Long hair is definitively a recessive trait in cats, meaning that the characteristic for short hair is the form that expresses itself more readily. Understanding this inheritance pattern requires a look at the basic rules of how traits are passed down from parent to offspring, known as Mendelian genetics. This simple genetic model explains why two short-haired cats can sometimes produce a litter of kittens with long, flowing coats.
Defining Dominance and Recessiveness
The physical characteristics of any organism are determined by genes, which are segments of DNA inherited from the parents. Each gene exists in different versions, called alleles, and a cat inherits one allele from each parent for every trait. The combination of these two alleles is the cat’s genotype, while the visible outcome, such as coat length, is its phenotype.
In simple Mendelian inheritance, one allele can mask the presence of another. A dominant allele requires only one copy to be present for that trait to be outwardly expressed in the cat’s appearance. Conversely, a recessive allele is the hidden form, which is only expressed if the cat inherits two copies of that specific allele, one from each parent. If a cat inherits one dominant and one recessive allele, the dominant trait will always be the one that is seen.
The Specific Gene Responsible for Cat Hair Length
The single gene that determines the length of a cat’s coat is the Fibroblast Growth Factor 5 (FGF5) gene. The non-mutated, “wild-type” version of this gene is the dominant allele, often symbolized as L, and it results in short hair. This dominant allele produces a protein that signals the hair shaft to stop growing relatively quickly, keeping the coat short.
The long-haired trait is caused by a mutation in the FGF5 gene, which is the recessive allele, symbolized as l. This mutation leads to a non-functional or less effective protein, which prevents the hair growth signal from being properly sent. As a result, the hair’s growth phase is prolonged, allowing the individual hairs to grow significantly longer.
A cat’s coat length is determined by one of three possible genotypes. A cat with two dominant alleles (LL) will have short hair, as the hair growth is promptly signaled to stop. A cat with one dominant and one recessive allele (Ll) will also have short hair because the single dominant allele is sufficient to produce enough of the stop-growth signal. Only a cat that inherits two copies of the recessive allele (ll) will have long hair, lacking the functional stop-growth signal entirely.
Scientific analysis has identified at least four different recessive mutations (M1 through M4) in the FGF5 gene that can independently cause the long-haired phenotype. The M4 variant is the most common across most long-haired breeds and non-pedigreed domestic cats.
Predicting Coat Lengths in Offspring
The recessive nature of the long-hair trait has direct implications for predicting the coat length of kittens. Short-haired cats that possess the Ll genotype are known as carriers, as they physically display the short coat but carry the hidden gene for long hair. Carriers are the reason the long-hair trait can unexpectedly appear generations after it was last seen.
When two carrier cats (Ll x Ll) are bred, there is a 25 percent chance that any given kitten will inherit two recessive long-hair alleles (ll), resulting in a long-haired coat. The remaining kittens will be short-haired, with 50 percent being carriers (Ll) and 25 percent being homozygous short-haired (LL) that do not carry the recessive trait at all. This explains how a long-haired cat can be born from two parents with short coats.
If a long-haired cat (ll) is bred with a short-haired cat that does not carry the gene (LL), all the offspring will be short-haired carriers (Ll). The dominant short-hair allele from one parent will override the recessive long-hair allele from the other, but every kitten will still carry the potential for long hair. Breeding two long-haired cats (ll x ll) will only ever produce long-haired kittens, as they can only pass on the recessive allele to their offspring.