Here are a list of the known horse color genes... they are in this format:

locus), alleles, series
effect of alleles

A) A, a, A+, At: Agouti

The dominant form is A, which, if present in a horse, restricts black to the points. In other words, it keeps black from the body parts, resulting in a bay or other color with black legs, mane and tail. The recessive or 'off' form is a-- it does not affect the black in any way; the horse may have black on entire body.
There may be other alleles for Agouti, too: A+ makes the horse a wild (primitive) bay, and At removes light pigment from soft body parts.

Cr) Cr, cr: Cream gene

When two cr genes are present, the horse is not a dilute (palomino, buckskin or the like). When one Cr gene is present, then the body is diluted (bay to buckskin, chestnut to palomino, etc.). When two Cr genes are present, then the horse is what is called a 'double dilute': a Cremello or a Perlino (lighter versions of the Palomino and Buckskin, respectively), or Smoky cream (black double dilute)

Ch) Ch, ch: Champagne

The Champagne gene, Ch, dilutes the color of the horse and also effects eye and skin color, as well as actual hair build. Horses with the Ch gene will be silvery-chocolate, reddish, or golden in color with a metallic sheen. Their eyes are amber, and their skin is a pink color, though not the pink of most horses-- it's a mottled orangey-pink. The hairs of a champagne are actually hollow, so this gene somehow affects more than color alone. ch is the regular gene, it has no affect.

D) D, d: Dun

The D gene creates a dun horse. It causes pigment to concentrate on one side of the hair, making it a diluted color. It also gives the horse a dorsal stripe, leg barring, and sometimes other primitive marks. A dun horse is either Dd or DD-- by just looking at that animal you cannot tell which. d is the regular allele, no effect.

E) E, e: Extension (black pigment)

The E gene creates black pigmentation; horse may be either EE or Ee. An ee horse will not have any black pigment (chestnut, red roan, palomino, etc.). A horse that is EE or Ee will have black pigment (black, buckskin, grulla, etc).

F) F, f: Flaxen

F creates a flaxen mane and tail, and perhaps lightens the body a bit, in ee horses.

G) G, g: Gray

G is the most dominant gene, and turns any color to gray. Foals may be born any color, but lighten with age. In other words, G is epistatic to all other genes. g has no effect.

Lp) Lp, lp: Leopard-complex

Lp creates mottling and spotting characteristics, and allows for different spotting patterns, such as those seen in the Appaloosa. lp has no effect.

 

N) N, n: White markings

One hypothesis is that the N gene prevents all white markings-- even if the horse had sabino or other pinto genes, this would mask them. n allows white markings to occur, and they will if pinto genes are present. It is not known for sure if the N gene exists.

Fr) Fr, fr: Frame

Fr creates the Frame pinto pattern in the horse. Homozygous form (FrFr) is lethal, resulting in an OLWS (overo lethal white syndrome) animal that dies.

P) P, p: Pangare

The Pangare gene, P, lightens the horse's muzzle, flanks and belly, creating a 'mealy' horse.

Rn) Rn, rn: Roan

Creates a mix of light and dark hairs, making roaning.

Sb) Sb, sb: Sabino

The genetic control of sabino is not completely known. It appears to be polygenic, and not a simple dominant like tobiano.

Sc) Sc, sc: Snowcap

Causes pigment loss in the form of snowcap spotting, in the presence of Lp.

Sd) Sd, sd: Dark spots

Causes dark spots in the presence of Lp.

Sn) Sn, sn: Snowflake

Causes white spots (snowflake pattern) in the presence of Lp.

Sv) Sv, sv: Varnish

Causes silvering/varnish effect in the presence of Lp.

Spl) Spl, spl: Splashed White

Causes pigment loss in the form of a Splashed White patterned pinto.

Sty) sty, Sty: Sooty

Causes mixture of black hairs, creating seasonal dapples, or a 'sooty' appearance.

To) To, to: Tobiano

Causes pigment loss in the form of a tobiano pinto pattern.

W) W, w: White

Causes pigment loss over entire body in hair and skin, but not eyes; dominant white. There is some doubt as to whether this gene actually exists.

Z) Z, z: Silver

Dilutes black pigment, has no effect on red pigment.