As the proverb goes, all cats are grey in the dark. But what about in the light of day? You may be surprised to learn that all cats have stripes, and that there are only 3 possible pigmentations to their fur. How is this possible? Are your eyes playing tricks on you?
There are a few genetic mechanisms at play in determining the appearance of your cat’s fur. If we were to decode cats’ DNA, we’d see that they’re all tabbies; this means that their fur is covered in stripes, which follow different patterns. Whether you can see those patterns or not depends on a gene named agouti. A gene is a section of DNA which is responsible for a specific characteristic in its carrier. Variations of a gene are called alleles. For example, the blood-type gene has A, B and O alleles.
Cats that are carriers of the agouti allele have hairs that are pale at their base and darker at their extremities, making their stripes more visible. Inversely, the hairs on cats that only carry the non-agouti allele have a consistent colour from root to tip, making their stripes very difficult, if not outright impossible, to perceive.
As for colours, cats produce black, brown and red pigments. These hues are associated with different types of melanine, which is the same substance behind skin colour in humans. Different genes can influence these three base colours. Dilution genes can turn black to grey or red to beige, for example, while white is created by genes that completely inhibit the production of melanine. Siamese cats are unique in the sense that their white fur comes from a mutation that prevents hair coloration in warmer areas of their bodies! This is why pigmentation is only present closer to parts of their bodies where their skin is coolest: tail, ears and paws.
Genetics are also responsible for some colour patterns only being found in females. Males cannot have both red and black hairs in their fur because the gene responsible for the red colour is only in the female sexual chromosome (X). A chromosome is the compact state of DNA in cellular nuclei. Chromosomes come in sets of two, inherited respectively from the father and the mother. For mammals, females have two X chromosomes while males have an X chromosome and a Y chromosome.
There are two alleles for the orange gene; one dominant and one recessive (non-dominant). When the dominant O allele is present, black and brown pigments are hidden. However, if a cat only has the recessive o allele, black and brown take over. Since males only have one X chromosome, they can only have one of these alleles (O or o) whereas females can have both the O and the o at once. This is what makes calico or tortoiseshell patterns in females possible.
Since females have two X chromosomes while only one is needed for proper cellular function, one of the two, selected randomly, must be inactivated. This is why black and red hairs are randomly distributed in the coat of females that carry both versions of the orange gene (O and o).
Genetics clearly play a key role in the surprisingly complex processes that regulate our beloved feline friends’ appearance. Who knows: your cat may not be showing you their true colours…
by Geneviève Coudé
Our Popsci author-in-chief, Geneviève Coudé, brings us a myriad of fascinating and astonishing facts about our feline companions. Geneviève has a Master of Science in Ecology from Sherbrooke University, in Quebec, Canada. Her passion for sharing scientific knowledge has brought her to write in a variety of scientific medias addressed toward the general public. She currently collaborates on two French-Canadian television shows geared towards introducing adolescents and young adults to the fascinating world of science.