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By special request from Emma S.
I am back with the genetics! A friend asked me if I was going to address the genetics of cat fur length in addition to my previous blogs on coat genetics. I wasn’t sure there was enough information to fill up a whole blog, but I dove in anyway. It turns out, there’s plenty. It’s not just the difference between a longhair and a shorthair, but also hairless cats and sort-of-hairless cats and curly-haired cats. Cats are complicated. Genetics is complicated. That’s kind of the fun of it, though. So, this one is for you, Emma. Enjoy.
Shorthair vs. Longhair
Let’s start with the basics. If you need a refresher on the principles of genetics, look over the first few paragraphs of my coat color genetics post. Ready? Let’s go. The wild-type, or default, fur for cats is short with no texture modifications. Long hair is recessive to short hair [1-4]. This has been common wisdom for a while, but genetic studies confirmed it in 2007 [1, 2].
The researchers discovered four mutations in the same gene, Fibroblast Growth Factor 5 (FGF5), which cause cats to have long fur [1, 2]. FGF5 mutations were already known to cause hair-length variability in mice and dogs at the time the studies were conducted, so this was not a surprising result [1, 2]. In studies with mice, the FGF5 protein signaled hair to exit the growth phase and enter the static phase [2]. When mutations in FGF5 cause the protein to be nonfunctional, the growth phase of the hair lasts longer [2]. Hence, longer fur.
The four FGF5 mutations in long-haired cats were, creatively, named Mutation 1-4 [2, 3]. Mutation 1 seems to be exclusive to Ragdoll cats [2]. Mutation 2 was only found in Norwegian Forest Cats [1, 2]. Maine Coons are most likely to have Mutation 3, but Ragdolls might as well [1, 2]. Mutation 4 belongs to all long-haired breeds and random-bred longhairs [1, 2]. So far, there is not indication that any gene besides FGF5 contributes to the longhair phenotype. The fact that at least four mutations in this gene exist, three of them breed-specific, suggests that the trait has arisen multiple times through a similar mechanism. This is probably because, in certain environments, long fur is advantageous. It has also sometimes been artificially selected for.
Now, we know that there are not just two lengths of cat fur, one long and one short. There are many variations within both the shorthair and longhair categories, and some cats are kind of in-between. This is because very few things are affected by just one gene. An unknown number of modifier genes determine the exact length of a cat’s hair relative to their genotype at the primary hair length locus [4].
Hairless, or Near Enough
Some cats or breeds are more or less hairless. True hairlessness is almost unheard of. Hairless cats present with a variety of traits that we call “hairless.” Most have very thin coats that look as if they aren’t there at all. Some have fur that thins drastically in only certain places. In either case, the degree of “hairlessness” can vary widely. As one would expect with so much natural variation, more than one mutation causes the different patterns of feline hairlessness. Sometimes the trait is dominant, and sometimes it is recessive [3, 4].
A recessive mutation dubbed hr gives the easily-identifiable Sphynx its hairless phenotype [3, 4, 5]. The Sphynx is a typical hairless breed with an extremely thin, downy coat that gives the appearance of baldness [5]. The hr allele is a mutation in the Keratin 71 (KRT71) gene [5]. The keratin 71 protein is expressed in the inner root sheath of the hair follicle and is important for the normal formation of the hair [5]. The hr mutation does not prevent hair from forming, but the hairs are easily dislodged because the hair bulbs are weak, resulting in the distinctive naked appearance of the Sphynx [5].
The Russian hairless breeds, Peterbald and Donskoy, carry the semi-dominant Hp mutation [4, 6]. Other genes affect how strongly the Hp hairlessness allele is expressed [4, 6]. The cats’ coats range from a brush coat–sparse, rigid, curly fur–to hairless all over [4]. Hairless mutations identified in British and French cats, hd and h respectively, are recessive [3, 4]. Sadly, hd is also a lethal mutation [4].
The Lykoi breed displays partial hairlessness. They have a thin coat all over and a hairless face, sometimes with other patches of hairless, especially on the legs. Lykoi molt all of their hair at least once in their life and may molt as often as every shedding season. This arguably makes them a breed that is truly hairless, albeit only for short periods of time. Six different mutations have been found to cause the Lykoi phenotype, but all of them are mutant alleles of the same gene, lysine demethylase and nuclear receptor corepressor, abbreviated HR for “hairless” [7]. Mutations in HR also cause hair abnormalities in humans, mice, sheep, and macaques [7].
Rexoid Variations
Curly fur occurs naturally in quite a few mammals, including dogs, rabbits, and cats. There are five recognized breeds of curly-coated cats, per the Cat Fanciers’ Association and The International Cat Association: American Wirehair, Cornish Rex, Devon Rex, LaPerm, and Selkirk Rex. Several other potential rexoid breeds have been proposed, including the German Rex and Oregon Rex. While all of these cats–and some random-bred cats–have curly coats, there are a lot of different causal mutations.
Remember the KRT71 gene? One kind of mutation in it causes the Sphynx’s hairlessness, but a different KRT71 mutation makes the Devon Rex’s pelt curly [5]. The Devon Rex mutant allele, re, is recessive to the wild-type allele but also to the hr Sphynx allele [5]. Devon Rex cats were used in the early Sphynx breeding programs, so some Sphynx still have one hr and one re allele, but they look just like any other Sphynx [5].
A recessive mutation also causes the Cornish Rex’s curly coat, but in their case the gene in question is lysophosphatidic acid receptor 6 (LPAR6) [8]. The mutant allele, known as r, causes the Cornish Rex to only develop an undercoat, one of the three layers of a normal pelt, and a curly one at that [8, 9]. Humans who are homozygous for LPAR6 mutations have a similar phenotype known as “wooly hair” [8]. Because the Cornish Rex was used to found the German Rex, r is in the genetic background of German Rexes, too [8].
Unlike the Devon and Cornish, the Selkirk Rex mutation, Se, is dominant, although incompletely dominant [9]. This means that cats homozygous for the mutation have a more dramatic phenotype: thinner coats, tighter curls [9]. And, wouldn’t you know it, Se is an unique mutation of our old friend, KRT71 [10]. Both KRT71 and LPAR6 are genes that maintain normal hair growth [5, 8]. It makes sense that different mutations in the same or similar genes would cause similar results.
Works Cited
- Drögemüller, C., Rüfenacht, S., Wichert, B., and Leeb, T. (2007). Mutations within the FGF5 gene are associated with hair length in cats. Animal Genetics, 38(3), 218-221. https://doi.org/10.1111/j.1365-2052.2007.01590.x
- Kehler, J.S., David, V.A., Schäffer, A.A., Bajema, K., et al. (2007). Four independent mutations in the feline Fibroblast Growth Factor 5 gene determine the long-haired phenotype in domestic cats. Journal of Heredity, 98(6), 555-566. https://doi.org/10.1093/jhered/esm072
- Basepaws. (2019, July 14). Cat coat genetics. Basepaws, Inc. https://www.basepaws.com/blog/cat-coat-genetics/
- Hartwell, S. (n.d.) Genetics of colour and conformation. MessyBeast. http://messybeast.com/cat-genetics-basics.htm
- Gandolfi, B., Outerbridge, C.A., Beresford, L.G., Myers, J.A., et al. (2010). The naked truth: Sphynx and Devon Rex cat breed mutations in KRT71. Mammalian Genome, 21, 509-515. https://doi.org/10.1007/s00335-010-9290-6
- Zhigachev, A.I., Vladimirova, M.V., and Katser, I. (2000). Phenotypic and genotypic characteristics of Russian hairless cats. Genetika, 36(4), 538-544. https://europepmc.org/article/med/10822817
- Buckley, R.M., Gandolfi, B., Creighton, E.K., Pyne, C.A., et al. (2020). Werewolf, there wolf: variants in Hairless associated with hypotrichia and roaning in the lykoi cat breed. Genes, 11(6), 682. https://doi.org/10.3390/genes11060682
- Gandolfi, B., Alhaddad, H., Affolter, V.K., Brockman, J., et al. (2013). To the root of the curl: A signature of a recent selective sweep identifies a mutation that defines the Cornish Rex cat breed. PloS One, 8(6), e67105. https://doi.org/10.1371/journal.pone.0067105
- Basepaws. (2019, July 15). Curly cat coat: A special kind of eye candy. Basepaws, Inc. https://www.basepaws.com/blog/curly-cat-coat/
- Gandolfi, B., Alhaddad, H., Joslin, S.E.K., Khan, R., et al. (2013). A splice variant in KRT71 is associated with curly coat phenotype of Selkirk Rex cats. Scientific Reports 3, 2000. https://doi.org/10.1038/srep02000
Published November 1st, 2020
Updated June 14th, 2023
