Genetic health in horses

A gene is a stretch of DNA that helps the body function. Genes can have different versions, called variants. Some variants can cause disease or make a horse more likely to develop a condition.

For a recessive condition, a horse generally needs to inherit the variant from both parents to be affected. For a dominant condition, one copy of the variant may be enough to cause signs of disease. The outcome depends on the particular variant.

Inherited conditions can affect muscles, skin, the nervous system and other parts of the body. Each has its own effects and pattern of inheritance.

These first guides explain each condition’s effects and inheritance, the populations where the condition or its variant has been documented, and the scope of its genetic test.

A DNA test looks for one or more specific variants. Depending on the condition, it may help identify a carrier, support a diagnosis or inform breeding decisions.

A negative result means the variant being tested for was not detected. It does not rule out every genetic condition. Equally, a result showing increased susceptibility does not mean the horse will definitely become ill.

For many genes, a foal receives one copy from its dam and one from its sire. Results from both parents help show which combinations the foal could inherit.

Two unaffected carriers of the same recessive condition can have an affected foal. The diagram below explains why, and shows how to read the chances for each pregnancy.

Some variants that influence coat color also affect other functions. The frame overo variant is one example: a foal that inherits two copies is affected by lethal white overo syndrome.

This link is specific to the variant. A horse having a lot of white, or looking similar to another horse, is not enough on its own to establish the genetic risk.

Related horses are more likely to carry versions of a gene inherited from a shared ancestor. Their foal can therefore be more likely to receive two copies of the same harmful recessive variant.

Inbreeding does not create that variant, but it can make the associated condition more likely to occur. Maintaining genetic diversity is part of caring for a population’s health over generations.