“Red cats are always male”, “White cats are deaf”, “Tortoiseshell cats are always female”. These are sayings most cat enthusiasts will have heard before. But are they a myth, or not? The answer is, yes and no. While these statements definitely have truth in them, they aren’t fully correct. It’s true that red cats are more often male than female, white cats are more often deaf than cats with different coat colours, and Tortoiseshell cats are almost always female. So, how is this possible? In this article we will look deeper into how being female or male is linked to coat colours in Cats.
Intro to genetics
Cat coat genetics are complicated and there are different genes involved that determine the overall look of the coat, like the colour, pattern, texture and length. To explain how coat colour and a cat being male, or female are related, one first needs to understand the following concept when it comes to genetics.
Living organisms are made out of cells. Some organisms are made out of a single cell, while others are made up out of millions. Within an organism’s cells you will find chromosomes, which are the structures that contain the genes of the organism. Genes are the genetic material that determine all the aspects that make up the organism, like instructions for growth and functioning, and physical appearance. Each chromosome contains hundreds to thousands of different genes.
Each organism has a set amount of chromosomes, unless a genetic mutation happens. Humans have 46 chromosomes, while cats have 38 chromosomes. Humans and cats are diploid, meaning that their chromosomes normally come in pairs which gives humans 23 pairs of chromosomes and cats 19 pairs. These pairs are formed at conception, where one half will come from the father, and the other half from the mother, which is how genetic material gets passed on to the next generation.
The chromosomes received from father and mother have the same genes, for example the gene for eye colour, but might have different versions of those genes, like brown eye colour, or blue eye colour. These different possible versions of the genes are called alleles. Alleles can be either dominant or recessive. Dominant alleles overrule recessive alleles and for recessive traits to show up, one will need to inherit the recessive allele from both parents.
So let’s put that in practice by looking at the trait for blue eye colour in humans in the image below. This is a simplified version to explain the basics of genetics, as eye colour in humans follows a polygenic inheritance pattern meaning there are more genes involved in determining eye colour.
Blue eye colour (b) in humans is a recessive trait. This means that a person would need to have both alleles for blue eye colour for their eyes to be blue (bb). Brown is a dominant allele, meaning that if a person carries a brown eye colour allele, their eyes will be brown.
Now let’s explain this further with the image above in which we will determine what the genetic make-up of their children could be. In example 1, you will see in the red squares the genetic make-up for that specific eye colour gene of parent 1 (BB), and parent 2 (bb). As explained above, alleles come in pairs which is why each parent has 2 alleles each. Parent 1 has brown eyes and only carries brown eye colour alleles (BB). Parent 2 has blue eyes and only carries blue eye colour alleles (bb). As they can only pass on 1 allele each (as each contributes half), in this case parent 1 will always pass on the brown allele, and parent 2 will always pass on the blue allele, as seen in example 1 in the image. The possible genetic make-up of the kids for this gene are circled. All kids will have 1 dominant allele for brown eyes (B), and one recessive allele(b) for blue eyes, making their genotype (Bb) which will result in them all having brown eyes. They also carry the allele for blue eyes, but as this allele is recessive, the brown allele overruled the blue allele.
Let’s have a look at example 2. Both parents carry a dominant brown eye colour allele (B), and a recessive blue eye colour allele (b), like the kids in example 1 (Bb). Both the parents will have brown eyes, as they carry the brown eye colour allele which is dominant. Their kids, however, as can be seen in the image 2, will have either only brown eye colour alleles which will result in brown eye (25% chance), a brown and one blue colour allele which will result in brown eyes (50% chance) or only blue eye colour alleles (25%) which will result in blue eyes. In example 3 and 4 you can see some more possibilities.
Chromosomes also determine if a cat, or human, is male or female. Female cats, and humans, have two X chromosomes (XX) and males have an X and a Y chromosome (XY). An important note is that this system does not apply to every animal.
Coat color in relation to being female, or male.
So how is it possible that being female or male is related to coat color? This is because cats and hamsters are the only mammals known where the genetic make-up for the coat color is partly on the X-chromosome. In other species they are on different chromosomes meaning they inherited independently of each other. So let’s explain this a bit more.
Just like with eye colour in humans, coat colour in cats has a polygenic inheritance pattern meaning there are more genes involved in determining coat colour. One of these genes is located on the X-Chromosome.The specific gene that is located on the X-chromosome has two possible alleles, either the first which is orange coat colour(O), and the other allele is for black coat colour (o).
As females have two X-Chromosomes (XX), they will have either the Orange or the Black allele on each of their X chromosomes making their possible genotypes either:
- (OO) – Orange coat
- (Oo) – Tortoiseshell coat (in combination with another gene this can turn Calico)
- (oo) – Black Coat
Normally alleles come in pairs, but because this specific gene is located on the X-chromosome and males only have one of these (XY), this is now not the case. This results in male cats only having one allele, compared to the female who had two. Making their possible genotypes:
- (O) – Orange coat
- (o) – Black Coat
As can be seen, in normal situations it will never be possible for a male cat to have the genotype Oo for the Calico or Tortoiseshell coat, as they cant have the 2 alleles needed due to them being male.
So how is it possible that some male cats are still Calico or Tortoiseshell? One known, but rare occurrence is when a sperm or egg are abnormal and the male cat ends up with an extra X chromosome, making their genetic makeup XXY, instead of XY, giving them the extra X chromosome needed to be able to get the Calico or Tortoiseshell colour. These Calico and Tortoiseshell males are known to be sterile.
Another rare occurrence that can happen giving a male cat a Calico or Tortoiseshell colour is Chimerism. We speak of Chimerism when two fertilised eggs fuse inside the womb. As these in normal cases would have been two different kittens, these two cells that fuse contain two different genetic makeups. When fusing, the two cells become one, making this one egg containing the genetic make-ups of two eggs. If one of the fertilised eggs was that for a black cat, and the other an orange cat, they cat can turn out Tortoiseshell or Calico while still being male.
So why don’t the orange and black colour mix into a different colour?
An interesting thing happens in females preventing them from expressing their double amount of x-linked genes. At a point in the embryonic development, in every cell, one of the two x chromosomes will inactivate randomly. This will leave one active chromosome in every cell. This means that in some cells the black allele will be active, while in other cells the orange allele will be active. This gives the Calico and Tortoiseshell their unique pattern.
Why are most Orange cats male?
So how come that most male cats are orange? As seen in the genotype example above, the possible genotypes in this case for males are (O) or (o), while for females they are (OO), (Oo) and (oo). As male cats (XY) only have one X chromosome, they only have to have the orange gene on this single chromosome to have an orange coat. The female cat (XX) on the other hand, will need the orange gene on both of the X chromosomes for the coat to turn orange(OO), because the mix Oo will lead to a calico or tortoiseshell coat. The same applies for the black coat colour, making full black cats also more likely to be male.
In addition to this, in the general cat population the orange gene is much rarer than the black gene. And as the female cat needs to inherit the orange gene from both parents to turn out orange, the chances are even lower that an orange cat is female.
But my cat is white?
As noted before, there are more genes at play that determine the final coat colour of cats. And while the red and black colour are considered the dominant colours and all colours originate from them, there are other genes at play that will determine the overall end look of the coat. For example, there is a recessive gene that will turn the black and red into their diluted versions, like grey, blue, cream etc. There is a dominant gene that might cover up all colours making the cat white. And there is a dominant gene for white spotting.
Cat coat genetics, complicated but interesting. And in cats, some coat colours are linked to being female or male.
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