an introduction to cat genetics
This introduction is meant to explain basic concepts that will be used in my articles on cat genetics. There is a lot more to learn, though, and I hope to do more in-depth articles in the future!
A gene is a physical section of DNA that codes for something. In this context, genes will either produce a protein, or regulate the expression of other genes.
A locus is the actual location of a gene. This is often synonymous with gene, but it is important because when doing genetic testing, if we can show that two genes are at the same locus, then they are actually the same gene.
In breeding terms, we usually refer to the gene and locus by a letter, such as B (for brown). There are often other, more technical names as well, which are mostly useful when comparing a gene’s effects across different species.
An allele is a variant of a gene, with its own unique version of the code and its own effects. For example, B, the black allele, and b, the chocolate allele, are both the same gene at the same locus, but their slight differences make them produce slightly different pigments.
Cats (and most other animals) have two copies of each gene, with one inherited from each parent. If they end up with two of the same allele, such as BB - two black alleles, we call them homozygous. If they end up with two different alleles, such as Bb, they are heterozygous.
We also have terms for what happens when a cat is heterozygous. The main ones to know are dominant and recessive. In this case, a Bb cat is black, making black dominant over chocolate, and chocolate recessive to black. Dominance does not always mean a trait is more common or more likely - it only means that when there are two different alleles, the more dominant one is the one that shows.
Generally, when there is a pair of alleles with the dominant-recessive relationship, we represent the dominant one with a capital letter (B, in this case), and the recessive one with a lowercase letter (b). If there are more alleles, they will typically be represented by adding superscripts onto the upper or lowercase letters - most on this website are written as non-superscripts for readability.
Remember, dominant and recessive are relative terms. The B locus actually has a third allele in cats - b1, which is cinnamon. Cinnamon is recessive to both black and chocolate. This makes chocolate recessive to black, but dominant to cinnamon. Dominant-recessive is not the only type of relationship that alleles can have, but we will talk about the others as we come to them.
It is important to differentiate between phenotype and genotype. Phenotype is the expression of a trait - for us, this will mostly mean what a cat looks like, but in other cases it can be whether the cat has a certain disease or not. Genotype is the combination of alleles causing that phenotype to occur. For instance, we know that a black cat has the black phenotype, but the genotype at the B locus could be BB, Bb, or Bb1.
Finally, there is the concept of carrying an allele. I use this term in a very specific way, so it is important to understand exactly what I mean by it. To carry a trait or allele means that a cat has one copy of a recessive allele that is masked by a more dominant allele. I will not use it in reference to traits that are masked by separate genes, as I consider that confusing. For example, a cat who is Bb is carrying chocolate, but a white cat who is BB is not “carrying” black, because the white is caused by a different gene. The proper term for one gene masking another is epistasis.
So let’s say we have two cats, and we want to know what their kittens will look like. Or, even better, what the chances are that their kittens will have certain appearances. How can we figure this out?
Well, we can make some predictions based on just their appearance, but we need the genotypes of the cats to be accurate. These can be determined by examining pedigrees or by direct genetic testing. Let’s say we have two black cats we know are Bb. Can any of their kittens be chocolate?
When cells divide to make gametes (egg or sperm cells), the two alleles from each gene separate, and each one goes into a different gamete. This means that if a cat is Bb, 50% of their gametes will randomly get the B allele, and 50% will randomly get the b allele. One of those gametes can then meet a random gamete from the other cat.
This means that for two Bb cats, the odds look like this.
25%: B egg + B sperm = BB kitten = black
25%: B egg + b sperm = Bb kitten = black
25%: b egg + B sperm = Bb kitten = black
25%: b egg + b sperm = bb kitten = chocolate
You can also use a Punnett square, which is more or less a table that organizes these outcomes
This means that each kitten has a one in four chance of being chocolate, or, if these cats had many litters, you’d expect about one in four kittens to be chocolate.
bigger breeding outcomes
If you want to make predictions based on more than one gene, you can make a bigger Punnett square...but that can get messy quick, and I’m sure you can easily find other sources on that (the excellent doggenetics.co.uk being one of them). When I am doing calculations, I instead use the rule of that the probability of two things occurring together is (probability of thing 1) X (probability of thing 2).
So, let’s say these two Bb cats are also Dd. The dilution gene, D, turns black to blue and chocolate to lilac when a cat is dd. I’ll first do separate Punnett squares for each gene. The black/chocolate one, as before, outputs a 3/4 chance of black and a 1/4 chance of chocolate. The dilution one outputs a 3/4 chance of non-dilute color and a 1/4 chance of dilute color.
I can now use the multiplication rule to find the chances for each color:
Black = black x non-dilute = ¾ x ¾ = 9/16
Blue = black x dilute = ¾ x ¼ = 3/16
Chocolate = chocolate x non-dilute = ¼ x ¾ = 3/16
Lilac = chocolate x dilute = ¼ x ¼ = 1/16
Unlike Punnett squares, this is easy to scale to include more genes. These calculations are the basis for all my calculators.