Why Do Cats Have Different Coloured Kittens?

Kittens come in a rainbow of colours and it can sometimes be confusing when a kitten is born to two parents who didn’t have the same coat colour, for example, two solid cats produce a pointed kitten or two black cats produce a grey kitten or two shorthaired cats produce a longhaired kitten.

It is not uncommon for a litter to contain kittens who are all different colours. This variation in colours and patterns comes down to several genes which determine how each kitten will look.

What are genes?

A gene is a small section of DNA that is located on thread-like structures called chromosomes which are made up of DNA and come in matching pairs, one from the mother and the other from the father. Chromosomes are present in every cell in the body. Each gene has a fixed location (loci) on its chromosome which helps to specify a certain trait, for example, coat length, eye colour, and hair colour.

Each location can contain different genes, and there is no fixed limit to the number of alternative genes that may occupy a locus. But each cat will only have two genes at a time.

Two basic colours

Cats display a huge number of coat colours, but there are only two pigments that determine coat colour. Eumelanin (black) and pheomelanin (red). White is omitted because it is not a colour, but a lack of colour.

Any other coat colour is a variation of these two pigments due to genetic mutations or modifier genes.

• Dilution: Changes black to blue and red to cream, chocolate to lilac and cinnamon to fawn.
• Dilute modifier: The dilute modifier (DM) modifies the dilute colours from blue to taupe, lilac to caramel, cream to apricot
• White spotting gene: This creates white areas on solid or tabby cats and can range from a few hairs to covering almost the entire coat. Therefore, if you ever see a cat who is a mix of a solid or tabby pattern, and white (no matter how large or small), the cat carries the white spotting gene. The gene(s) responsible for white spotting in the domestic cat has not yet been identified. Cats with large areas of white tend to have two copies of the white spotting gene.
• Dominant white: This dominant gene masks out all other colours, no matter what colour genes are present.
• Albino: A complete absence of pigmentation in the hair, skin and eyes due to a mutation in the gene encoding for tyrosinase. Tyrosine is necessary to make melanin, the major pigment in the skin and hair; it gives us wonderful colours.
• Partial albino: This unusual coat pattern is most commonly seen in the Siamese and other pointed cats have a defective form of tyrosine which does not function at normal body temperature. Genetically, a seal point Siamese is a black cat but due to the tyrosine defect, pigmentation can only appear on areas of the body that are cooler, such as the nose, ears, feet and tail.

Types of genes

Many genes can impact the coat colour and pattern a cat exhibits (phenotype), which may be dominant or recessive.

A dominant gene overrides the different variants on the same gene, and a recessive gene is a gene that can be masked by a dominant gene. For example, a cat may be black but carry the recessive blue gene (Bb). If that cat mated with another cat who also carried the recessive blue gene and the offspring inherited both copies, the kitten would be blue (bb).

A dominant gene is represented with an UPPERCASE letter, and a recessive gene is in lowercase. Remember, the cat has two copies of each gene, one from the mother and one from the father.

• B/B – Homozygous black (the cat has two copies of the dominant black gene and is black)
• B/b – Heterozygous black (the cat has one copy of the dominant black gene and one copy of the recessive blue gene, and is black)
• b/b – Homozygous blue (the cat has two copies of the recessive gene and is blue)

How these genes are passed on to the next generation:

• B/B cat can only pass on the black coat colour (B or B)
• B/b can pass either black or blue (B or b)
• b/b can only pass on blue (b or b)

Sex-linked orange gene

Kittens display coat colour inherited from the mother or father, but in the case of female cats who inherit the red (O) gene from one parent and the black (B) gene from another, she can display both colours (B/O) at the same time, this pattern is known as tortoiseshell, or if the cat has the white spotting gene as well, calico.

Why only females? Because the O is carried on the X chromosome (which makes it sex-linked) and as females have two X chromosomes (XX) they can show both black and orange. The random colour pattern is due to X inactivation which silences one of the copies in each cell to ensure the female only has one functional copy of each gene in each cell. As males (XY) only have one X chromosome, they will either be black or red but not both. So, we know that a tortoiseshell cat has one black parent and one red parent.

Let’s take a look at the four kittens in the photo above. We have a tortie and white, a black and white, a blue and white and another black and white.

Assuming this litter all shares the same father, the parents would be:

• Parent 1: Red, red and white, carrying the recessive blue
• Parent 2: Black, black and white, carrying the recessive blue
• One or both of the parents had to carry the white spotting gene

Outcome

• Two kittens inherited the black gene with white spotting (black and white, also known as piebald or tuxedo)
• One kitten inherited the dilute blue with white spotting (blue and white, also known as piebald)
• The female on the left inherited the black, red and white spotting gene (tortie and white)

Tabby patterns

The tabby gene is known as agouti (A) and is dominant over non-agouti or solid cats (a). The wild-type gene for the domestic coat colour and pattern is a brown tabby. All cats are tabby, but the tabby pattern is masked in solid coloured cats. You can sometimes see ‘ghosting’ on a bright day or in young kittens.

Kittens can have different fathers

As if coat colour genetics isn’t confusing enough, a litter of kittens may also have more than one father as a female cat in heat will mate with one male if given the opportunity. This is known as superfecundation.

Between dominant genes, modifier genes, white spotting, and different fathers, no wonder kittens in a litter can have a variety of coat colours.

Determining what colour potential kittens will be used to boil down to some genetics knowledge, test matings and with modern advancements, DNA tests to determine what coat colour a cat is and if it carries recessive genes.