The genetics of bay, black and chestnut

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
Struck me that it might be an idea to explain how the base colours work too! :D

These are a lot simpler than you'd think. To start with, all horses are genetically either black or chestnut as a starting point. It works like this....

Strictly speaking, a gene isn't dominant or recessive. A gene contains codes for development, nothing more. Each gene, however, contains what's called 'alleles' - these can be dominant or reccessive. There is no separate gene for red and black - they're both caused by the same gene. It depends on whether the horse carries 2 reccessive alleles of that gene (e) or a dominant allele of the gene (E). Put very simply - it depends whether the gene is 'switched on' for black or not.

If a horse carries the dominant allele - E - he'll be black. If not, and he has instead two reccessive alleles - e - he'll be red (or chestnut). Dominant means he only needs one E allele to show that colour - in order to show red he must have two copies of the reccessive form e.

So, there's the start - if a horse has ee, he'll have a red base. This horse can only ever contribute e (red) to a foal.

If a horse has Ee, he'll have a black base - but if he bred, he could contribute either E (black) or e (red) to the foal. This is why black can throw non-black foals.

If a horse is EE, he's homozygous for black. This mean that he can only contribute E to a foal - breed two EE blacks and you'll only ever get black foals.

And onto bay....

Bay is caused by a gene called Agouti. Again, whether a horse carries bay depends on whether the allele for this gene is dominant (A) or reccessive (a) - whether bay is 'switched on', in other words.

A horse with the switched on copy of bay (A) will carry bay. A horse without (a) won't.

Agouti works by restricting black pigment to the points - legs, mane and tail. Because it only works on black, a chestnut horse can carry bay without showing it. A black will always be bay if it carries bay.

This is how a bay foal can be born to a black parent and a chestnut parent - the black passes on E for black, the chestnut (carrying an 'invisible' A) passes on e, which doesn't show on the coat colour, and A, which causes the black pigment to be restricted to legs, mane and tail - so there's your bay foal.

A horse that carries two A alleles will only ever throw bay or chestnut foals, never black, even to a homozygous black - because they will always pass on A that causes black to become bay.

All other coat colours are caused by genes and their alleles that modify these base colours. For example, cream on a red base to give palomino; tobiano on a black base to give piebald; roan on a red base to give strawberry roan.
 

casey

Certified equine dentist
Mar 27, 2003
3,233
0
0
NSW Australia
s6.photobucket.com
Woah....Ok.

I understand, that Black and Chestnut are the only true colours.

I bred my black mare to a chestnut stallion and got a dark bay colt:confused: Now, i know NOTHING about genitics, but thought maybe, he would come out either or.

How has his colour been influenced? I genuinely am interested.

My mare's dam was chestnut, and her sire black.

My colts Sire Luidam's dad was also chestnut.:)
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
casey - my dark bay colt came from a black sire and chestnut dam... :D Works like this.

The black parent (the dam in your colt's case) is E - one allele for black - possibly two, we don't know, and it doesn't honestly matter. The black allele is dominant to the chestnut anyway so the horse appears black regardless of whether it's Ee or EE.

The chestnut parent is ee - has to be in order to appear chestnut. In this case (and in the case of my colt too) the chestnut also carries a switched on allele for bay - that is, A. It's not visible on the chestnut because there's no black pigment to restrict.

So; your colt got e from his sire, and E from his dam - making him black. He also got A from his sire, which restricts the black to the points.... and hey presto you get bay from black x chestnut.

The genetics of dark bay are a bit murkier - we don't honestly know what causes dark bay as opposed to bright bay, for example.

Your colt is genetically EeAa - this means he can throw bay, black or chestnut (depending obviously on the mare he's put to).
 

Arabmare

Active Member
Nov 1, 2002
4,805
0
36
41
Ashford, Kent
www.kbequinephotography.co.uk
Brilliant thread!

Weve always wondered (and been asked) If AH Mahal is homozygous black. His breeding is almost all black all the way through! How would we find out if he has that dominant gene? Does that mean if he is he can only produce black (all the time) if the mare is proven Homozygous too? Hes had 2 foals out of bay mares and the foals were bay does that mean he is not Homozygous black or it was the mares genes making the foal bay? Confuzzled!:eek:
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
Honozygous black will only breed black every time when put to another black, or a chestnut that doesn't carry bay. On a bay, or a chestnut with the bay gene, there's a chance of a bay foal - if put to a homozygous bay, the foal will always be bay no matter how many black genes the parents carry!

There is a test available to ascertain whether a horse is homozygous for black - but it's important to remember that even a homozygous black won't throw black every time unless they're used on another black.

So - if AH Mahal is homozygous for black, he'd be EE. He can only pass on the allele for black, not red. On a chestnut mare that's ee (two red alleles) and aa (no switched on bay allele) you're guaranteed a heterozygous black foal (Ee).

On a chestnut mare that carries one bay allele (eeAa) there's a 50/50 chance she'll pass the A on - so although you're guaranteed the base colour will be black, there's also a 50% chance the foal will get A, and be bay.

On a chestnut mare with two A genes, the foal will only ever be bay - it'll get E from AH Mahal, and eA from mum - no blacks at all from this pair.

If AH Mahal is Ee (and it is possible, even with generations of black breeding. Both parents would have to pass on e for baby to be chestnut) then there's only ever a 50/50 chance he'll pass E for black on - so the chances of a black foal are reduced again. If he is Ee, and put to another black that's also Ee, there's even a 25% chance that the foal would be chestnut!
 

horseygal90

Going
Aug 27, 2004
7,759
5
0
Hm... Intereesting! Makes good reading that.

If I've read right, then a piebald would be EE and then tobiano, so would throw either a black, or piebald? Gah. Confusing. (But I'm pretty sure, not a chestnut or bay)

I'm probably wrong.
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
That's one possible black tobiano. It could also be Ee and T (tobiano). They have the same chances of throwing black as a black with no tobiano, but there's also a 50/50 chance they'll throw tobiano (or, if they're homozygous, the foals will always be tobiano patterned).

A piebald is a black horse (so Ee, or EE) with a Tobiano gene on top of that.
 

Alle

Horse Crazy
May 18, 2004
1,225
0
0
Somewhere out there
Visit site
I've been reading all your color threads with interst! Very confusing and facinating.

My mare is a red bay and has a faint dorsel stripe which actually shows much darker in the winter. Does she have a dunning gene somewhere to cause the dorsel do you think?

What a great subject!

Alle
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
Dorsal stripes are not just a dun thing - they can occur in any colour. They are usually very sharply defined in a dun though - as if someone's drawn a line down the back in felt pen. On non-duns, they do tend to be a bit more blurry and indistinct - this type of dorsal marking is known as countershading.

I'd say your mare is probably a bay - certain red duns can have a very red body, but their legs, manes and tails won't be black, as dun also dilutes black pigment.

Dun is a simple gene - a horse either has dun, or it doesn't. If it does, it'll be dun in colour - with at least two of the dun charcateristics.
 

virtuallyhorses

NZ TB owner
Mar 1, 2002
2,785
0
0
57
New Zealand
www.virtuallyhorses.com
You're forgetting brown ;) which is also EE or eE or Ee and black\brown horses may also be aaeE (from Bay)

Strictly speaking there is only one base colour - red or black (whether that is e or E is irrelevent as that nomenclature simply defines whether the gene is dominant or recessive so the red and black 'gene' is identical) depending on whether you believe that red is washed out black or black is very undiluted red :)
 

WelshJumper

New Member
Jan 14, 2002
830
0
0
39
Essex, epping forest
Visit site
How do the geans {sp}work? As we have had the family of Duns,

Dam was a Dark Golden dun, her foal Golden dun, her foal Dark dun {mouse} and agane from the same mare {golden} a Chestnut Dun {almost creem} so would the next be lighter or does it depend on the sires col?
 

Alle

Horse Crazy
May 18, 2004
1,225
0
0
Somewhere out there
Visit site
Originally posted by chev
Dorsal stripes are not just a dun thing - they can occur in any colour. They are usually very sharply defined in a dun though - as if someone's drawn a line down the back in felt pen. On non-duns, they do tend to be a bit more blurry and indistinct - this type of dorsal marking is known as countershading.

I'd say your mare is probably a bay - certain red duns can have a very red body, but their legs, manes and tails won't be black, as dun also dilutes black pigment.

Dun is a simple gene - a horse either has dun, or it doesn't. If it does, it'll be dun in colour - with at least two of the dun charcateristics.

Heh, so her color is nothing very special then? ;) I was wondering about the stripe, b/c I have seen the faint, indistinct dorsels on several horses, so this clears it up. Thanks!
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
Originally posted by virtuallyhorses
You're forgetting brown ;) which is also EE or eE or Ee and black\brown horses may also be aaeE (from Bay)

Strictly speaking there is only one base colour - red or black (whether that is e or E is irrelevent as that nomenclature simply defines whether the gene is dominant or recessive so the red and black 'gene' is identical) depending on whether you believe that red is washed out black or black is very undiluted red :)

How could I forget brown! :eek:

Or seal... thought to possibly be a variety of bay modified to some extent (a large extent in the case of brown) by the sooty gene.

And yes - strictly speaking there is only one base gene - but I didn't fancy explaining how a horse is only one base colour - red or black....

I stand corrected Viv! ;) :D
 

chev

Moderator
May 7, 2002
10,840
1,171
113
right here, right now
Welshjumper - it depends on the base colour. Remember that dun is only a modifier - it dilutes whatever base colour a horse is.

So, if you take dun out of the equation, your family of duns would be a family of bays, black and chestnut. Add the dun gene that's obviously been passed down from the mare, and you have your family of duns.

As an example;

Bay mare has bay foal

Bay mare has chestnut foal

Bay mare has bay foal

Bay foal has black foal

Now add dun -

Golden dun mare has golden dun foal

Golden dun mare has red dun foal

Golden dun mare has golden dun foal

Golden dun foal has mouse dun foal

See how it works?
 

virtuallyhorses

NZ TB owner
Mar 1, 2002
2,785
0
0
57
New Zealand
www.virtuallyhorses.com
Originally posted by chev

And yes - strictly speaking there is only one base gene - but I didn't fancy explaining how a horse is only one base colour - red or black....

:D I understand completely, its often so much easier to give the half answer when on a subject where a reasonable amount of background info is required - I was only teasing ;) :D

I would correct the person who said that dorsal stripes are not a dun thing - indeed they are! when you see them in other colours they are the evidence (usually) of a recessive dun gene for instance EeAaCCDnndDnnd would be a bay horse and may or may not show any traits of the recessive Dun gene. The exception is a foal which starts out with a dorsal stripe which then fades.

The dorsal stripe (and dun colouring) are known as 'primitive' because they tend to appear on those older breeds such as tarpans which still show coat colourings that help them blend in to the steppes and grasslands from which horses evolved and also to reappear on horses when they are bred back to those roots. Sort of the equine equivalent of the 'little yellow dog' to which most mongrels revert.
 
Last edited:
Similar threads
Thread starter Title Forum Replies Date
Frances144 Dog Colour Genetics HELP Breeds, Colouring and Genetics 2
Jane&Ziggy Went to see puppies this evening... Dog colour genetics, anyone? Cats, Dogs and other Animals 6
S Colour Genetics Experts, Help Please :) Breeds, Colouring and Genetics 4
MagicSix Dun genetics? Breeds, Colouring and Genetics 10
Soot Need help with coloureds genetics Breeds, Colouring and Genetics 0
Soot Need help with coloureds genetics Breeds, Colouring and Genetics 16
Jen_e_Jen Genetics for tri colored horses? Breeds, Colouring and Genetics 5
helenc Genetics of white horses Breeds, Colouring and Genetics 3
Troi Introduction to genetics Breeds, Colouring and Genetics 0
Jen_e_Jen Grulla genetics Breeds, Colouring and Genetics 3
Midnight Rose Drum horse genetics Breeds, Colouring and Genetics 22
No_Angel Confused again by colour genetics Breeds, Colouring and Genetics 5
chev Tobiano genetics identified Breeds, Colouring and Genetics 2
Troi Coat Colour Genetics Breeds, Colouring and Genetics 4
my_horse_zinc Colour and Genetics Breeds, Colouring and Genetics 1
Vicki&Milo Fjord Genetics Breeds, Colouring and Genetics 2
Wally Genetics, how to get rid of black! Chev? Breeds, Colouring and Genetics 1
casey Chev... genetics question General 19
fiesty_filly Articles on Genetics and Biochemistry of Horses? General 0
Stella2 Colour Genetics: Grey & Chestnut (Chev?) Breeds, Colouring and Genetics 7
ambatt Chocolate Dun - genetics please Breeds, Colouring and Genetics 10
ponylover88 Chev, or someone who knows about genetics etc Breeds, Colouring and Genetics 16
S genetics of chocolate brown horses? Breeds, Colouring and Genetics 11
CMR Genetics... Breeds, Colouring and Genetics 1
S red roan with black points - what's the genetics? Breeds, Colouring and Genetics 3
Horsesarelife Colour genetics- Chev? Breeds, Colouring and Genetics 3
~elizabeth~ Silver bay? Breeds, Colouring and Genetics 0
domane To quote the Bay City Rollers..... Cafe 4
Expatmum Numnah colour for Brown tack on a bay horse Tack & Saddlery 21
C Dressage throughbreed bay mare & happy hacker rider Confidence Club 5
ponylover88 Bay horses and 'ears' colours Breeds, Colouring and Genetics 4
Sunshine-x Inter hunt relay on the bay beastie! Pics! Cafe 8
Sunshine-x I would like some bay horses please Cafe 15
T STOLEN - Arrius, 15.3hh, bay gelding. Freezemark K9C5 Cafe 7
T (trace) Destiny, 15.2hh-16hh, dark bay/black mare. Freezemark DESI Cafe 0
Mary Poppins E bay tips needed Cafe 10
pepsimaxrock Beautiful Druridge Bay in Northumberland Cafe 6
T Murphy, 17.3hh, bay gelding. Freezemark UD70 Cafe 0
T Stolen - Rupert, 15.3hh, bay gelding. Freezemark - 2DX1 Cafe 0
T i miss my irish bay gelding! Cafe 1
Wally E-bay scam update. Cafe 5
S Black spots all over a bay horse?????? Breeds, Colouring and Genetics 6
Wally E-bay scam on a horse trailer- beware. Transport and Travelling 21
T (Stolen) 14.3hh approx, bay gelding. Freezemark - 26DU Cafe 0
Jane&Ziggy Anybody want a Cleveland Bay? Cafe 15
fairlady UMMMMM ! Barefoot on E BAY Tack & Saddlery 5
Befnee's Hero Sand Bay - Somerset? Hacking 1
fairlady Western saddle from e bay - bargain or not? Tack & Saddlery 6
daftdraught Brown or Dark Bay. Breeds, Colouring and Genetics 12
Brodies_Girl Anybody have a barefoot Cleveland Bay? Hoof Care 4

Similar threads

newrider.com