Siamese Genetics: Why That Cute Pointed Kitten May Not Be One After All!

Ah yes, kitten season is officially upon us once again. With it comes all the cute descriptions on rescue sites and shelters attaching “probable” breeds to babies that sorta, kinda look like them — all in a bid to make them seem more rare and thus higher in demand for adoption. However, that isn’t how it works. A coat color or a fur type alone does not dictate what breed something is.

Siamese is a breed in and of itself — it is based not just on coat color but structure, conformation, pedigree, and other genetics. So having points alone does not make a cat a Siamese nor mean they have a Siamese parent, or any other purebred lineage.

Get ready to learn some really nerdy stuff!

The actual pointed coat coloring that Siamese and other breeds are known for (as well as rats, mice, and rabbits) is simply a recessive form of partial albinism called “Himalayan”. It has nothing to do with the specific breed of cat called Himalayan, as that is something entirely separate — other than the fact that the Himalayan breed ALSO has the Himalayan genetic mutation. 😊

The Himalayan mutation alters the enzyme responsible for melanin production, which is called Tyrosinase.

Melanin: any of various black, dark brown, reddish-brown, or yellow pigments of animal or plant structures (such as skin or hair). [1]

Tyrosinase: a copper-containing enzyme that promotes the oxidation of phenols (such as tyrosine) and is widespread in plants and animals. [2]

Tyrosine: a phenolic amino acid (C9H11NO3) that is a precursor of several important substances such as epinephrine and melanin. [3]

The mutation restricts the ability of tyrosinase to oxidize tyrosine only at cooler temperatures; warmer temperatures deactivate the enzyme. This results in little to no melanin production within each hair follicle located over the warmer areas of the body (chest, abdomen, upper legs, neck), and more melanin production in hair follicles over cooler areas of the body (ears, face, paws, tail — the “points”).

Because this mutation only creates partial albinism, it overshadows the normal base coat color phenotype that is located on a different allele — i.e. black, blue, red, etc. — while still allowing it to be visible.

Phenotype: the observable properties of an organism that are produced by the interaction of the genotype and the environment. [4]

Genotype: A set of alleles that determine the expression of a particular characteristic or phenotype. [5]

Allele: One of a pair of genes occupying a specific spot on a chromosome. That spot is called a locus. [6]

For example, a Seal point cat is actually a black cat with the Himalayan mutation altering how the melanin is developed and displayed. A Chocolate point is actually a brown cat, a Flame point is actually an orange cat, etc. Blue points and Lilac points are dilute forms of black and brown; they require an additional separate allele mutation to make them dilute. Lynx points, Tortoiseshell (Tortie) points, and Showshoes have yet another additional gene being expressed that gives them their stripes, orange streaks, and masks/socks. These are the same genes as in non-Himalayan mutation cats, just expressed differently. I won’t go into the full cat coat and color genetics explanation, as that will have to be another article on its own!

The Himalayan mutation has a simple recessive hereditary pattern:

Recessive Inheritance: When a trait is recessive, an individual must have two copies of a specific allele to express the trait. Recessive alleles are denoted by a lowercase letter (“a” versus “A”). Only individuals with an “aa” genotype will express a recessive trait; therefore, offspring must receive one recessive allele from each parent to exhibit a recessive trait. [7]

Heterozygous: containing two different alleles [or only one copy] for a particular trait at the same locus. [8]

Homozygous: containing two copies of the same allele for a particular trait, located at similar positions (loci) on paired chromosomes.

In a diploid organism, there are two sets of chromosomes. One of the sets comes from the mother and the other set from the father. Each maternal chromosome has a corresponding paternal chromosome to match with based on their loci. When the loci in the matching chromosomes bear the same alleles, it is described as homozygous. When the loci only contain one of the alleles, it is described as heterozygous. [9]

This means that it can be genetically carried WITHOUT being phenotypically visible (Heterozygous). Thus two parents of any average coat pattern/type would simply need to each carry one copy of the Himalayan gene to potentially have babies with the Himalayan coat pattern. The baby with the mutation would be the Homozygous form with 2 copies of the gene, one inherited from each parent.

Have I lost you yet? Hang in there!

The Himalayan gene can be carried through the parental lines for decades without any expression until the carrier is mated with a fellow carrier to create a Homozygous (visible) offspring. The trait also happens randomly in spontaneous genetic mutation during DNA replication in the first stages of embryonic development.

So what does all of this fancy stuff mean?

The parents of any kitten/cat with colored points could just be run of the mill DSH or DLH cats; black or tabby or orange or grey or what have you, that happened to have an ancestor with the Himalayan mutation 50 years ago or 100 years ago. This is why you can have a random pointed Himalayan mutation pop up in an otherwise normal DSH litter (what many people call a “Siamese” baby within an otherwise domestic litter). It doesn’t necessarily mean they had Siamese or any other purebred breed in their lineage, it just means the parents carried the genetic mutation and that individual happened to inherit 2 copies in order to express the phenotype.

Phew!

So unless a cat has breed-specific traits/conformation, and you know precisely what his or her pedigree is — their parents, their grandparents, their great-grandparents, etc. — proving purebred lineage, then they are simply a beautiful DSH/DLH displaying the Homozygous form of a recessive gene at the C locus…a mutt with an inherited coloration mutation. 😊

Lily, my beautiful mutt with a Homozygous pair of alleles for the Himalayan mutation at the C-locus. Rejected and brought back to the humane society twice at just 18 months old — she was clearly meant to be my sweet, smart, loyal girl. Best $25 I’ve ever spent!

Here’s a fantastic peer-reviewed research article about the Himalayan mutation in mice if you’re into science and genetic chemistry:

Molecular basis of mouse Himalayan mutation.

[Featured Image: A photo of one of my Mom’s purebred Applehead Siamese kittens that I took several years ago. His eyes were mid-change from neonate deep blue to adult turquoise.]

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