The outward appearance of an organism (phenotype) is determined by its genetic code (genotype). In cats, hair presence is primarily governed by a single gene that dictates hair structure. The presence of hair is the standard, genetically dominant condition for most felines. This blueprint ensures the cat produces the necessary proteins to create a strong, fully developed hair shaft and functional hair follicle.
The Dominant Gene for Hair
A normal, full coat of hair is the dominant trait, requiring only one copy of the functional gene. The primary gene responsible for hair structure is Keratin 71 (\(KRT71\)). This gene provides instructions for building keratin, a fibrous structural protein that makes up the bulk of hair. The functional allele, represented as \(KRT71^+\), codes for the correct keratin protein, forming a straight, strong hair shaft securely anchored in the follicle.
A cat with at least one \(KRT71^+\) allele will have hair. This dominant allele acts as the “on switch” for normal hair production. The \(KRT71\) gene ensures the hair follicle completes its cycle correctly, producing the guard, awn, and down hairs of a typical coat. Therefore, the genotype for a cat with hair includes any combination where the dominant allele is present, such as \(KRT71^+KRT71^+\) or \(KRT71^+KRT71^{hr}\).
How Hairlessness Occurs
Hairlessness, seen in breeds like the Sphynx, results from a specific recessive mutation in the \(KRT71\) gene. The physical effect (lack of hair) only appears when the cat inherits two copies of the non-functional allele. This hairless allele is denoted as \(KRT71^{hr}\). This genetic change disrupts the normal function of the keratin protein required to build the hair shaft.
The non-functional keratin protein causes structural defects, resulting in weak, brittle, or improperly formed hair shafts. Hair may start to grow but quickly breaks off or falls out because it is not securely anchored in the follicle. Although the cat may have a fine downy layer of fuzz, the two copies of the \(KRT71^{hr}\) allele override instructions for a full coat. This recessive genotype (\(KRT71^{hr}KRT71^{hr}\)) results in the hairless phenotype.
Understanding Inheritance Patterns
The inheritance of hair or hairlessness follows Mendelian genetics, specifically autosomal recessive inheritance for the hairless trait. A cat with hair can be either homozygous dominant (\(KRT71^+KRT71^+\)) or heterozygous (\(KRT71^+KRT71^{hr}\)). A heterozygous cat will have a full coat because the single dominant \(KRT71^+\) allele is sufficient to produce the correct keratin.
The heterozygous cat is known as a carrier, as it carries the recessive \(KRT71^{hr}\) allele without displaying the hairless trait. If two carrier cats mate, the combination of alleles determines the kitten’s phenotype. There is a 25% chance for the offspring to inherit the hairless genotype (\(KRT71^{hr}KRT71^{hr}\)) and be hairless. There is also a 50% chance they will be carriers with hair (\(KRT71^+KRT71^{hr}\)) and a 25% chance they will be homozygous dominant with hair (\(KRT71^+KRT71^+\)).