Why do some horses have blue eyes?

Blue eyes in horses come from the genes that place pigment, and there is more than one route to them. The clearest documented one is splashed white. UC Davis’s Veterinary Genetics Laboratory describes it as “a variable white spotting pattern characterized by a broad blaze, extended white markings on the legs, variable white spotting on the belly, and often one or both blue eyes”, and has identified eight separate mutations that cause it, across two genes.

Dilution genes are the other route. The VGL records that two copies of the cream allele give cremellos and perlinos, and that the pearl allele “interacts with cream dilution to produce pseudo-double cream dilute phenotypes including pale skin and blue/green eyes”. So a blue eye on a very pale horse and a blue eye beside a broad blaze aren’t the same thing happening.

One thing worth stating carefully, because it’s often overstated: the VGL says “some, but not all, splashed white horses are also deaf”. It’s an association to be aware of, not a diagnosis. Britain’s older word for a pale or odd eye is wall-eyed.

A blue eye is the detail people notice first. A portrait made from your own photograph keeps it.

Written by Craig Fearn, Stable & Hoof.