Grey horses, sometimes called white horses, are much less common than horses of other coat colors. There are a few key reasons why grey horses are rarer than bay, chestnut, black or other coat colors.
Low occurrence of the grey gene
The grey coat color in horses is caused by a dominant gene known as the grey gene (or gray gene). For a horse to have a grey coat, it must have at least one copy of this gene variant.
The grey gene is relatively uncommon compared to other coat color genes. Most horses do not carry a copy of the grey gene. Without the presence of the grey gene, a horse’s coat will be determined by other color genes in its genetic makeup.
Research has found the grey gene occurs naturally in about 3-10% of horse breeds. That is much lower than the frequency of the chestnut gene, for example, which occurs in over 50% of many breeds.
With such a low rate of occurrence, the odds of two horses that both carry the grey gene breeding and producing a grey foal are low. That is why grey is one of the rarest horse coat colors overall.
Breed prevalence differences
While grey horses are uncommon in general, the percentages can vary quite a bit between breeds. Some breeds have higher rates of the grey gene than others.
Breeds with a relatively high frequency of grey horses include:
- Lipizzaner – around 90% are grey
- Andalusian – around 60% are grey
- Arabian – up to 45% are grey
- Percheron – around 10% are grey
Meanwhile, other breeds like the Thoroughbred and Quarter Horse have grey coat rates of only 1-3%.
So while the grey coat is rarer overall, it is quite common in a handful of breeds. The high rates in those breeds indicates the grey gene has been selectively bred for in those populations.
Gradual greying process
Another factor that contributes to the rarity of grey horses is the unusual way their coat color changes over time.
Grey horses are not born grey. Almost all grey horses are born with a dark base coat color like bay, black or chestnut. They then gradually lighten to a white or near-white shade over several years.
This gradual greying process means young grey horses often do not appear white. A 6 month old grey foal may still be quite dark. Only over time does their grey genetics become visibly evident.
This unusual aging process likely contributes to the perceived rarity of grey horses. Many grey horses only become fully white after the age of 6 or older. So the number of visibly grey horses is smaller than the number that genetically carry the grey gene.
Breeding challenges
The genetics behind grey coat color also lead to some challenges when breeding for grey. This can reduce the numbers of grey horses being produced.
While the grey gene is dominant, there are complex factors when breeding greys. If a homozygous grey (carrying two grey genes) is bred to a non-grey horse, 100% of the foals will be grey. But when two heterozygous greys (each carrying one grey gene) are bred, only about 50% of the foals will be grey.
The gradual greying process also makes it hard to identify horses carrying the grey gene when they are young. A dark foal may still turn grey later on. This genetic uncertainty can dissuade some breeders from breeding for greys.
Furthermore, the dominance of the grey gene means it can override and erase other coat color genes when passed on. Some breeders avoid using grey horses to prevent losing coat color diversity in their breeding program over generations.
Loss of pigmentation over time
As grey horses age, most will become progressively lighter in shade. Many will eventually turn completely white by 12-15 years old.
This is because the grey gene prevents pigment-producing melanocyte cells from functioning properly. As a grey horse gets older, more and more of its melanocytes stop producing pigment.
With less pigment being produced as the years go by, grey horses lose the pigmentation in their hair and skin. This causes their coat to become increasingly white as they enter their teens and 20s.
This tendency to turn white means older grey horses are easier to identify. It likely contributes to the perception that greys are common when in reality they make up less than 10% of horses overall.
Novelty
The pale, almost luminous look of mature grey horses adds to their appeal. Their striking white hair coats often stand out compared to the common bay and chestnut horses in a field or barn.
Many horse enthusiasts find the white coat color very eye-catching and beautiful. This novelty factor means grey horses tend to get more attention and interest at events and shows.
The extra interest in greys adds to the perception that they are more popular or numerous than they really are. In reality, their pale coat color just makes them more noticeable.
Conclusion
In summary, grey is one of the rarest horse coat colors for several key reasons:
- Low occurrence of the grey gene in most breeds
- Gradual greying process makes young grey horses hard to identify
- Breeding challenges due to grey genetics
- Loss of pigment over time leads to white coats in older horses
- Novelty and appeal of the pale coat color
While grey horses attract a lot of interest for their striking white coats, they truly are quite rare in the equine population. Less than 10% of horses worldwide are genetically grey. Certain breeds like Lipizzaners and Andalusians have higher percentages, while other breeds have rates of only 1-3%.
So next time you come across a dapple grey horse, appreciate it! That pale coat is the result of some unique genetics and aging effects that make truly white horses harder to come by.
