Can Kittens Be Identical Twins?

Can two cats be identical twins?

Identical or monozygotic twins are possible in kittens, but rare. Almost all kittens in a litter are not identical, even if they have the same physical appearance.

What are identical twins?

Identical (monozygotic) twins occur when a fertilised egg splits in two during early development resulting in two kittens who share the same DNA. This differs from non-identical (dizygotic twins) who have their own unique DNA. The worldwide incidence of identical twin births in humans is 3 – 4 per 1,000 births. We don’t tend to refer to kittens as twins, triplets, etc, in animals who give birth to multiple offspring, instead, they’re known as littermates.

Most kitten littermates aren’t twins, in fact, kittens within the same litter may not even share the same father. A female cat in heat may mate with more than one male, the resulting litter will have different fathers (heteropaternal superfecundation). There is only one reference to monozygotic twins in reference to monozygotic cojoined twins (commonly referred to as Siamese twins) in a litter of kittens.

Differences between identical twins

Identical twin kittens will always be the same gender and share the same DNA, as they arose from a single fertilised egg, but may not always look the same.  Coat patterns are unique to each cat, even identical twins, as the genes responsible for the coat pattern may be expressed differently in two genetically identical cats.

Because female cats receive two copies of the X chromosome (XX), one is randomly inactivated to prevent the expression of them both. This is known as X inactivation and occurs at random. Due to the random nature of X inactivation, one female kitten may display more traits from the father while the other expresses more from the mother. This is especially true in tortoiseshell, calico and caliby cats. These cats, who are almost always female, carry one copy of the orange gene and one copy of the black or brown gene in every cell. So, even if two cats share exactly the same DNA, their coat pattern will be different.

Why do some littermates look so similar if they’re not twins? 

Kittens inherit their coat colour and pattern from their parents. Cats carry 38 chromosomes, made up of 18 autosomal chromosomes and the sex chromosomes (XY).  Each egg and sperm carry 19 chromosomes. When they combine, the resulting zygote has 38 chromosomes. All of the information to make a new organism is in the DNA.

Littermates can look almost identical because they have inherited the same colour genes from their parents, and therefore have the same phenotype (outward appearance). But there is more to a kitten’s DNA than coat and eye colour. The kittens may look the same, but their genotype (DNA makeup) is different. For example, two littermates may be black, but one cat carries copies of the black gene, while their sibling is black but carries the recessive blue (grey) gene. We can’t see the recessive gene unless it appears in future generations.

Grey is recessive, and therefore kittens have to inherit two copies of this gene. Russian Blues, Negelungs and Korats all have blue coats, and the entire litter will all look very similar. They have the same phenotype (outward appearance), but each kitten will have a unique genotype (DNA makeup).

How to determine if littermates are identical twins?

The only way to know is by witnessing the birth of two kittens who share a placenta or carrying out a DNA test.  DNA tests use a swab, blood or hair samples for analysis.  

How common are twins in cats?

There appears to be no data on the incidence of identical twins in cats but it is thought to be rare. Unless somebody is present during the birth of kittens and witnesses a shared placenta, or littermates are DNA tested, we really won’t know if littermates are twins. We can’t conclude two similar-looking kittens are identical. All it tells us is they share the same phenotype.

How do you tell identical kittens apart?

Most of us will never encounter a pair of identical twin kittens. Breeders and rescuers may have a litter of kittens with the same coat colour, and therefore look very similar. A coloured collar or nail polish can help identify each kitten. Over time, each kitten will develop its own personality, and unique features which make them easier to tell who is who.