FAQ — Understanding horse genetics terms

A piece of hereditary information passed down by the parents. Each gene modelled here (Extension, Agouti, Cream…) affects one part of the coat: the base color, a dilution, a white pattern…

A horse carries two copies of each gene: one from the sire, one from the dam. Each copy is called an allele. Depending on the two alleles received, the visible result can differ.

Having received two identical alleles for a gene (one from each parent). For example, a Cremello horse is homozygous for the Cream gene — it received one copy from each side.

Having received two different alleles for a gene. The visible effect depends on the gene: sometimes invisible (a chestnut horse can be heterozygous without it showing), sometimes partly visible (a Palomino is heterozygous for Cream on a chestnut base).

Whether a horse is homozygous or heterozygous for a given gene. In expert mode, you can specify the known zygosity (from a pedigree or a DNA test) for each gene to refine the result.

A dominant allele shows up as soon as it's present once. A recessive allele only shows up if the horse has two copies (i.e. is homozygous recessive). Example: a chestnut horse is homozygous recessive for Extension, since the dominant black pigment allele is absent on both sides.

The genotype is a horse's exact combination of alleles, invisible to the eye. The phenotype is what you see: its coat. Two horses with the same coat can have a different genotype — that's why the simplified mode computes an average estimate.

Without a DNA test or pedigree, there's no way to know for certain whether a horse is homozygous or heterozygous for a gene that doesn't show in its coat. The simplified mode therefore averages across every possible genotype. If you know a parent's real zygosity, use expert mode for a more precise result.