If you live with a cat – I envy you. I miss my old feline companions Hal and Storm who live with my parents. Recently I’ve been satisfying my cat-drive by spending hours watching kittens on Youtube researching some cool quirks of the genetics of our feline friends.
Did you know that cats can’t taste sweet things? The protein that is responsible for detecting sweet sugars is made of two parts called T1R2 and T1R3. The gene which codes for T1R3 works just fine, and is also used in detecting amino acids – very important when your diet is primarily protein in the form of mice and pigeon meat! However at some point in the evolutionary history of cats, T1R2 broke.
If you study the first half of the gene that codes for T1R2 protein in cats, you’ll find it looks very similar to that found in humans, dogs and mice. Then you find that there’s a chunk removed which amounts to about 10% of the gene’s total coding length. The rest of the gene ends up being completely garbled because of frame-shifting (see note 1). The receptor for sweet carbohydrates simply cannot form, and Kitty becomes completely indifferent to sugar.
Broken genes like this are called pseudogenes, and are found littered throughout our genomes. I’ve heard them compared to almost-perfect ships preserved beneath the waves with one single cannonball hole telling the story of its demise (I wish I could remember where I got that from!)
All members of the cat family, tigers to tabbies, have this particular pseudogene. The breakage probably happened at about the time the cat-ancestors became strictly carnivorous. Once they were eating only raw meat, being able to taste sweet stuff had no bearing on their survival at all. If gene is not selected for, inevitable mutation and decay take over, and the ship sinks for good.
Sometimes on my way to work I meet a gorgeous tortoiseshell cat who rubs my legs and follows me down the road for a while when I try to leave her – I call her Suzy. I haven’t looked at her nether-regions thoroughly enough to say for sure but statistics is definitely on my side when I claim she’s a lady cat – here’s why.
Sex determination in mammals comes entirely from the chromosomes (in turtles and some other reptiles, the temperature of the nest is the deciding factor). Generally speaking, females have two X-chromosomes (XX) and males have an X and a Y chromosome (XY). The Y-chromosome only contains about 50 genes, most of which are useful only in the testes. Meanwhile the X-chromosome holds 1500 genes used for all sorts of things, but the imbalance in chromosome number between girls and guys has the potential to cause big problems.
Proteins have to be produced in just the right amount at the right time for cells to work efficiently, and changing the number of genes that code for those proteins greatly upsets this balance. In humans, the least severe case of an extra chromosome is Trisomy 21, which people with Down’s syndrome have. It’s very rare for babies to survive beyond birth with even one chromosome missing.
There is a system in place which eliminates this dosage issue between boys and girls – simply hush-up one of the X-chromosomes at random from each cell of the female during development. This will leave one fully active X-chromosome and one which is coiled up tight and tucked away for good. This is called X-inactivation or lyonisation (not to be confused with actual lions…), and normally it goes unnoticed.
Cats have a locus (region) on their X-chromosome which is very important in fur colour, so it’s easy to see X-inactivation in action. There are two alleles (forms) possible, one which results in orange fur and one which results in black fur. Jezebel on the left here has one of each allele. Because the X-inactivation that took place before she was born was random, her fur colour at any given place also random. The result is a gorgeous mottled mess that is tortie fur (see Note 2).
Siamese cats were one of the first recognised breeds of cats. They sport a dark mask, tail, ears and paws. This distinctive pattern is known as “Himalayan”, or “pointed”, and it’s no coincidence that the darker areas are also the coolest parts of kitty’s body.
In cats, humans and other mammals, skin or hair is darkened by a pigment called eumelanin (the orange in torties and redheads is a related molecule called pheomelanin). The quantity of eumelanin production is determined by the activity of an enzyme called tyrosinase.
Siamese have a slightly mutated form of tyrosinase which works perfectly well, but only at temperatures slightly lower than that found inside a cat. The mutation is only a tiny one – only one base change in the entire gene – but the protein it produces is destabilised just enough that it unfolds at cat body temperature and stops working. Therefore the cool extremities are really dark but the belly is the warmest and hence the palest place.
Note1 – DNA is a code consisting entirely of three-letter words. Each word is code for a specific amino acid, which string together to make a protein. There is very little punctuation in this code apart from STOP messages, and there is certainly no space bar. When DNA is transcribed, it’s like the space bar is automatically pushed every three letters – whether or not the message ends up makes sense. If DNA was English, it might read: THECATATETHERAT which could be decoded into THE-CAT-ATE-THE-RAT. If letters are added or removed together in multiples of three, there’s not a big problem and you have a basically coherent sentence. THE-ATE-THE-RAT. However removal or addition in any other number will garble the message entirely, and the earlier it happens, the worse: THE-TAT-ETH-ERA-T. The deletion in the T1R2 gene is 247 bases or letters long – not a multiple of three.
Note2 – Genetics very rarely deals with absolutes – it’s rare but possible to have male tortoiseshell cats. If he has an extra X chromosome, lyonisation will still take place before birth and you’ll have a tortie boy.
Source papers – Subscription / paid access only 🙁
A stain upon the silence: genes escaping X inactivation – Brown and Greally, 2003
Chocolate coated cats: TYRP1 mutations for brown color in domestic cats – Lyons et al, 2005
Tyrosinase mutations associated with Siamese and Burmese patterns in the domestic cat (Felis catus) – Lyons et al, 2005
Source paper – Open access and freely available online 🙂
Pseudogenization of a Sweet-Receptor Gene Accounts for Cats’ Indifference toward Sugar– Li et al, 2005
A Domestic cat X chromosome linkage map and the sex-linked orange locus – Schmidt-Kuntzel et al