Understanding genetics is fundamental to a breeding program

Nature passes hereditary material from one generation to the next (e.g. eye colour) by using genes. Genes are a set of guidelines that help define the way the body looks and to some extent the character of the cat. The gene is the basic unit of hereditary and achieve their effects by directing the synthesis of proteins. Our environment (including the food we eat, and our social and family experiences) also play a part in affecting our looks and character

Chromosomes
Genes are laid out like a necklace on chromosomes. The number of chromosomes are different for different species. It is an even number because chromosomes come in pairs.  One of each pair comes from Dad, the other part of the pair from Mum. Humans have 23 pairs holding about 100,000 genes. Cats have 19 pairs of chromosomes. The dog has 39 pairs. It is known that not all genetic material is used in defining the organism. It's estimated that about 75% is junk in the human

Genes
Genes, like chromosomes, come in pairs, they occupy the matching position on each of the pair of chromosomes. The 2 genes in a pair are not always the same as they come from different parents. These variations are known as alleles but are often called genes.
If the paired alleles are the same, the organism is said to be homozygous for that trait; if they are different, the organism is heterozygous. A dominant allele will override the traits of a recessive allele in a heterozygous pairing. In some traits, however, alleles may be co-dominant i.e. neither acts as dominant or recessive. An example is the human ABO blood system; persons with type AB blood have one allele for A and one for B. (Persons with neither are type O.)

Inheritance
Inheritance is a game of chance based on simple probabilities. Which of the 2 genes an individual receives from a parent is like tossing a coin and seeing if it comes up head or tails. Here's an example of eye colour

 

 

Allele from parent 1

 

 

B

b

Allele from parent 2

B

BB = Brown

Bb = Brown

b

Bb = Brown

bb = Blue

Brown eye colour is dominant over blue eye colour so the brown eyed individual only needs 1 brown eye allele to be brown eyed but the blue eyed individual needs 2 blue eye alleles to be blue eyed. This is usually represented as B_ and bb respectively. The capital letter is used to denote dominance. The _ means that it does not matter what the allele is as it will not affect the appearance of the cat, also known as phenotype. The underlying genes in the cat are known as the genotype
As ever, life is never that simple! There are many characteristics that are influenced by more than 1 gene pair. For example, human skin colour is influenced by at least 4 gene pairs - skin colour is a result of the effects of several separate genes

Sex Linkage
The chromosomes that make up each pair are similar in size & shape except for one pair which is very different: this pair is called the
sex chromosomes. The other chromosomes are known as autosomes.
In the male, one sex chromosome is medium sized (called the
X chromosome & comes from Mum), the other is quite small (called the Y chromosome & comes from Dad), so the male is XY.
In the female, the sex chromosomes are the same medium sized X chromosomes (one from Mum & one from Dad), she is XX .
Research has determined that maleness is determined by a single gene carried on the Y chromosome

Orange (red) gene
The sex-linked orange gene in the cat is carried on the X chromosome.
If a male receives this sex-linked orange gene on his one and only X chromosome he will be red (or cream if he also receives the dilute gene but that's another story).
If a female receives 2 sex-linked orange genes (one from Mum & one from Dad) she will be red (or cream). If she receives only one then she will be a tortie.
This gene is inherited separately from the other coat colours
It is possible to have tortie males but these are a genetic aberration and are infertile

Dilute gene
The dilute colour gene is recessive so for a cat to be a dilute colour it must have received this gene from both its parents. It is represented as dd. If both genes are the same it is described as homogenous. If it is not it is known as heterogeneous.

This gene is inherited separately from the other coat colours

Want to read more?
A useful book to read is 'Get a Grip on Genetics' by Martin Brookes ISBN 0 297 82699 9