The red base colour that makes chestnut is a combination of TWO red genes. Because of this, chestnut horses DO NOT carry black. 
A horse that has only 1 or no red genes, will be black based.

EVERY time you breed two red based horses together the foal will also be red based.

Many horse breeds in America use the term Sorrel for the brighter shades of red and chestnut for deeper shades.  In Australia the term chestnut is used for all shades of chestnuts, however with the importation of American horses you do see the term Sorrel on registrations also.

• The mane and tail can range in colour from "white" or flaxen to very dark red.

• The points are usually the same colour as the body coat or will vary a shade or two, but are never black. Chestnut horses can not make black pigment.

• Chestnut horses can carry the silver gene and the bay gene without displaying them - as these two genes do not effect red hair.

• Chestnut and other red based foals are born with pinkish coloured skin, especially around the eye lid, in the nostrils and the genital area... over several days the skin will change to dark brown or black.

   
The two foals above show the pink tinge to the skin around their eyes,
and then the eye once their true pigment has darkened the skin.

>> Chestnut Examples

Want to get Technical?

Red pigment or the Chestnut colour is caused by the pigment called Pheomelanin (fee-oh-MEL-a-nin). 
A Chestnut horses entire body, including the mane, tail and legs, is made up of pheomelanin, regardless of the shade of the colour. 
Chestnut based colours such as Palomino and Cremello are the same, the intensity of the phenomelanin is just diluted.

To be physically displayed, Chestnut requires a horse to have two doses of the same gene at the Extension locus. This is because Chestnut is recessive. Since the other base colour (black) at the Extension locus is dominant it means the only way for chestnut to be visible is if there are NO Black genes.

You can have your black based horse lab tested to confirm if it carries red (Ee) or not (EE). Or if you are not sure of your horses base colour - such as an extreme sabino you can have it tested to see if it is red or black and if it carries bay.
UC Davis is one lab that provides this test. The test costs US $50.00 and is done with mane hairs that you pull yourself, no vet required and no blood required.

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