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The flaxen gene is a genetic mechanism that causes the mane and tail of chestnut-colored horses to be noticeably lighter than the body coat color, often a golden blonde shade. Certain horse breeds such as the Haflinger carry flaxen chestnut coloration as a breed trait. It is common in chestnut-colored animals of other horse breeds that may not be exclusively chestnut.
Studies on Morgan horses have indicated that the flaxen trait is inherited, but the genetic mechanism that produces the trait is not yet mapped. The trait may be produced by a homozygous recessive allele. Evidence of a recessive mode of Mendelian inheritance exists in preliminary studies showing that flaxen chestnut horses mated with other flaxen chestnut horses consistently produce only flaxen chestnuts. The degree of expression of the trait is highly variable, however: some chestnut horses that do not exhibit much flaxen may nonetheless produce strongly flaxen offspring. Thus, some researchers hypothesize that there is more than one gene involved. The flaxen gene does not affect black or bay horses, only chestnuts. However, as there are examples of flaxen chestnuts born to parents that are black or bay, it is hypothesized that it can be masked in darker-colored horses but still passed on to their offspring.
Flaxen mimics
Coloration of a light mane and tail with darker body color may be created by other genetic mechanisms known as dilution genes, most of which can be detected by DNA testing. The most common is palomino, created by one copy of the cream gene acting on a chestnut coat, resulting in a gold-colored coat with a white or cream colored mane and tail. Deep gold palominos may be hard to distinguish from light chestnuts with significant expression of flaxen, but genetic tests can aid determination of color. The Champagne gene acting on a chestnut coat may also produce a horse with a gold coat and ivory mane and tail, distinguishable from palomino by freckled skin and light-colored eyes. In most cases, a chestnut with flaxen can be distinguished from other colors by the presence of some reddish, chestnut hairs in the mane or tail.
The silver or silver dapple gene acts only on black hair. On a black base coat, it lightens the body to a brown color and the mane/tail to a cream or silver shade; on a bay base coat, it lightens the mane and tail to cream or silver. It does not affect chestnut (red) coloring. A genetic test exists for the silver gene.