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.
- The basic colours black, bay and chestnut
- Colours caused by the CCcr-locus
- Other colour genes
- Crossing schemes
- Icelandic and Dutch names for colour patterns
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
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
- 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.
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.
- 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.
- 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.
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
- 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.
Silver dapple yellow dun/vindbleikálótt.
The mare Líf.
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
A. E. CCcr R. = buckskin roan
Jarplitföróttur/bay roan:
the stallion Yngri
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
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. |
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 |
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 zwartsilver 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
We intend to get similar articles online, so feedback at this stage
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