The Basics of Equine Color Genetics

The Foundations: I find that the easiest way to think about horse color is to imagine it as a series of building blocks. Each color is built from a group of genes that act differently when combined with different bases. One key thing to understand is that horses inherit color genes from each parent---each parent has several pairs of color genes, and half of each pair is passed on to the foal. If you are not familiar with basic Mendelian inheritance, please read this wiki page for more information.

(Please note that for the sake of simplicity and easier understanding of these pages, I generally refer to genes, alleles, and loci as simply genes.)

All horse colors are built on only two base colors, black (E) and chestnut (e). Black is dominant to chestnut, and chestnut is thus recessive. That means that a horse carrying 2 black genes (EE) will be (homozygous) black; a horse carrying one black gene and one chestnut gene (Ee) will also be black (but heterozygous); and a horse carrying two chestnut genes (ee) will be chestnut (always homozygous). So if two heterozygous black horses are bred together (Ee +Ee), they have a 25% chance of producing a homozygous black (EE), a 50% chance of producing a heterozygous black (Ee), and a 25% chance of producing a chestnut. Two chestnuts bred together (ee + ee) can only produce chestnut.

Chestnuts vary in shade from light golden red color to coppery red to dark liver. Some of the darkest liver chestnuts, often called "black chestnuts," are nearly indistinguishable from true black horses. Some chestnuts have flaxen manes and tails.

This is Affirmed, the 1978 Triple Crown winner, a light or golden chestnut.

This is top TB sire Rahy, a slightly darker golden chestnut.

This is the South African champion racehorse Horse Chestnut, a red chestnut.

This is top TB sire Giant's Causeway, a liver chestnut.

This is Merwin All A Breeze, a black chestnut (ee) Morgan.

This is Kerry Top Hat, a flaxen black chestnut Morgan.

This is a flaxen light chestnut.

This is a flaxen liver chestnut.

Black horses can also vary in shade---from blue-black to dusty black to sun-faded black. Not all black horses fade in the sun, but those that do generally resemble brown, dark bay, or even liver chestnut horses. The ends of their manes and tails often fade to a burnt reddish shade. The genetic factor governing non-fading black vs fading black has not yet been discovered.

Australian superstar Lonhro, a true, non-fading black TB.

A true black like Lonhro will not show any brown hairs on the muzzle, the telltale sign of the presence of the agouti gene which is discussed on the next page.

This is K-One, a fading black Arabian gelding bred by Deal Arabians.

This is Burn's Shadow Dancer, a very faded black TWH gelding.


On to the Dilution Genes