, Small Town Scandal (2009), moderate white on face and legs only.

W33: Standardbred, original horse is presumably Art Major (1999). Almost completely white as a foal that turns completely white as an adult.

W34: multiple breeds. Small white markings on the face and legs.

W35: multiple breeds. Minimal white markings on face and legs, reference horse also had a small white marking on the back.

Dominant white combinations:

W5/W20: near white to completely white.

W13/W20: completely white.

W13/W0: completely white.

W3/W19: white with base topline as foal to almost completely white with base ears and dark hooves as an adult.

W15/W19: almost completely white save for a base fleck on the hindquarter.

W31/W20: completely white.

Leopard complex (Lp)

Blanket:

Heterozygous Lp with a PATN gene that causes the pattern to cover the hindquarters.

Bronzing:

Lp sometimes causes black hairs to turn brown as the horse ages. This can make a black or bay horse appear chestnut.

Fewspot:

Homozygous Lp with a PATN that causes the pattern to cover the whole (or most of the) body.

Leopard:

heterozygous Lp with a PATN that causes the pattern to cover the whole (or most of the) body.

Leopard Complex (Lp):

A very old gene that predates domestication. Incomplete dominant. At least one copy of this gene is required for any spotted patterns to express. Leopard complex with no pattern genes is called varnish roan. The horse is born its base colour and 'varnishes' out as it ages. The gene causes mottled skin, striped hooves and wall eyes (when the sclera of the eyes is white instead of coloured.)

Homozygous Lp horses have Congenital Stationary Night Blindness (CSNB) and are at elevated risk of Equine Recurrent Uveitis (ERU). Heterozygous horses are still at risk of eye problems but to a lesser extent.

PATN1:

A very old gene that predates domestication. Causes the leopard pattern. Dominant. Requires at least one copy of Lp to express. Skin is dark underneath spots.

Snowcap:

Homozygous Lp with a PATN gene that causes the pattern to cover the hindquarters.

Snowflake:

White spots caused by Lp.

Varnish roan:

See: Leopard Complex

Metallic

Some Akhal Teke have a metallic sheen to their coat, caused by individual hairs being hollow. The gene that causes this is unknown. Some champagne horses also have a metallic sheen.

Mushroom (mu)

A rare, recessive gene found in Shetland ponies and (rarely) Miniature horses that lightens and desaturates red hair into a taupe colour and has no effect on black hairs.

Overo

Frame, splash and sabino horses are all overo.

Frame Overo:

An incomplete dominant gene that is lethal when homozygous. Horses can express little to no markings and, thus, a horse of any breed that has overo in it (and is not clear by parentage) must be tested for overo prior to breeding.

Overo tends not to express unless the horse has another white spotting gene, such as W20. When frame is combined with another white spotting gene, the markings tend to express as jagged splotches of white that tend to NOT cross the topline or underside of the horse (thus creating the image of a white 'framed' by the base colour.) The skin is pink under white markings. The eyes may be dark or blue (typically when white markings cover the eye.) May be deaf. NOT a KIT gene.

Lethal White Syndrome:

Homozygous frame horses have LWO, where the foal is born completely white with pink skin and blue eyes and dies painfully a few days after birth from intestinal abnormalities.

Pangaré

A type of counter-shading where the underside, muzzle and area around the eyes of the horse is lighter than the top. May be very minimal or extremely extensive. Also called mealy. Does not affect black. Potentially wild-type.

Foal vs adult coat: non-grey foals are typically born desaturated compared to their adult colour with a pangaré pattern.

Rabicano

Subsection (skunk tail, stripes)

Unknown gene(s) that cause limited roaning on the horse. Notable features include white hairs in the tail (skunk tail), limited roaning (typically on the flank) . Red based horses may express rabicano more extensively than black and bay based horses. Many horses are misidentified as rabicano.

Silver (Z)

An uncommon, dominant gene that only affects black hairs, diluting them. Sometimes causes dapples on black coats. Silver is associated with Multiple Congenital Ocular Abnormalities, heterozygous horses typically experience less severe defects compared to homozygous horses. Vision may progressively worsen as the horse ages.

Silver black, also called silver dapple or taffy (in Australia). The horse is either a dark grey or brown-grey colour, may or may not have dapples.

Silver bay: silver on a bay coat. The brown part of the coat is unaffected, while the black hairs on the legs and ears are usually diluted to a brownish colour. The mane and tail are flaxen.

A chestnut horse with silver is not visibly changed, however, it will still be at risk of silver-related eye problems.

Sooty

Sooty is when there are black hairs interspersed throughout the coat. The cause of this is unknown. The amount of sooty may change with age and between winter and summer coats. Not visible on black horses. Sometimes, the sooty forms a dapple pattern, called 'sooty dapple'.

Splash white

Splash causes well-defined, white markings that spread from the bottom up (as if the horse has been splashed with white paint.) The eyes are usually blue and the skin under white markings is pink. Extensive white markings are associated with deafness. The various splash whites tend to look similar.

Possible embryonic lethal combinations: SW3/SW3, SW4/SW4, SW5/SW5, SW6/SW6, SW1/SW5, SW3/SW5, SW3/SW6, SW5/SW6

There are two locations where splash occurs: MITF and PAX3. A horse can have two MITF and two PAX3 genes at the same time but cannot have three or more MITF or three or more PAX3 genes at the same time. NOT a KIT gene.

MITF^prom1, SW1: incomplete dominant. Heterozygous have minimal white while homozygous have extensive white. The most common and oldest of the splash mutations and predates modern breeds.

MITF^C280Sfs5*20, SW3: very rare, found in certain American Quarter Horse and Paint lines. Possibly homozygous lethal.

MITF^del, SW5: found in American Paint Horses, possible homozygous lethal and very rare.

SW6: found in dual-registered American Quarter Horse-Paint. Possibly homozygous lethal and very rare.

MITF^N3105, Macchiato: informally named mutation found in one, deaf, sterile horse.

PAX3^C70Y, SW2: a fairly old mutation.

PAX3:p.Pro32Arg, SW4: found in Appaloosas, possibly homozygous lethal.

Spotted (see leopard complex)

Tiger eye

Very rare and can be caused by two different genes (found on the same locus). These genes are both recessive and only found in Puerto Rican Paso Fino horses.

TE1: the more common of the two tiger eye genes. It causes the eyes to be yellow, amber or bright orange.

TE2: VERY rare, only found in one palomino horse that had very light yellow/blue eyes.

Section 6, white markings

Face markings

Snip: white marking on the nose.

Star: white marking between the eyes.

Stripe: narrow white marking along the nose bridge.

Blaze: broad white marking along the nose bridge.

Bald: a mostly white head.

Medicine hat: an almost completely white head with base colour ears.

Badger face: white markings that go up the side of the face, like the markings of an American badger.

Reverse badger face: badger face but the base and white markings are inverted.

Leg markings

Coronet: small white marking above the hoof. Typically has no effect on hoof colour.

Pastern: small white marking that is more extensive than a coronet but does not cover the fetlock of a horse. Usually turns the hoof pale.

Sock: a white marking that covers at least the fetlock but does no cover the knee or hock. Hoof is almost always pale.

Stocking: an extensive white marking that covers at least the knee or hock and may cover the entire leg. Hoof is almost always pale.

Section 7, interesting combinations:

Grey-and-chrome causes an interesting effect when the base coat has dark skin. When the horse is fully greyed, one can usually see where the base coat used to be, as the darker skin causes the white fur to have a cool, blueish tone, while the chrome areas will be warm and pinkish.

A similar effect can be observed with grey-and-spotted, where one can easily see where the spots used to be.

Cream-pearl: horses that have one cream gene and one pearl gene are called cream-pearl and are phenotypically identical to double cream.

Tovero: a common combination of tobiano and overo.

Peacock: is a leopard complex pattern where the spots are 'ringed' with a lighter colour. This pattern fades as the horse ages.

Section 8, incorrect, outdated and inaccurate terms:

Albino: albino horses do not exist. People often incorrectly call double dilute horses albino.

Appaloosa: people sometimes refer to any spotted horse as Appaloosa, however, this is incorrect as Appaloosas are a breed and not a colour.

Chocolate: incorrect term for a sooty palomino or silver black

Chocolate palomino: sometimes incorrectly used to refer to silver black

Chocolate silver: incorrect term for a sooty palomino

Cremello: incorrectly used to refer to any double cream but is only correct when referring to red double creams.

Grullo: incorrect term for grulla

Isabella: light palominos are mistakenly called isabella. Isabella is what pearl horses are called in Spain.

Red roan: red roan used to refer to bay+roan however that is considered outdated and genetically inaccurate. Red roan now refers to chestnut+roan while bay roan is used for bay+roan.

White: (only correct when referring to dominant white)

Outdated:

Seal brown: old term for dark bay, when it was thought to be a separate gene from 'regular' bay.

Strawberry roan: outdated term for red roan (chestnut roan).

Wild bay: outdated term for light bay, from when it was thought to be a separate gene.

Section 9: additional explanations

Why bay is considered it's own base colour and not a modified form of black.

Firstly, this has to do with bay (dun) being the wild-type colour. The original base colour is bay, the original colour genetics are E/E A/A. Horses then had a recessive mutations on the agouti and extension genes - the mutation of A to a, leading to horses being able to be E/E a/a and thus black and the mutation of E to e, allowing horses to be e/e A/A and thus chestnut. Bay is not modified black, it is more correct to say that black and chestnut are modified bay.

Secondly: base colour are determined by the base pigments - phaeomelanin and eumelanin, thus chestnut, black and bay, as the only colours that modify the production of those pigments, are thus the base colours. This is also why dun is not considered to be the base despite being the wild-type - dun dilutes the base pigments, but does not change the production of them.