Genetic health · the essentials

Understanding genetic conditions in horses

Two apparently healthy horses can have a foal with an inherited condition. To understand how, we need to look at what the parents pass on, as well as what we can see. A few basic ideas make this much easier to follow.

Horses can inherit different versions of the same gene

DNA contains the biological information the body uses. A gene is a stretch of DNA involved in how the body functions, for example by helping produce a protein needed by muscles or skin. Genes do not work in isolation: several genes can contribute to the same function.

DNA is not exactly the same in every horse. A difference in its sequence is called a genetic variant. You will also see the word allele used for different versions of the same gene. A variant is not necessarily harmful; many have no known effect on health.

For genes on chromosomes other than the sex chromosomes, a horse usually has two copies: one from its dam and one from its sire. These are autosomal genes. The copies can carry the same version or different versions. The pair of versions at the location being studied is part of the horse’s genotype.

A foal usually receives two copies of the same gene, one from each parent

The mare

One copy passed on to the foal

The stallion

One copy passed on to the foal

The foal

One copy inherited from the dam

One copy inherited from the sire

This diagram follows a single autosomal gene. Both copies belong to the same gene, but their versions can be identical or different. The foal does not receive both copies from each parent.

Sources [1] · [2]

What genetic, inherited and congenital mean for a horse’s health

These words answer different questions. Genetic describes a cause, inherited describes how information passes between generations, and congenital describes when a condition is present.

A DNA change can occur for the first time in a horse without having been inherited from either parent. Equally, an inherited condition may not cause visible signs for months or years. A foal looking healthy at birth does not rule out every inherited condition.

Genetic: The cause involves a DNA change or characteristic. It may have been inherited or arisen for the first time in that individual.
Inherited: Genetic information can pass from parents to their offspring. Signs of the associated condition do not necessarily appear at birth.
Congenital: The condition is present at birth, even if it is recognized later. Its cause may be genetic, environmental or a combination of factors.

Sources [1] · [3] · [4]

A horse can carry one copy of the HERDA variant without having HERDA

In a simple autosomal recessive condition, one copy of the variant is generally not enough to cause the condition. A horse can therefore be unaffected while still passing the variant on. This is what unaffected carrier means here: it refers to this particular condition, rather than the horse’s overall health.

HERDA is an example. A horse with one copy of the tested variant does not have HERDA. If it inherits a copy from each parent, it has two copies and develops the condition. Two carriers without signs of HERDA can therefore have an affected foal.

Each parent passes on just one of its two copies. The foal’s outcome depends on the copies it receives from both parents. The following diagram illustrates a simple recessive condition. The circles and diamonds represent two versions of the same gene, not a gene being present or missing.

Recessive inheritance in horses

A recessive example in horses: two parents without the disease can have an affected foal

Consider a recessive condition inherited in the same way by male and female foals. This is called autosomal inheritance. In this example, a foal is affected if it inherits the variant from both parents. The mare and stallion each have one copy of the variant, but neither has this condition.

Gene version without the variant being studied
Gene version with the variant being studied

The mare

Has one copy of the variant without having this condition

The stallion

Has one copy of the variant without having this condition

Each parent passes one of its two copies of the gene to the foal at random.

25%

Recessive example: foal without the disease-associated variant

The foal has not inherited this variant. It is unaffected by this condition and cannot pass this variant on.

50%

Recessive example: foal with one disease-associated variant copy, without the condition

The foal has inherited one copy. It does not have this condition, but can pass the variant on.

25%

Recessive example: foal with two copies of the same disease-associated variant and the condition

The foal has inherited two copies. In this example, it has the condition.

These chances apply to every pregnancy. Four foals will not necessarily include exactly one without the variant, two carriers and one affected foal.

This diagram shows a simple recessive example. Other conditions have different inheritance patterns, or do not affect every horse with the variant.

Sources [1] · [4]

One copy of the PSSM1-associated variant can cause muscle signs in a horse

A variant is dominant for a particular effect when that effect can occur with one copy. Dominant does not mean more common, stronger or more severe. It describes how an effect can appear when two gene versions are paired.

PSSM1, a condition that disrupts glycogen storage in muscle, is one example. Glycogen is a stored form of sugar that the body uses. One copy of the PSSM1-associated variant can be enough to cause muscle signs, but horses with the variant do not all show the same signs, and some show none.

The number of copies and the horse’s actual condition need to be considered separately. A horse without visible signs that has a disease-associated dominant variant should not automatically be called an unaffected carrier in the sense used for HERDA.

Comparing the HERDA variant and the distinct PSSM1 variant in horses

In these two separate examples, the horse has one copy of the disease-associated variant. How the variant works changes what that means.

Gene version without the variant being studied
Gene version with the variant being studied

One copy of the HERDA variant is not enough to cause HERDA

With one copy of the tested variant, the horse does not have HERDA. It can pass the variant on.

One copy of the PSSM1 variant: muscle signs are possible

With one copy of the associated variant, the horse may show PSSM1 signs. Whether signs appear and how severe they are can vary.

The symbols are learning aids. HERDA and PSSM1 involve different genes and variants. You need to know which variant was tested before interpreting one copy.

Sources [1] · [5]

Some genetic variants increase a horse’s disease risk without making illness certain

Genetic susceptibility means a feature of a horse’s DNA makes a condition more likely. It does not always tell us whether the condition will develop, at what age or how severely. Other variants, age and environmental factors can also matter, depending on the condition.

When not every horse with the disease-associated genotype develops the condition, scientists call this incomplete penetrance. When the condition does develop but its signs or their severity differ between horses, this is variable expression. These ideas help explain why identical test results do not necessarily mean identical health outcomes.

Some conditions depend mainly on one gene; others involve several genes and external factors. The simple diagrams in this article explain inheritance. They cannot calculate the risk of every equine condition.

Sources [1] · [2] · [5]

Read a horse’s DNA result by checking which condition and variant were tested

Before interpreting a report, check the condition and variant the test looked for. The meaning of zero, one or two copies depends on that variant. One copy has different consequences in the HERDA and PSSM1 examples.

A negative result means the tested variant was not detected. Other variants, sometimes in the same gene, may not have been tested for. This is not an overall health assessment. Likewise, a horse without visible signs cannot be assumed to have a negative test result if it has not been tested.

If a horse shows signs of illness or you are planning a mating, a veterinarian can consider genetic information alongside an examination, the horse’s history and results from both parents. The test provides a specific piece of information; interpreting it depends on the context.

Sources [6] · [4] · [5]

Questions about inherited conditions and DNA tests in horses

Can two apparently healthy horses have a foal with an inherited condition?

Yes. For some recessive conditions, each parent can pass on a variant without having the condition itself. A foal that receives the two copies needed for the condition can be affected even though neither parent is.

Does a dominant condition cause more severe disease in horses than a recessive condition?

No. It means one copy may be enough to produce an effect. Severity depends on the condition, the variant and sometimes other factors. Recessive conditions can be very severe.

Does a negative DNA test rule out every genetic condition in a horse?

No. The result only covers what the laboratory tested for. It cannot rule out all other variants or every other cause of disease.

Key points for understanding inherited disease risks and DNA tests in horses

A variant may have no known effect, cause a condition or increase susceptibility.
An inherited condition is not always visible at birth or in the parents.
The meaning of a copy count depends on the variant’s inheritance pattern and effects.
A DNA test adds information about a horse; it does not describe its entire health.

Scientific sources on genes, inherited conditions and DNA tests in horses

The explanations and examples draw on university resources, a public research institute and a scientific review. References appear beside the explanations they support.

Back to genetic health Explore inherited conditions

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