How Genes Work

A gene comes in two copies, one from each parent. Each copy is called an allele, and a gene is written as two letters, such as Ee.

Dominant and recessive#

A capital letter is a dominant allele and a lowercase letter is a recessive allele. A dominant allele shows up in the horse even if it only has one copy. A recessive allele only shows up when the horse has two copies.

Genotype	What it means
EE	Two dominant copies
Ee	One of each. The dominant copy wins, but the horse still carries the recessive one
ee	Two recessive copies

A horse with Ee looks the same as a horse with EE, but it can pass the recessive e on to a foal.

The six genes#

Every horse carries these six genes.

Gene	Letters	What it controls
Extension	E / e	Whether the horse can have a black base coat or has a red base (chestnut)
Agouti	A / a	Whether black is restricted to the points, making a bay
Cream	n / Cr	Dilution of the coat colour
Gray	g / G	Whether the horse is gray
Leopard	n / L	Spotted markings
Pinto	n / P	Large white patches

The letter n stands for normal, meaning the plain version of the gene.

Colours and markings are covered in Coat Colours and Markings. Health genes are listed separately in Hereditary Diseases.

Reading a horse's genes#

A horse's genes are listed in a fixed order: Extension, Agouti, Cream, Gray, Leopard, Pinto. A default horse reads EeAannggnnnn.