The field of genetic research is a relatively new addition to the world of science. We’ve known the basics for a while – parents pass traits down to their offspring, and some traits are more dominant than others – but most of the biggest advances have only been made within the past 200 years or so. Thanks to technological advancements such as computers, high-powered microscopes and specialized lab machines like thermal cyclers, we’re better equipped than ever to extract and analyze genetic material from all species, including cats.
You may remember learning about chromosomes in biology class – they’re the structures found in each and every cell’s nucleus that are made of DNA. Each species of plant and animal has its own unique chromosomes. The exact number varies wildly depending on the species: male jack jumper ants have just one chromosome, while certain species of Adder’s tongue ferns have as many as 1260!
So how many chromosomes do cats have? What do we know about their genes and how do they compare to other animals? You don’t need a biotech lab or even a microscope to learn more about cat genetics – just read on!
Building Blocks: The Construction of Chromosomes
Breaking it Down
The body is made up of cells, each of which is centered around a nucleus. Within this nucleus reside the chromosomes. They form the very basis of who we are, so they’re kept well-protected inside the nucleus, and since they’re present in every cell there are plenty of backup copies available.
A chromosome is often shaped like an “X” with two short arms and two long arms, though some are shaped like upside-down “V”s and others like a single bar. Whatever shape the chromosome takes, it’s made up of DNA wound tightly around proteins. The resulting material is called chromatin and loops around itself to form the chromosome.
An individual’s DNA, which contains its genes — from Marriam-Webster: a unit of DNA that is usually located on a chromosome and that controls the development of one or more traits and is the basic unit by which genetic information is passed from parent to offspring — is divided up between the chromosomes, so all of them are needed to form a complete genetic portrait. If a chromosome is damaged or incomplete, it won’t usually effect genes on other chromosomes but can result in a genetic disorder. Genetic disorders have such all-encompassing symptoms because genes aren’t arranged categorically on the chromosome; a gene that controls eye color may be found right next to one that dictates toe length and another that determines metabolic rate.
Chromosomes normally come in pairs, one from the mother and one from the father. Sperm and egg cells contain a random half of their respective individual’s DNA, and when they combine with each other a complete set is formed, creating a fetus with its own uniquely paired chromosomes. Individual genes are located at the same points on each chromosome, and the expression of a given gene depends on how it corresponds to its counterpart on the matching chromosome.
Sometimes one chromosome wins out and the other’s gene isn’t expressed at all. The prevailing gene is known as the dominant gene, which is a relative term; while some genes are usually dominant, they may lose out if paired with an even more dominant gene. The losing gene is called the recessive gene and though it isn’t expressed, it can still be passed down to offspring.
Other times, both genes mingle with each other and are expressed simultaneously. This is called codominance, and a well-known example is human blood type. Blood can be type A, type B or type O, but sometimes both the A gene and the B gene are expressed, resulting in type AB blood.
Kitty’s Karyotype: All About Cat Chromosomes
Cat Genetics at a Glance
Cats have 38 chromosomes, combining to a total of 19 pairs. One of these pairs determines the cat’s sex; its components are the aptly-named sex chromosomes, which can be either X or Y. Females have two X chromosomes while males have one X and one Y – mothers always contribute an X chromosome, while fathers can contribute either an X (resulting in a female) or a Y (resulting in a male.)
While the sex chromosomes also carry genes that aren’t directly relevant to sex, most of a cat’s genetic makeup is determined by the remaining 18 chromosome pairs, known as autosomes. This is why male and female cats look mostly identical – only 5% of their DNA is tied to their sex.
All members of the cat family have very similar chromosomal makeups, which are called karyotypes. In fact, the karyotypes of various cat species are so similar that they can sometimes successfully breed with one another. This interspecies mating results in hybrids like the Bengal (a cross between a domestic cat and an Asian leopard cat) and the liger (a cross between a lion and a tiger.)
The slight chromosomal differences, however, can be enough to cause problems such as infertility. As such, many of these hybrids cannot themselves reproduce. They also carry a higher risk of severe birth defects and other health problems due to mismatched genes within their chromosome pairs.
Not So Different
At first glance, humans and cats have very different genes. For starters, humans have 23 pairs of chromosomes, four more than cats have. And on a less microscopic level, we just look completely different: we have fewer legs, less hair, no tails, no whiskers, opposable thumbs … the list goes on and on.
Humans and cats branched off from each other on the great tree of evolution over 100 million years ago, so it seems obvious that our two species are so radically different. But despite the staggering amount of time that’s passed since we were one, surprisingly little has changed deep down in our chromosomes.
We share a shocking 90% of our DNA with our pet cats. For every way we differ from cats, there are nine ways in which we’re the same.
The most obvious similarity is our sex chromosomes: we also receive an X chromosome from our mother while our father’s contribution determines our sex. If we end up with an XX pair, we’re female, and if we get an XY pair, we’re male. The exact genes that comprise our chromosomes are different than cats’, but the way our sexes are determined is identical.
Additionally, many of our chromosomes are laid out similarly or even mostly identical to cats’ chromosomes. They’re named and ordered differently, but the same sequences of genes can be found in both of our species. This makes cats invaluable to science, particularly the fields of medicine and biotechnology, as their chromosomal similarity to humans allows us to better understand diseases and medications.
Genetic Oddities: Common Cat Gene Variations
Cats of a Different Color
Have you ever noticed that calico and tortoiseshell cats are almost always female, while orange cats are almost always male? It’s not your mind playing tricks on you – it’s just genetics!
Cat fur color is affected by many genes spread out over the various chromosomes, but the base color is determined by a gene on the X chromosome. This gene can either be black or orange, but not both on the same chromosome. All other cat colors are based on one of these two colors.
Female cats have two X chromosomes, which means that one can carry the black gene while the other carries the orange gene. The genes are then expressed codominantly, creating the black and orange tortoiseshell pattern. Calico cats are tortoiseshells with white spots or patches; white fur is not linked to the sex chromosomes but rather to the KIT gene found on chromosome B1.
Because male cats only have one X chromosome, they can’t display both black and orange coloration at once. However, they do have the upper hand when it comes to full orange coats. 81% of orange cats are male, and the reason for this comes down to probability.
Male cats only have one fur color gene; if that gene is for orange fur, then the cat will be orange. Females, on the other hand, will only be orange if both of their parents contributed the orange gene – effectively halving their probability of being orange.
Odd Ones Out
Despite these sex-linked differences, some male calicos and tortoiseshells still exist. Several different genetic anomalies can be responsible for this rare phenomenon. If you meet a male cat with black and orange fur, consider yourself lucky: he may quite literally be one in a million!
The orange gene is considered a mutation of the black gene, meaning that the gene’s original state was black. Sometimes a male cat fetus with the orange gene will suddenly revert back to the black gene early on in the womb. When this happens, the cat will have black and orange fur, with the amount of black varying depending on when the gene reverted – any skin that developed before the change will be orange, and any that developed after will be black.
Sometimes an egg or sperm cell is abnormal but still produces a viable fetus. One such abnormality is an extra X chromosome, resulting in a set of XXY sex chromosomes; two X chromosomes mean two opportunities for fur color gene expression. Cats with this condition, which is known as Klinefelter syndrome and also occurs in humans, have male reproductive organs but are infertile and can have other health conditions such as low bone density and cognitive problems.
Finally, male cats can be black and orange due to chimerism, a rare condition in which two fertilized eggs merge together in utero. A cat’s DNA is determined at the moment of fertilization, so if one egg would have been a black male and the other an orange male, the chimera cat will contain both sets of DNA in different parts of its body. In other words, the orange and black parts of the cat will be genetically distinct from one another – literally two cats in one!
White Fur, Blue Eyes, Can’t Hear
It’s often claimed that all-white cats with blue eyes are deaf. While this isn’t strictly true, it’s a reliable rule of thumb, as up to 85% of white cats with two blue eyes are completely deaf. It’s not limited to blue eyes, either – even white cats without blue eyes experience deafness at a rate of around 20%, far higher than cats with other fur colors.
All-white cats with one blue eye and one non-blue eye (a condition called heterochromia) may only be deaf in one ear. In these cases, the deaf ear is usually on the same side as the blue eye. Total deafness is still a strong possibility, occuring at a rate of around 40% in all-white heterochromic cats.
Congenital deafness appears to be highly correlated with the KIT gene on autosome B1, which determines how much (if any) of a cat’s fur is white. The gene can take one of four states: no white, white gloves, white spotting and dominant white. The dominant white gene makes the cat completely white and also dramatically increases its likelihood of having blue eyes, though the reason for this is not yet known.
In cats with the dominant white gene, the hearing receptor may completely disintegrate within a few weeks after birth, before the ears even open. These cats never hear anything and face extra difficulties throughout their lives, but they still display that trademark feline resourcefulness and may even experience the other senses at heightened levels. Cats with congenital deafness frequently go on to lead happy lives that are just as long as those of cats without it.
"In ancient times cats were worshipped as gods; they have not forgotten this."
-- Terry Pratchett