Long hair in cats is 1 recessive trait with 4 known off switches, all of them inside the same gene, FGF5. That gene makes a growth factor which tells a hair follicle to stop growing and let the hair go. Break it and the growth phase runs long: 30-60 days in a normal cat, 90-180 in a long-haired one. Two short-haired carriers produce about 1 long-haired kitten in 4, which is why a fluffy kitten can appear in a litter of sleek parents.
FGF5 is the stop signal, not the start signal
A hair follicle cycles through 3 phases: anagen (growth), catagen (transition) and telogen (rest). FGF5 is switched on late in anagen and acts as the brake; when its level rises, the follicle shuts down and the hair is shed a few weeks later. Hair growth rate barely changes between breeds, at roughly 0.25-0.35 mm a day. Length is set almost entirely by how long anagen lasts, which is why 1 broken brake adds 3-5 cm rather than making the hair thicker. The follicle count, the diameter and the density of the coat are decided by other genes.
4 variants, 1 coat
Commercial panels test 4 loss-of-function variants in FGF5, labeled M1 to M4. Each one arose in a different population, and each one shortens or breaks the protein enough to remove the brake. A cat needs 2 broken copies to be long-haired; the 2 copies can be the same variant or 2 different ones.
- M1: the variant carried by the Persian family and by Ragdolls, a small insertion that shifts the reading frame.
- M2: the variant found in Maine Coons and in some Ragdoll lines, a separate insertion at a different position.
- M3: the Norwegian Forest Cat variant, a substitution that removes a splice site.
- M4: the Siberian variant, a deletion that truncates the protein.
Coat length differs by variant and by background: cats with 2 M1 copies typically run 4-6 cm on the body, M3 cats 5-7 cm, and the same genotype on a smaller frame looks longer still. Coat length by breed group is one of the first things the breed quiz asks you to look at.
The arithmetic: 1 in 4, 1 in 2, all
Write the alleles as L (short, dominant) and l (long, recessive). A cat with LL and a cat with Ll both have short hair, and the second is a carrier that looks identical. Only ll grows a long coat. What a pairing produces follows directly from that, and the table below is inheritance arithmetic, not a plan for anyone's cats.
| Genotypes paired | Long-haired (%) | Carrier (%) | Clear (%) | Kittens to see 1 long |
|---|---|---|---|---|
| Ll x Ll | 25 | 50 | 25 | 4 |
| Ll x ll | 50 | 50 | 0 | 2 |
| ll x ll | 100 | 0 | 0 | 1 |
| LL x ll | 0 | 100 | 0 | none |
Why 1 test site is not enough
The 4 variants are independent, so a cat can carry M1 on 1 chromosome and M2 on the other and still be ll. A single-site test would read that cat as clear on the site it checks and miss the second copy entirely. A 4-site panel on 1 cheek swab takes 10-14 days and returns 1 of 3 answers per site: clear, carrier, or 2 copies. The result is worth having if you are buying a long-haired kitten advertised as guaranteed, and it settles arguments about a kitten from 2 short-haired parents.
3 numbers that keep a long coat mat-free
The undercoat sheds at 2-3 cm of loose length, and that is exactly where mats start: the shed hair wraps around the growing hair instead of falling out. Brush 3-5 minutes a day for a 4-6 cm coat, 10 minutes a day during the 2 annual molts, which last 2-4 weeks each. A hairball every 1-2 months is ordinary for a long-haired cat; 2 or more a month, or any hairball in a cat that has stopped eating, is a reason to call rather than to buy malt paste.
What changes at home
A clipper patch of 5 cm over a surgical site takes 6-8 weeks to cover, so plan any procedure with the coat in mind. Heat matters more than most owners expect: a 4-6 cm coat at a room temperature of 28-30 °C is a real load, and long-haired cats drink and groom more in summer. A sanitary trim of about 2 cm around the rear saves 5 minutes of cleaning a day in an older cat. Litter reaches the coat within 1 day of a box change, so a low-dust, larger-grain litter pays for itself in brushing time.
When long hair is hiding a problem
The coat hides 3 things well: weight, breathing effort and skin. A 10% weight loss disappears under 5 cm of fur, so weigh the cat monthly on a kitchen scale. Mats that appear in 1 week in a cat that groomed fine last month usually mean joint pain or dental pain, not laziness. A mat that will not lift, with red or weeping skin underneath, is a burn and needs a vet, not scissors. Breathing that is visible through the flank at rest, or a chest film read by lung pattern because the coat hides the shape of the rib cage, both belong in the same visit. A cat that stops grooming for 3 days usually gets a urine check too, since pain is the usual reason (reading a urinalysis).
What to do next
- Weigh the cat monthly and write it down; the coat will hide 500 g of change from your hands.
- Brush 3-5 minutes daily, 10 minutes in the 2 molt windows of 2-4 weeks.
- Check behind the ears, in the armpits and around the groin every week, where mats start first.
- If you are buying a long-haired kitten, ask for the 4-site FGF5 result in writing, not a photo.
Terms mentioned in this article
Frequently asked
Can 2 short-haired cats have a long-haired kitten?
Yes, and it is the normal outcome of 2 carriers. Short hair is dominant, so a cat with 1 long-hair allele and 1 short-hair allele looks short-haired and is invisible by eye. Pair 2 such cats and about 25% of the kittens get a copy from each parent and grow a long coat. That is the single most common reason a fluffy kitten appears in a litter nobody expected it in.
Do all 4 FGF5 variants give the same coat?
No. All 4 remove the stop signal and produce long hair, but the length and the texture differ with the variant and with the rest of the cat's genetics. Cats carrying 2 M1 copies tend to run 4-6 cm on the body, M3 cats 5-7 cm, and a small body makes the same length look longer. The 4 variants are also independent, so a cat can inherit 2 different ones and still be long-haired.
How often does a long-haired cat need brushing?
Three to five minutes a day for a 4-6 cm coat, and about 10 minutes a day during the 2 annual molts, each of which lasts 2-4 weeks. The schedule matters more than the tool, because mats start at 2-3 cm of loose undercoat and form fastest behind the ears, in the armpits and around the groin. Brushing less often and longer does not work as well as a short daily pass.