How Genetics Are Inherited
When you breed two horses, the foal inherits a combination of genetic information from both parents. This determines everything from its base coat color to its markings.
Every horse has 64 chromosomes, arranged in 32 pairs. During breeding, each parent contributes 32 chromosomes, giving the foal its own complete set of 64.
For most genes, the foal inherits one allele from the mare and one from the stallion, giving it two alleles that form a complete pair.
For example, if a horse carrying one copy of the Cream dilution gene (CR/n) is bred to a horse without Cream (n/n), the foal has a 50% chance of inheriting the dilution. This is because the parent carrying Cream can pass down either CR (Cream) or n (no Cream), but never both.
Homozygous & Heterozygous Genes
Horses can be either homozygous or heterozygous for a particular gene.
Homozygous means the horse has two copies of the same allele, such as G/G or n/n.
Heterozygous means it has two different alleles, such as G/n.
A horse with two copies of the same allele will always pass one of those copies to its offspring, whereas a heterozygous horse has a 50% chance of passing either allele.
So a homozygous grey horse (G/G) will always produce a heterozygous grey foal (G/n) when bred to a non-grey horse (n/n).
Dominant & Recessive Genes
When a foal inherits genes from its parents, some traits are dominant, while others are recessive. This determines whether a particular genetic trait will be visible on the horse or remain hidden.
Just note that hidden genetics can still be passed on to future offspring.
Dominant Genes
A dominant trait only requires one copy of the allele to be expressed.
For example, Grey (G) is dominant. A horse only needs to inherit one copy of Grey from either parent to progressively turn grey as it ages.
- G/G - The horse has two copies of Grey and will display it.
- G/n - The horse carries one copy of Grey and will display it.
- n/n - The horse does not have or carry Grey, and it will not display it.
Recessive Genes
A recessive trait generally requires two copies of the recessive allele to be expressed. If a horse only inherits one copy, it can carry that allele without showing the recessive trait.
For example, the Pearl dilution (PRL) is recessive, meaning a horse needs two copies for the dilution to be visible.
- PRL/PRL - The horse has two copies of Pearl and visibly expresses the dilution.
- PRL/n - The horse carries one copy of Pearl but does not visibly express the dilution.
- n/n - The horse does not have or carry Pearl.
Understanding Breeding Probabilities
It's also important to understand that breeding probabilities apply to each individual foal.
If a pairing has a 25% chance of producing a certain genetic combination, that doesn't mean one in every four foals is guaranteed to inherit it.
Each breeding is a separate event, meaning you could produce several foals with the same genetic combination before seeing another outcome.
Just because a foal inherits a particular gene doesn't make the next foal more or less likely to inherit it.
Genotype & Phenotype
A horse's genotype is its actual genetic makeup, including any hidden genes it may carry. Its phenotype is what you can physically see.
This means two horses can look identical but have different genotypes, and potentially produce very different foals when bred.
For example, two palomino horses might look identical, but one could carry the recessive rabicano gene while the other does not.
One horse might have CR/n and RAB/n, while the other has CR/n and does not carry rabicano. Both horses appear palomino, meaning they share the same phenotype.
However, their genotypes are different. The first horse has a 50% chance of passing the rabicano gene to its foal, while the second cannot pass rabicano at all.
Both horses look the same, but their hidden genetics can lead to different breeding outcomes.