A cat’s body cells all have 2 sets of chromosomes (called diploid), one set inherited from each parent. When body tissues are renewed, the cells undergo cell division to produce genetically identical cells with the same number of chromosomes as the parent cells. This is called mitosis.

When germ cells are produced, the ova (egg cells) and sperm cells must only have one set of chromosomes (called haploid) in order to make a diploid embryo when an egg is fertilised. This process is called meiosis and is part of sexual reproduction in cats (and in humans, dogs etc). This is called meiosis and allows genetic traits to be shuffled so that the offspring have a different set of traits to either parent.

The following description of cell division is greatly simplified and provided as an aid to understanding how genetic traits are shuffled and how mutation occurs. For more detailed information on the different stages and processes involved in each type of cell division, there are many resources on the web.

In mitosis, one cell diploid divides into 2 daughter cells while preserving the diploid chromosome count. Each daughter cell is genetically identical to the parent cell and to the other daughter cell (barring mutation due to random transcription errors).

A diploid cell has a full set of chromosome pairs. Each pair contains one chromosome from each parent. These chromosome pairs are not identical, but are homologous (equivalent) chromosomes as they contain different alleles of genes. For example, one particular locus (location – a bit like a house number on a street) on one of the father's chromosomes may pass on black fur. The same locus on the mother's chromosome may pass on grey fur.

Firstly, the parent cell must make copies of each chromosome. The two identical copies are called sister chromatids and are attached together at a specialized region of the chromosome called the centromere. At this point, the cell contains twice the normal amount of genetic material. The chromosomes then align themselves in a line across the cell. The chromotids are pulled apart and one chromatid, now called a chromosome again, migrates to each pole of the cell and the cell itself splits into 2 cells with each cell having the normal amount of genetic material.

When DNA is replicated, each chromosome makes an identical copy of itself. One copy will end up in each daughter cell so that the daughter cells have the same combination of genetic material that the parent cell had. Homologous chromosomes are kept together. In the diagram, the parent cell has one red and one blue chromosome. Both daughter cells will also get one red and one blue chromosomes.

Sometimes there are errors in mitosis. This affects not only the immediate daughter cells, but the future offspring of those daughter cells. For example, in non-disjunction, the chromatids fail to separate and one daughter cell gets both sister chromosomes while the other daughter cell gets none. This can result in cancer. Chromosomes may also become damaged with parts broken off, wrongly copied, duplicated or reattached wrongly (looped or reversed). The effects range from no noticeable effect to lethal (especially in the embryo which is rapidly dividing and where early mistakes are multiplied many, many times). These mistakes are called somatic mutation because they affect the body cells and not the germ cells.

Meiosis produces four genetically varied gametes (egg cells or sperm cells) out of a diploid cell. A diploid cell has a full set of chromosome pairs. Each pair contains one chromosome from each parent. These chromosome pairs are not identical, but are homologous (equivalent) chromosomes as they contain different alleles of genes. For example, one particular locus (location – a bit like a house number on a street) on one of the father's chromosomes may pass on black fur. The same locus on the mother's chromosome may pass on grey fur.

First, the chromosomes are replicated. Each chromosome comprises two identical chromatids joined together by a stretch of DNA called the centromere. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. The end result is 4 haploid cells each containing slightly different mixes of genes.

During meiosis I, the chromosomes are replicated and genes may be shuffled from one chromatid to another. The chromosomes pair up together for a short time during which chromosomal crossover occurs – a gene on one chromosome may get swapped for the gene on the corresponding chromosome (a bit like 2 people crossing the road in opposite directions at a pedestrian crossing). In the diagram, there are 2 chromosome pairs (big chromosome and little chromosome). The red part was inherited from the father and the blue part from the mother, so the pairs of chromosomes in the offspring have one red and one blue chromosome. Crossing over of genes is shown as red and blue patches. This results in 2 diploid cells, each containing a slightly different mix to the original parent cell.

During meiosis II, each of the 2 diploid cells splits into 2 haploid cells. It skips the stage of replicating chromosomes and goes straight into the division stage. As a result of this, the cells have half the number of chromosomes. These are the ova (egg cells) and sperm cells, known collectively as germ cells. When they meet up in a fertilised egg, the resulting embryo will have the normal diploid number of chromosomes.

If the number of chromosomes was not halved, each generation would have twice as many chromosomes as its parents. Some hybrids are triploid (3 sets of chromosomes) or tetraploid (four sets of chromosomes) but are usually sterile because meiosis cannot occur. Having three or more sets of chromosomes is known as polyploidy and can result in developmental abnormalities. Sometimes there are errors during DNA replication – parts of the DNA may be lost, damaged, duplicated or reversed. This is mutation. If a germ cell with mutant DNA fertilises (or is fertilised by) another germ cell and results in an embryo, the embryo inherits the mutation. This can range from not noticeable (recessive genes or "redundant" DNA) to cosmetic (fur colour) to lethal (body structures are not properly formed). Mutations which are not noticeable in the offspring may show up in later generations if two identical mutant genes meet up.

Sometimes chromosomes do not separate properly during meiosis I or chromatids do not separate properly during meiosis II. As a result the germ cells have either more or less genetic material than normal. A chromosome might be duplicated or absent. The most obvious instances in cats are when one of the sex chromosomes is duplicated or missing. Normal females are XX and normal males are XY. An XXY cat is a sterile male with a tendency to giantism or overweight; this is one cause of the uncommon tortoiseshell male cat. An XO (Turner syndrome) cat is a sterile undersized female, often with heart problems.

A chromosome is a length of DNA. In cats, as in humans, autosomes (chromosomes other than X or Y) occur in pairs. Cats have 19 pairs of chromosomes. The matching strands of DNA form a double helix. The exception to this are the sex-determining chromosomes X and Y. A gene is a stretch of DNA that, in simple terms, encodes for a trait such as long or short fur. On the autosomes, genes occur in pairs - one on each strand. These genes may be identical or they may be slightly different variations of the same basic gene. The variants are called properly called alleles of genes, but for simplicity I shall call them genes (they should also be called "gene for black", "gene for grey" etc, but I shall refer to them as "black gene", "grey gene").

The "locus" of a gene refers to the place it occupies on the chromosome. Think of the chromosome as being like a street, the gene being a house and the locus being the house number. The street has 2 sides and the houses opposite each other may be identical or different.

Most cells have pairs of chromosomes (called diploid). The exceptions are egg cells and sperm cells which have a single set of chromosomes (called haploid). This is so that egg and sperm cells can fuse into a fertilised egg that has the correct number of paired chromosomes, one set from the mother and one set from the father. The offspring gets half of its genes from each parent and will be genetically different from each. Working out which animals to mate together to achieve the desired results in the offspring is both a science and an art (and the subject of many full-length books).

Named after the studies of Gregor Mendel, simple inheritance relates to a trait governed by a single gene. In cats, the inheritance of black and blue (grey) is an example. Genes on the autosomes (the chromosomes other than the X and Y sex chromosomes) are inherited in pairs, one copy from each parent. If a cat inherits 2 blue genes, it will be grey. If it inherits 2 black genes it will be black. If it inherits 1 black gene and 1 blue gene it will be black but can pass on the blue gene to its offspring.

The black gene is dominant - this gene will always be expressed (show up in the phenotype i.e. outward appearance) even if only 1 copy is present. The blue gene is recessive and will only be expressed if 2 copies are present. An animal where both copies of the gene are the same is homozygous for that trait. An animals where the copies are different is heterozygous for that trait.

The blue gene is recessive. It is only expressed if the cat didn't inherit a black gene. Blue cats are homozygous for the blue colour. Its genotype (genetic make-up) is the same as its phenotype unlike a black cat whose genotype can be either heterozygous or homozygous.

By convention, a dominant gene is shown in uppercase and a recessive gene is shown as the same letter, but in lowercase. I have also shown the letters in colour.

In some cases, neither gene is dominant. The genotype Xx would then give a phenotype (appearance) intermediate between XX and xx in the same way that pink is intermediate between red and white. In the 19th Century, breeders believed all genes worked this way and called it "blending".

A dominant gene is always expressed even if only one copy is inherited. A recessive gene is only expressed if two copies are inherited.

Another type of gene sometimes called "dominant" is an "epistatic" gene. This is a gene that resides at a different locus of the chromosome or on a completely different chromosome and it masks the effect of certain other genes. An example in cats is "dominant white" (properly called "epistatic white"). When present, dominant white masks out any other colour genes. The cat has a white phenotype but also has the genes for other colours and passes on those colour genes to its offspring.

Orange (or red) is another epistatic colour as it influences the non-agouti gene. Non-agouti (solid colour) cats that also have the orange gene will turn out tabby. Even the so-called solid red (red self) cat has tabby markings, but breeders have worked to decrease the contrast between background and marking colours to give the impression of no markings.

Agouti (Abyssinian tabby) is epistatic over other tabby patterns. Until recently it was thought to be an allele of tabby. Recently it was found to be on a different locus.

Although genes are inherited in pairs, there may be more than 2 variants of a gene carried by different individual. Some of those genes are dominant, some are recessive and some are co-dominant (neither dominant nor recessive). An example in cats is the colourpoint series. This is a form of temperature dependent albinism where the cooler parts of the cat (legs, tail, ears, muzzle) are darker than the warmer parts (torso). The colourpoint gene is recessive to the solid colour gene.

There are many possible combinations because some genes are more recessive than others!

* If C (full colour) is inherited, the cat will be solid colour and any other genes will be hidden.

* If C is not inherited, the other combinations will show up as below:

* If cs is inherited, the cat is colour point and any c or ca genes are hidden
* If cb is inherited, the cat is sepia and any c or ca genes are hidden.
* If neither cs nor cb are present, ca is inherited, the cat is blue-eyed albino (recessive white) and the c gene (if present) is hidden.
* If none of cs, cb or ca are present, 2 c genes are inherited, the cat will be pink-eyed albino.

There is one complication: cs, cm and cb are co-dominant (dominant over other genes, but not dominant over each other). If a cat inherits one cs + one cb, the pattern is intermediate between colourpoint and sepia and is called mink. The Tonkinese breed was created by breeding Siamese and Burmese cats together. In the same way cs + cm creates intermediate form Siamocha, while cb + cm creates intermediate form Burmocha.

Inbreeding (breeding together closely related individuals) tends to bring out recessive genes because the animals carry similar combinations of genes and there is more likelihood of any hidden genes meeting up and, therefore, showing up. Inbreeding does not create recessive traits, it just increases the probability of recessive genes being manifested.

A gene series must occupy the same loci on pairs of chromosomes. Some websites based on older texts incorrectly show tabby as a series of genes in the following order of (most dominant first): Abyssinian (ticked) Tabby, Mackerel Tabby, Classic Tabby. This is incorrect because Abyssinian Tabby is on a different locus.

Most traits are inherited separately from each other because the genes occupy different loci (plural of locus) on the chromosomes. Some traits are inherited together because the genes are located close to each other and tend to be passed along together. Think of it as being like terraced houses on a street - you can't move one of the terraced units without moving all of them because they share some walls.

Two grey cats mated together produce more grey cats because they only have the recessive genes. Two black cats mated together might produce grey kittens if both parents carry a hidden recessive grey gene. Looking at the pedigree of the parents, it will turn out that each cat has a grey ancestor. The hidden recessive gene has been passed down over several generations, but only shows up when a kitten inherits 2 recessive genes and no dominant gene to mask it. This is sometimes called a throwback because the kitten is a throwback to an earlier ancestor.

Some genes are carried on the X and Y sex chromosomes. Because these chromosomes do not form pairs in the same way as the autosomes, the traits affected show up unequally in males (XY) and females (XX). These traits are known as sex-linked. In cats, the tortoiseshell pattern is sex-linked (this includes the calico pattern which is simply tortoiseshell with the addition of the white spotting gene) and the majority of tortie cats are female. Male torties are uncommon and are due to developmental or genetic abnormalities.

The orange gene (O) changes black pigment into a reddish pigment. The orange gene is carried on the X chromosome. A normal male cat has XY genetic makeup so he only needs to inherit one orange gene for him to be a ginger cat. A normal female is XX genetic makeup. She must inherit 2 orange genes to be a ginger cat. If a female inherits only one orange gene, she will be tortoiseshell and will be heterozygous for both orange and black. (The actual formation of orange and black patches in a tortie female is because, early in embryo development, some cells randomly activate the orange gene from one X chromosome while others activate the black gene from the other X chromosome. This happens in all cells, but only shows up visibly in skin cells as these produce pigment.)

More accurately speaking, O = "epistatic orange" and o = "non-orange" meaning that non-orange allows whatever colour is carried on the other chromosomes to be displayed (black, blue, chocolate etc). For simplicity, I am only considering red/black tortie and have referred to o as "black". "Orange" is also known as "red" depending on the age of the text you are reading. A "-" means there is no corresponding gene because the sex chromosomes do not form a pair.

Some traits are influenced by multiple genes carried on different loci of different chromosomes. Ginger cats vary from show-quality deep red through to sandy yellow depending on which polygenes are modifying the basic colour. Some genes intensify the colour while others weaken it. All ginger cats have the O (orange) gene, but the different mix of polygenes causes the great variation in shade. Likewise, grey cats range from a dark grey through to much paler versions.

Mating the palest ginger cats together means some offspring may inherit several genes that lighten the overall colour. In exhibition quality red cats, breeders have mated together the deepest coloured cats over many generations to produce richly coloured cats. These cats have the same gene for orange that is found in sandy coloured non-pedigree cats, but they also have more of the polygenes that intensify the colour. Ginger cats vary from deep red through to sandy yellow depending on what polygenes are modifying the basic colour.

Co-dominance, where two genes are equally dominant and the result of having one of each is a halfway point, has already been mentioned. Both genes are expressed (at least partially). The Tonkinese mink colour is intermediate between the Burmese sepia and Siamese colourpoint colour and pattern because the cb and cs genes are neither dominant to, nor recessive to, each other.

In incomplete dominance (partial dominance), a heterozygous individual (one who has both the dominant gene and the recessive gene) also has an appearance intermediate between a homozygous dominant individual and a homozygous recessive individual. The result is a blend of traits. If two individuals with an incompletely dominant trait are bred together, there will be a mix of offspring. Some will have the dominant trait, some will have the recessive and some will have a blend of traits.

Penetrance means the extent to which a gene is expressed. Even a dominant gene may not be fully expressed because of the effects of other genes and of the environment. A highly penetrant gene is almost always expressed regardless of other genes or the environment. A low penetrant gene is only sometimes expressed. Highly penetrant genes are easily noticed and the mode of inheritance (dominant or recessive) more easily worked out. Some genes have age-related penetrance and start showing up as the individual ages.

Classic polydactyly in cats is dominant with incomplete penetrance. Cats with the normal number of toes sometimes produce polydactyl offspring. Barring random mutation, the parents are genetically polydactyl, but other genes or environmental factors affecting embryo development meant they didn't develop the extra toes. In this case, although the trait was suppressed in the parents, it showed up in the offspring. Cases of incomplete can resemble a recessive trait until enough breeding statistics are gathered.

The process of duplicating DNA in a cell can result in errors. These are comparable to spelling errors when copying out a chunk of text. Mutation is where a change to the DNA affects a gene and the change shows up in individuals inheriting that change. Over many generations, mutations accumulate. Mutations in body cells are called somatic mutations. They may show up in the individual in whose body they occur, but they are not passed on to the next generation (for example some skin cancers). Mutations in sperm or egg cells are germ-line mutations. They are passed onto the offspring.

Mutations can affect physical traits such as colour, fur type etc. Some cause devastating damage and the individual dies or is deformed. Others have no apparent effect. Some have a cosmetic effect such as rex (curled) fur. Some have a beneficial effect, better suiting the individual to survival in a changing environment.

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