Colour and Colour Inheritance in Horses:
a brief essay
© Kim Middel 1995


A very special colour: ljósmoldótt, ljósfext. 
The mare Hulda Hnöll frá Torfunesi 

Colour inheritance in horses is not half as difficult as it seems: there is a very logical explanation to almost anything you see. When one looks at a very colourful species of horses such as the Icelandic horse the range of colours and colour combinations seems endless and impossible to tag names onto; in fact, it is only a matter of sorting out gene patterns.

Most genes which influence horse colouring are dominant, some are recessive. These two basic forms in which the genes occur are called dominant and recessive alleles; another name for colour genes is loci (singular: locus), referring to the place where they start working on the pigment. Dominant alleles are referred to with capitals and recessive alleles with minuscles; since the horse always has a pair of compatible genes, or rather alleles, it can, for instance, have Aa in its genetic colour pattern: a dominant A-allele and the recessive a as its counterpart. This is the way the genotype or gene pattern is denotated; the phenotype means what you see on the outside of what's in the genotype. When we know one dominant allele is present and we are not sure whether the other is or isn't, the second allele is denotated with a period mark: A. (meaning AA or Aa). We call a pair of the same alleles homozygous (e.g. AA) and of different alleles heterozygous (e.g. Aa).

Sorts of colouring and the genes that cause them

Basic colours

First of all, it is necessary to know that all colours are based on the basic colours chestnut, bay and black. These three colours are based on the presence of the following genes:

E - causes black and brown pigment, eumelanin, to be formed when present in dominant form.

A - causes red and yellow pigment on top of black (EE or Ee), phæomelanin to be formed when present in dominant form.

These are fully dominant, i.e. only one of the alleles has to be dominant to effect full forming of the pigment involved. Thus:

aa E. =
black; aa means no red/yellow pigment is formed, B. means blackpigment is formed
A. E. =
bay/brown; A has considerable impact on E causing red on top of black, with exception of mane, tail and legs.
aa ee =
chestnut; ee doesn't form black pigment; aa will not interfere on black. The horse will then always be red over its entire body.
A. ee=

chestnut; if a horse has a A. ee genotype, for instance if it is born out of a bay parent and a chestnut parent, its phenotype will still be chestnut, for A only interferes with E, not with e! This accounts for the fact that out of a chestnut and a black horse a bay foal can be born.

 

Colours caused by the Ccr-locus


Son of Buck TWH
palomino
Now that we've seen the basic pattern to all colours, we must look at the C-locus because it is most important: the three basic colours and the six colours formed by the Ccr allele of the C-locus (cr for cremello) are at the basis of all other genes. All other genes will work on these nine colours.

Let us first look at what the C-locus does. The C-locus has three alleles: two dominant ones and a recessive one. The C-allele does nothing, the Ccr-allele effects dilution of colours. Ccr is not fully dominant: its influence on the basic colours is more intense when homozygously dominant than when heterozygously dominant. It cannot overrule the strength of C completely. Thus:
 
CC CCcr  CcrCcr
aa E. black  smoky silversmoky
A. E. bay buckskin perlino
aa ee/A. ee chestnut palomino cremello


Longview Takes No Chances TWH
cremello
aa E. CCcr:
smoky is in its most expressive form steel grey with a metal gloss to it or black with red shining through the colour all over the body; it can also look like a very dark bay or light black; there is something yellowish to the coat.
aa E. CcrCcr:
silversmoky is light steal grey or a very light clay colour with pink skin and light brown eyes. It's very rare!
A. E. CCcr:
buckskin is a yellow or cream colour with black mane and tail.
A. E. CcrCcr:
perlino is a cream colour with cream coloured mane and tail and blue eyes.
aa ee CCcr:
palomino/isabel is yellow/cream with creamy white mane and tail. The eyes are the usual brown.
aa ee CcrCcr:
cremello is the same phenotype as perlino.
NOTA BENE: these genotypes NEVER cause a dorsal stripe on the back!!!!!


The other colour genes

The D-locus

The D-locus (D for dun) causes dun colours when present in dominant form: the colour becomes lighter and a dorsal stripe to appear on the back; perhaps also wild stripes on the legs and a stripe crossing from one shoulder over the back to another. Legs and head are always of darker colour than the rest of the body. The dominancy is full; homozygous dominant alleles are denotated as DD, heterozygous dominant alleles as Dd.

Unfortunately, when this gene works on chestnut, bay and black the result is often almost the same as when CCcr is present (with dorsal stripe of course) and therefore diluted and dun colours are often confused. To go short:
aa E. D. =
mouse dun/blue dun/grulla: steal grey with black&white mane and tail and a black dorsal stripe (Tarpan horses).
A. E. D. =
yellow dun: light bay to yellow with black or black- &white mane and tail and a black or brown dorsal stripe (most Norwegian Fjord horses).
aa ee D. =
red dun: light chestnut with mane and tail ranging from chestnut to yellowish orange; also yellow with cream mane and tail; dorsal stripe brown or chestnut.
Of course, the Ccr-colours can also be affected by the D-locus. We'll have to describe these with the Dutch term wild colour, used to denotate the presence of this gene. We then see:
aa E. CCcr D. =
wild smoky; a lighter variant of the mouse dun, the main body colouring being yellowish grey.
A. E. CCcr D. =
wild buckskin; a lighter variant of the yellow dun with the main body colouring being cream.
aa ee CCcr D. =
wild dun/palomino; light yellow to cream body colouring; mane and tail cream; brown or yellow dorsal stripe.
Should it occur on perlino or cremello it is not likely to be visible or only just. But what if for instance you have a dark wild buckskin, whose colour look almost like yellow dun? In such cases, it's worth while waiting until the horse is in its winter coat: D-locus colouring has a tendency to become lighter in wintercoats than in summercoats, so a A. E. CCcr D. horse would still get a lighter coat in the winter and a A. E. D. horse wouldn't.

In Amrican terminology there are more than one way for describing dun horses, e.g. apricot dun (red dun with orange body and red mane, tail and eel stripe), golden dun (either light red dun or palomino dun), cream dun (dun palomino) and silver dun (light blue dun). Mind you these are only terms for fenotypes, not for genotypes!

The P-locus

The P-locus causes skew- or piebald (Am.: tobiano) spotted patterns; it is a dominant gene and the most common of the genes that cause spotting. The white fields on the horse are mainly on the shoulders, back and flanks. White fields on the back will also interrupt dorsal stripes if present.
aa E. P.    = piebald (black tobiano)
A. E. D. P. = skewbald (yellow dun tobiano)

The Z-locus


Thrymur frá Geirshlíd
black silver dapple
The Z-locus causes the rare colour of silver dapple, found in Iceland horses, Dutch warmblood horses, Groningen horses, Welsh ponies, Falabella ponies, Rocky mountain horses and Shetland ponies. It colours a deep, dappled chocolate brown colour of the body with silver white or silver grey mane and tail. It is dominant but only works when one or two dominant E-alleles are present. Therefore, it cannot be seen on a chestnut even when it is present in dominant form. Thus:
aa E. Z. =
silver dapple black; deep dappled blueish brown with white mane and tail
A. E. Z. =
silver dapple bay; brownish chestnut with white mane and tail; to distinguish it from a sorrel one has to look at the fetlocks: these are grey or white, but never yellowish.
I know some Icelandhorses who are silver dapple mouse dun, silver dapple yellow dun, silver dapple smoky or silver dapple buckskin; even one that is silver dapple wild smoky. These are all lighter than their counterpart without the D-locus or the D-locus or both and of course all have white mane and tail. If dilution genes or dun genes are present, the colour may tend towards yellow. A silver dapple buckskin and a silver dapple yellow dun look almost like palomino's, except that they may have extensive dappling.
 
 


Silver dapple yellow dun/vindbleikálótt. 
The mare Líf.

The F-locus

The F-locus causes what is called white-headed paint or splashed white horses. It seems to colour the parts of the horse white that would probably have been coloured in a normal tobiano pattern: The belly, the neck and, most important, the head. The colour only seems to occur in species from Northern Europe (F for Finnish paint, since it was first found in Finland). It works when homozygous recessive present. Therefore, out of two normally coloured looking parents, a whiteheaded paint can be born.
aa ee F.  = chestnut
aa ee ff  = splashed white chestnut

The O-locus

The O-locus causes sabino paint colouring. It is best seen in Shire horses: unsharp, mottled white big spots on the belly and large white "boots" covering the legs. The white fields have not a very distinguished end butget diluted into the coloured fields. Even the sheer presence of the completely white legs is enough to spot a sabino paint. The presence of white can vary from one single little white spot to a white horse with vague coloured spots coming through the white. It is fully dominant.

aa E. O. = sabino paint black (Shire)

The Om-locus

The Om-locus is a mutant form of the O-locus, causing overo spot- ting: a coloured back, white fields on the sides marked by frea- kishly coloured fields on the edges. This is a common sort of paint in American horses. It works in homozygous recessive form.

A. E. CCcr omom = buckskin overo paint

The R-locus


Generator's Pushin' Blue THW
black roan
The R-locus (R for Roan) causes the occurence of white hairs among the coloured, thus causing an effect of mildew or mould over the horse's body. In Dutch and Belgian draft horses this gene occurs very often. It is dominant.

A. E. CCcr R. = buckskin roan


Jarplitföróttur/bay roan: 
the stallion Yngri

The G-locus

The G-locus is fully dominant and causes decoloration of any colour unto dapple grey or white in the first 3 to 7 years of its life.
A grey horse can be born in any colour, for instance a mouse dun skewbald, but will turn grey or white over the years. Most
Lipizzaner horses are born black or bay, but very few stay coloured (unless of course if their parents were not homozygously
grey and both handed a recessive allele down). The famous Icelandic stallion Gáski 920 frá Hofsstöðum, who died in 1995,
was born a silver dapple bay skewbald but carried the G-allele which turned him white: 

A. E. Z. P. G. = grey, born silver dapple bay skewbald 

The W-locus (lethal white)

The W-locus causes white colouring with blue eyes in heterozygous dominant form; these horses are not albino's for albino
colouring implies there's no pigment forming at all and such horses wouldn't be able to live: homozygous dominant alleles cause serious intestine defects in a foal and it will die shortly after birth.There is too little pigment in a WW-horse to be able to live. 

 aa E. W.     = White, born black
(aa E. WW)    = White, born black; not viable

Appaloosa colouring


Fairwind's Standing Ovation Tiger Horse
white body, coloured spots everywhere except in the tail area 
Unfortunately, the genes that cause Appaloos colouring are quite complexe. Basically, there is need for two different loci at
least: a mutant of the W-locus, Wap (ap for appaloosa), and the S-locus (S for spotting). The dominant Wap-allele will cause both white colouring and spotting, but they will only come out well if the horses is homozygous recessive for S. If either one of them is missing, no appaloosa spotting will come out. Thus: 

A. E. Wap. ss = appaloosa, white with brown spots. 

On top of this, there are five more loci that influence and af- fect the appaloosa patterns: 

* the Sl-locus (Sl for silver); 
     it causes in dominant form a frosting like pattern over the colour. 
* the M-locus (M for mottled); 
     when heterozygously present (Mm), it causes confined white mottled region at the end of the back. 
* the Bl-locus (Bl for blanket); 
     when heterozygously present it does the same as Mm but more extendedly over the back and the flanks. 
* the Rap-locus, a mutant of the R-locus; 
     when homozygously dominant it causes roancolouring in the white area's. 
* the Eap-locus, a mutant of the E-locus; 
     Eap extends the white area's on the body. Homozygously dominant alleles cause white exterior; heterozygously dominant alleles can cause white snowflake spots. 

If two or three of the first mentioned loci work, with or without Eap or Rap, so-called blankets of different size and colour
appear. A scheme of patterns: 
 
Genotype Phenotype
WapWap ss eapeap Mm Blbl Slsl  entire blanket
WapWap ss eapeap Mm Blbl slsl  or:
"              "          Bl. Sl. / Mm Sl.
half a blanket
Wapwap ss eapeap Mm Blbl Slsl spotted blanket, big spots
Wapwap ss eapeap Mm Blbl slsl or
"              "          Bl. Sl. / M. Sl.
spotted blanket, big spots
Wapwap ss eapeap Mm blbl slsl white spots on the hips
Wapwap ss eapeap mm Blbl slsl marmer-like spotting on the flanks
Wapwap ss eapeap mm Blbl Slsl roanish spotted blanket
WapWap ss Eapeap Mm Blbl Slsl entire blanket, white snowflakes on the body
Wapwap ss Eapeap Mm Blbl Slsl  spotted blanket, snowflakes on the body
Wapwap ss Eapeap mm blbl Slsl white snowflakes and coloured spots on the body
WapWap ss EapEap Mm Blbl Slsl white body, coloured spots everywhere except in the tail area
Wapwap ss EapEap Mm Blbl Slsl white body, coloured spots everywhere
Wapwap Ss Eapeap Mm Blbl Slsl few or none appaloosa features at all
Wapwap ss Eap. RapRap M. Bl. Sl. or: 
Wapwap ss EapEap RapRap mm blbl slsl
roan leopard.
(from: "Het paard in zijn kleurenrijkdom", J.K. Wiersema, Den Haag 1977.)

Crossing Schemes
Do you still get it? I hope you do. Now then, to make it interesting (so you can predict what colours of foals your mare will be able to give birth to next year or what colours you sire can produce with which dams), let us now look at some crossing schemes. These have all been derived from real examples.

Since a foal receives one gene from each parent, their gene pairs have been split so when the single gene from the male meets the single gene of the female they will again form a pair. It is therefore that pairs of alleles are split when you want to make a crossing scheme; you have to take all the possibilities of allele-combinations.

For example, a bay horse which has a AaEE genotype can give the following combinations after splitting its pair of genes: AE, AE, aE and aE.

The red dun (aa ee Dd) sire Gustur 754 frá Hrafnkelsstöðum and the piebald dam Skjóna frá Ólafsvöllum (aa Ee Pp) produced a mouse dun foal called Björk. Since only the father carried the D-locus, and only the mother carried E and P, Björk was heterozygous for nearly all loci: aa Ee Dd Pp.
 
 
aEP  aEp aeP  aep
aeD  aaEeDdPp  aaEeDd aaeeDdPp aaeeDd
aed aaEePp aaEe aaeePp aaee

There was a 1 in 8 chance of this colour.
 

The palomino dun mare Leira (aa ee CCcr Dd) produced three offspring, two of which buckskin and one palomino. Leira was born out of a palomino stallion, Amor (Lýsingsson) 793 frá Ásgeirsbrekku, and a dun mare. She therefore had only one D-allele which she did not pass on to these three foals. They were born out of a bay, a black and a chestnut stallion. Let us look how great the odds were for the colour of the first foal born.

The first foal, a buckskin mare named Drottning, was by Jarpur fá Sandhólaferju, who is supposedly AaEe.
 
 
AE Ae aE ae
aeCD  AaEeCCDd AaeeCCDd aaEeCCDd aaeeCCDd
aeCd AaEeCC AaeeCC aaEeCC aaeeCC
aeCcrD AaEeCCcrDd AaeeCCcrDd aaEeCCcrDd aaeeCCcrDd
aeCcrd AaEeCCcr AaeeCCcr aaEeCCcr aaeeCCcr
There was a one in 16 chance to this colour being born.

Icelandic and Dutch names for colour patterns

Both the Icelandic and Dutch language have a wide range of names for colour patterns in horses. Here's a scheme of their variety:

ENGLISH                 ICELANDIC                DUTCH
 
black                   svartur/brúnn            zwart
bay                     jarpur                   bruin
chestnut                rauður                   vos

smoky                   mosóttur/glóbrúnn        smoky
buckskin                moldóttur                valk
palomino                leirljós                 isabel

silversmoky             glóbrúnn/mórauður        zilversmoky
perlino                 hvítingur                perlino
cremello                hvítingur                cremello

blue/mouse dun          móálóttur                wildkleur zwart
yellow dun              bleikálóttur             wildkleur bruin
red dun                 (fífill-)bleikur         wildkleur vos/leemvos

smoky dun               -                        wildkleur smoky
buckskin dun            bleikmoldóttur           wildkleur valk
palomino dun            bleikleirljós            wildkleur isabel

grey                    grár                     grijs/schimmel

roan                    -litföróttur             -schimmel
* e.g. red roan         rauðlitföróttur          roodschimmel

pie-/skewbald           -skjóttur                -bont
*e.g. buckskin skewbald moldskjóttur             valkbont

Finnish paint/          kápóttur                 witkopbont
splashed white          sléttuskjóttur

silver dapple black     móvindóttur              zilverappelzwart
silver dapple bay       rauðvindóttur            zilverappelbruin

but:

silver dapple mouse vindmóalóttur zilverappel wildkleur zwart 
silver dapple grey vindgrár zilverappel grijs
 

Post scriptum

I really hope you have enjoyed this bit of genetics and that the story has been clear and will be happy to receive comments or questions.
Kim Middel

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