Each of a cat’s distinguishing characteristics is determined by the genes it inherits from its parents. Although each breed is governed by similar groups of genes that are peculiar to the type and make them appear similar, the individual also inherits specific genes that make it unique. In this way, Siamese cats will always be recognisable as Siamese, but will differ slightly from other Siamese in some characteristics, such as length of tail or shape of ears.
Genes are physical structures, but they can be thought of as a set of instructions that passes from parent to offspring. The genes are carried in chromosomes – genetic material that is contained in the nucleus of every cell in the body. Every time a cell divides, the complete set of chromosomes are copied, so that the information they contain is constantly passed on from one generation of cells to the other.
Animals do not all have the same number of chromosomes. Cats, for example, have 38 chromosomes, arranged in pairs of 19. Humans have 46, arranged in pairs of 23.Both parents contribute a complete set of genes to their offspring, which govern all aspects of the cat’s physical appearance. So, for example, a kitten will have a gene (known as the Manx gene) determining whether or not it has a tail from one parent, and another gene determining whether or not it will have a tail from the other parent. The gene for a normal tail is given the symbol m, and the one for ‘no tail’, is M.
DOMINANT AND RECESSIVE
As with all genes, some are dominant, or dense, and others recessive, or dilute. Dominant genes, as the name suggests, take precedence over recessive genes. The notation for a dominant gene is always a capital letter, while that for a recessive gene is a lowercase letter. So, in the matter of the tail, M is dominant and m recessive. Therefore, if the kitten inherits an M gene from one parent and an m gene from the other, the kitten will have no tail, or hardly any tail at all.
Genes determine such matters as the size and shape of the cat’s body and head, the position and shape of the eyes, ears and so on, and the length and thickness of the tail. The genes occur singly or in a group. Genes that occur in groups are known as polygenes. Polygenes determine body structure, bone and muscle development and the distribution of fat around the body.
Very often, a single trait, such as albino, is caused by a single gene, in this case, c. However, a single gene sometimes has multiple effects. For example, the gene W responsible for a white coat, also determines the eye colour, which will be blue or orange. It can also cause deafness.
Cats that inherit the same gene from each parent – for example, the genes for a black coat (DD) – are known as homozygous. Those that inherit a different gene – for example (Dd) , which produces a blue coat – are called heterozygous.
The ‘ideal’ Manx cat is one that has a hollow at the point where the tail would normally be attached to the body. Such cats are known as ‘rumpies’ . Those Manx cats with some tail are known variously as `rumpierisers’ , `stumpies’ or ‘longies’ , depending on how much (or in fact how little) tail they have.