EXPLANATIONS OF THE GENES
Note: If you found this page through any means other than via the Pure Felinity site, you should be aware of the fact that this page was written about cyber cat breeding, not real cats, and that it is tailored to the breeding rules of this site. The genetics of this site are a simplified version of reality, so there may be some principles that do not hold true in real life.
Pure Felinity have no claim to copyright on any of the linked images - all images are copyright of the owner and should not be copied without their prior consent. Pure Felinity are in no way responsible for the content of external sites. If you know of a better image than any of those used, please let me know.
Under each gene is a description of its action/effect on Pure Felinity, and the 'Real Life' genetics behind this gene. Wherever you see 'RL:', this is either information about genetic effects that only apply to 'Real Life', or information included simply for those who are interested. These 'RL' sections have no direct relevance to Pure Felinity (PF), and have been italicised to differentiate them from the surrounding PF detail.
The genes are:
- Hair Length - Shorthair/Longhair
- Hair Type:-
- Cornish or German Rex (Rex1/RexI)
- Devon Rex (Rex2/RexII)
- Selkirk Rex (Rex3/RexIII)
- Tail Type:-
- Manx (Tail-less)
- Bobtail (Short tail)
- Leg Length - Munchkin/Normal
- Ear Type:-
- Hair Colour:-
- Base Colour (Black/Chocolate/Cinnamon)
- Red (Red & Tortie)
- Dilute (Blue/Lilac/Fawn/Cream)
- Agouti (Tabby/Shaded/Chinchilla)
- Base Tabby Pattern (Ticked/Mackerel/Classic)
- Spotted Tabby
- Inhibitor (Silver)
- Wideband (Chinchilla/Silver Shaded/Silver Tabby)
- Rufusing (Rufused/Golden)
- Albino (Sepia/Point/Mink)
- White Spot (Bicolour/Harlequin/Van)
- Dominant White (Solid White)
This gene controls the length of a cat's hair. It does not affect any of the other genes - both long and shorthair cats can be Rex or LaPerm, for example, depending upon what other genes that they have.
RL: In 'real life' the recessive longhair allele 'l' is a mutation of the normal, dominant allele 'L'. Long and shorthair both exist in a variety of length, texture and density, which are not caused by different genes, but by selective breeding. A Persian, for example, has the same gene for hair length as a Turkish Angora, but the Persian coat is much longer and fuller. This is due to selective breeding - a hundred years ago the Persian coat looked like that of the Turkish Angora. There are four different longhair alleles, but these have no apparent external variation.
Rex cats are known for their unique curled fur. There are four different types of 'Rex' cats, including LaPerms, which are all caused by different genes - the Cornish and German Rex (caused by the same gene), the Devon Rex, the Selkirk Rex and the LaPerm. Because these different 'rex types' are caused by different genes, they respond separately. Having a 'non-rex' gene for one does not mean that a cat cannot have a different type of rex coat; nor does having one type of rex mean that a cat cannot have another - it is entirely possible to have a LaPerm rex1 rex2 rex3.
This link has a timeline of the discovery of various rex types.
This link provides more details of the development of curly coats in cats.
You can also visit the website of the Rex Cat Association for more information on the breed histories.
RL: The Cornish/German rex coat falls in 'washboard waves', with curls lying close to the body and very soft to the touch.
RL: The Devon rex coat is very light (making these cats warm to the touch), and varies from a 'shaggy mop' of loose curls to a thin suede-like coat that leaves some areas almost free of hair.
RL: The Selkirk coat is plush and loosely curled, showing up more dramatically on longhaired cats.
RL: LaPerm kittens are born bald and begin to grow soft curly hair at the age of about 8 weeks. This hair can be anything from loose waves to curls that can range from long ringlets to tight corkscrew curls.
RL: Satin coats have virtually no undercoat, are very silky and have a satin-like sheen.
The hairless allele is a recessive mutation of the normal 'H' allele. It causes the cat to appear hairless, as in the case of the Sphynx, but this is not actually the case. These cats simply have very short coats that are therefore virtually unseen.
Because the hairless gene is separate to the other coat-type genes, it can be present at the same time as these other genes. Obviously, though, you cannot tell what 'type' of hair a cat has if it has no hair. This means that a hairless cat can 'mask' a dominant hair type such as Selkirk Rex or LaPerm.
(M/m) Manx (Tail-less)
The manx gene controls whether a cat is born with or without a tail. It is totally separate to the bobtail gene, so a cat can actually be bobtail and manx, but you would only know about the manx because the bob-tail wouldn't be seen. Homozygous cats will be stillborn (dead at birth), so it is always wise to breed a tail-less cat to a normal or bobtail cat.
RL: The tail-less allele is a semi-dominant mutation of the 'm' or normal-tail allele. It is caused by a spinal defect that is harmless in its heterozygous form (Mm). However, if it is homozygous (MM), this defect becomes so severe that the kitten will die before birth. Because it is semi-dominant, different tail lengths can occur, from completely tail-less to a partial tail, depending upon how dominant it is in each case.
The genetic differences between bob-tail and Manx are also unclear, and there is little agreement as to how the two are related (or not).
(Jp/jp) Bobtail (Short tail)
The bobtail gene controls the length of the tail (if there is a tail). It causes the cat to have a short or 'stumpy' tail, that is often kinked (as is seen in the Japanese Bobtail).
RL: In 'real life' it is assumed not to be a version of the manx gene, but this is not entirely clear.
(Mu/mu) Munchkin (Short Legs)
The munchkin gene causes cats to have shorter than normal legs.
RL: It is caused by a genetic mutation that shortens the long bone in the legs, but it generally causes no ill effects in the cat (although some will suffer slight movement difficulties). It is a dominant gene, so only short-legged cats can produce short-legged offspring.
These links look at the history and breeding of short-legged cats:
RL: It is caused by a genetic mutation that seems to have no side effects.
RL: In 'Real life' kittens are born with normal ears and folding occurs from 4 weeks of age, completing by the age of 3 months. It is caused by a genetic mutation that has serious or even fatal side effects if homozygous (FdFd), but in pf it has no such side-effects.
A cat's hair colour is decided by many different genes which all interact in different ways. These are as follows:
(B/b/b1) Base Colour
Black is the normal, dominant, base colour.
Chocolate (dark brown) is a genetic mutation of Black, and is recessive to Black.
Cinnamon (mid brown) is a genetic mutation of Chocolate (brown) and is totally recessive to the other two.
The base colour is the 'genetic' colour of the cat. A Black tabby can look brown, as can a Black (self/solid), or 'Sable' burmese, but both of these are still 'genetically' Black. A solid White or Red cat will also have a 'genetic' colour that is one of these base colours, but the base colour is 'masked' by the White or Red and will only come through if the White or Red is bred out (or if the cat is a Tortie).
RL: Although cats on purefelinity are just 'Black', 'Chocolate' or 'Cinnamon', in 'real life', these genes are believed to be slightly inter-related. Carrying Chocolate may cause a 'rusty' tint in a Black cat, or brownish points in a Blue point cat. A pure Chocolate point cat may have a cold, dark coat, whereas a Chocolate that carries Cinnamon should have a more desirable warmer, lighter-chocolate coat.
(x/xo) Red (Red & Tortie)
Red is known as a 'sex-linked' colour because it is 'carried' on the 'x-chromosome'. The sex of a cat is decided by two chromosomes, the 'x' and the 'y'; a male cat has the genetic combination 'xy', while a female cat will be 'xx'.
The 'x-chromosome' tells the coat whether it is Red, or non-Red. Obviously, a male can therefore have a maximum of one Red 'gene', while a female can either one or two.
The symbol used to denote an 'x-chromosome' that carries the red gene is xo (or sometimes simply O).
A non-Red (Black, Blue, Chocolate etc. but not necessarily white) male is 'xy'.
A Red male is 'xoy'. He is red because he has the chromosome telling his coat to be red. He still has another 'base' colour underneath, but it cannot be seen.
A non-Red (Black, Blue, Chocolate etc. but not necessarily white) female is 'xx'.
A Red female is 'xoxo'. She is red because both her 'x-chromosomes' are telling her coat to be red. She still has another 'base' colour underneath, but it cannot be seen.
If, however, a female receives one Red 'x-chromosome' and one non-Red ('xox'), then her coat receives both messages, so part will become Red, and part will become her 'base' colour. This results in a 'tortie' or 'tortoishell' cat, where the coat is 'blotched' with Black, Blue, Chocolate etc. and Red or Cream.
To understand this better, see the 'Some Examples' section.
RL: Note that it is virtually impossible to get a true, completely 'solid' Red or Cream cat, because the 'non-agouti' allele does not work properly on 'phaeomelanin' (Red pigmentation). Breeders have succeeded in creating 'solid' red cats by selectively breeding for cats with rufusing (which tends to 'wash out' the contrast between the 'marking' and 'ground' colour), and an underlying base pattern of Ticked Tabby, to minimise marking. The 'trademark' tabby 'M' can, however, almost always still be seen on the forehead of 'solid' Red cats, and most show some level of tabby 'ghostmarkings', giving away their Agouti heritage.
(D/d) Dilute (Blue/Lilac/Fawn/Cream)
This gene 'dilutes' the main colour to a different shade and is recessive to non-dilute (or density). If diluted, Black becomes 'Blue', Chocolate becomes 'Lilac', Cinnamon becomes 'Fawn' and Red becomes 'Cream' (Tortie becomes 'Blue Tortie', 'Lilac Tortie' or 'Fawn Tortie', also known as 'Blue-Cream', 'Lilac-Cream' and 'Fawn-Cream'). The dilute gene does not affect the appearance of a white cat.
RL: In 'real life', as with the base colours, the dilute gene is not as simplistic as it appears on purefelinity. A chocolate cat, which should have a warm chocolate colour, can have a cold, dull colour if it carries the dilute gene.
(In 'real life' there is another gene in this set, called the 'dilution modifier'. It is a dominant mutation of the dilute gene, which affects the appearance of dilute coats. It only shows if the cat is homozygous for dilute (i.e. would show dilution if it didn't have the modifier gene). It changes Blue, Lilac and Fawn to Caramel (only slight differences can be seen between the different Caramel forms) and Cream to Apricot. This gene is NOT present on purefelinity, because it is too rare in 'real life' to be worth considering here). To see what these colours look like, see the gallery of the Caramel Apricot Federation.
(A/a) Agouti (Tabby/Shaded/Chinchilla)
The agouti gene controls whether or not a cat is tabby, but also shaded/chinchilla, by causing the hairs to become 'banded' with a 'marking' colour and a 'ground' colour.
The type of agouti patterning shown depends firstly on whether or not the cat also has Inhibitor/Silver. If the cat has Inhibitor/Silver then it will show a pattern based on what combination of the 'Wideband' gene (see below) that it has. If it doesn't have silver, then it will be a Tabby. In this case, each hair has two or three of the 'bands', which group themselves differently depending upon which 'Tabby pattern' (see below) the cat has.
A non-Agouti cat will have no Tabby markings, and is the only form of 'Silvering' (Smoke) to be unaffected by the Wideband gene.
RL: The Agouti allele is dominant and is actually a protein that 'attacks' the melanin (pigment/colour) production, causing the colour production to 'turn on' and 'turn off' periodically as the hair grows, giving bands of 'marking' and 'ground' colour. The non-Agouti allele is a mutation of the 'Agouti' allele, which suppresses the 'attack' on the melanin, allowing the whole hair to be one colour, but the Tabby pattern is still genetically present 'underneath' the sold colour.
(Ta/t/tb) Base Tabby Pattern
All cats, whether Solid or not, have a Tabby pattern, but in Solid or White cats this pattern is hidden by another gene. Like base colour, the Tabby base pattern is a 'multiple allele' gene, meaning that it has three alternative alleles, rather than two. The Tabby 'colour' is given according to the 'marking' colour, rather than the 'ground' colour.
Ticked Tabby is the most dominant, causing each hair to be 'ticked' with bands of both 'marking' colour and 'ground' colour. There is little or no grouping of like-coloured hairs, resulting in the cat being free of 'typical' Tabby markings. Most of the body appears the same colour, although the underside will be lighter and the face and tail will show slight 'Tabby striping' and the lower legs will have no 'marking' colour at all. The most obvious visible difference between a Ticked Tabby and a Solid-coloured cat is that a Ticked Tabby's coat appears 'dusted' or flecked with the ground colour instead of being truly Solid.
In both Mackerel and Classic tabby, like-coloured hairs are gathered together to produce the Tabby markings.
Mackerel Tabby is recessive to Ticked, but dominant to Classic. It causes the coat to become 'striped' with clearly defined, narrow 'pencillings' of 'marking' and 'ground' colour. The legs and tail are ringed with narrow 'bracelets' of colour.
Classic Tabby, also known as 'Blotched' Tabby, is the most recessive, being recessive to both the other two patterns. It causes clearly defined, broad markings in the form of swirls, blotches and 'butterfly wings'. Like Mackerel Tabbies, the tail and legs are ringed with 'bracelets' of colour.
(Sp/sp) Spotted Tabby
The 'Spotted' gene causes the 'marking' colour to appear as spots on the 'ground' colour background. It is hidden by the Ticked Tabby gene, but is dominant to Mackerel and Classic. The reason for this is obvious once you understand how the 'real life' gene operates:
RL: In 'Real life', it is not clear whether the normal Spotted gene is actually a separate gene, or a set of 'polygenes', but it appears to act on the other tabby patterns, 'breaking' the bands of colour. It causes the underlying tabby pattern (Mackerel or Classic) to break up into spots, so it is often possible to tell by looking at the cat which tabby pattern it 'masks'. For this reason, the 'Real life' gene has no effect on ticked tabbies, because they have no set 'pattern' to break up. This is also the reason that Classic is normally preferred as the 'underlying' pattern in Spotted breeds - it gives better distribution of spots, and tends to result in larger, clearer spots.
The following links illustrate the concept of the Spotted Tabby gene 'breaking up' the other patterns:
The spots on this cat clearly show a Mackerel pattern.
Scroll down this page about 2/3 of the way until you get to the Spotted Tabby section - the Blue Spotted Tabby clearly shows its underlying Classic pattern.
There is some evidence to suggest that the Spotted gene seen in Bengals is different to the gene seen in other breeds (presumably coming from its Asian Leopard Cat roots), and this appears to have no link to the underlying pattern, instead producing a random spattering of spots.
(I/i) Inhibitor (Silver)
This gene causes 'Silvering' of the coat in one of four forms: Chinchilla, Silver Shaded, Silver Tabby or Smoke. Which one occurs is controlled by the Agouti and Wideband genes.
In Agouti cats with the 'I' allele, the Silver replaces the 'ground' colour of the Tabby pattern, and the cat will be 'Chinchilla', 'Silver Shaded' or 'Silver Tabby', depending upon which combination of the Wideband (see below) gene they also have.
A Non-Agouti cat with the 'I' allele will always be a 'Smoke', where only the very base (approximately 1/3) of the hair is 'Inhibited'; the rest of the hair having colour as normal. The Wideband gene seems to have no effect on Smokes.
RL: The Inhibitor allele (I) is a dominant mutation of the Normal (i) allele, causing several degrees of 'Silvering'. It works by inhibiting the production of the normal 'melanin' or colour pigments, meaning that bands of Silver (white) occur on the hairs.
A Smoke appears virtually 'full coloured' when sitting but when in motion (or if the hairs are very long, as in the case of the Persian), this silver 'ground' will 'flash' through.
(Wb/wb) Wideband (Chinchilla/Silver Shaded/Silver Tabby)
This gene controls the type of Silvering that will occur in Agouti cats that also have the Inhibitor gene: Chinchilla, Silver Shaded or Silver Tabby.
The Wideband gene only operates on cats that have the Inhibitor (I) gene, and are Agouti; it has no effect on Non-Agouti (Smoke) cats. As explained above, there are three types of Agouti with Silver: Chinchilla, Silver Shaded and Silver Tabby. The Wideband allele works by increasing the width of the band(s) of Silver 'ground' colour on the Tabby coat.
If the dominant Wideband allele is homozygous (WbWb), the cat will be Chinchilla. In this case, the Silver 'bands' are increased in size until they become one band that covers the whole lower part of the hair, leaving only the tip of about 1/8 of the hair, with colour. Chinchillas appear almost white, with only a very slight 'dusting' of colour over the top, resembling the coat of the small rodent by the same name.
If the Wideband gene is heterozygous (Wbwb), the cat will be Silver Shaded. In this case, the Silver 'bands' again become one band, but covering the lower 2/3 of the hair, leaving the tip (the remaining 1/3) with colour as normal. These cats also appear mainly white, but have a more definite 'Shading' of colour.
If there is no Wideband allele (wbwb), the cat will be a Silver Tabby. In this case, the Inhibitor only operates on the 'weakened' areas of melanin (pigment/colour) that have been 'attacked' by the Agouti protein. This means that the 'bands' of the hair that would normally be 'ground' colour are replaced with Silver, resulting in a Tabby appearing as normal, but with Silver instead of the normal tan/beige/ivory etc.
RL: In 'real life', there is much controversy over the presence or absense of a discrete Wideband gene. The most commonly accepted assumption is that the shift from Silver Tabby to Silver Shaded to Chinchilla is actually a continuous scale caused by a series of polygenes, rather than three distinct 'Silvering' types controlled by one gene. It can be purely a matter of opinion whether a cat is a heavily 'tipped' Chinchilla, or a lightly 'tipped' Shaded. Equally, it can be very difficult to tell the difference between a Shaded and a Tabby, particular with the Ticked Tabby, where the only difference is that the Ticked Tabby will have multiple bands of colour down the hair, instead of it being restricted to the tips. Even where the base pattern is Classic Tabby, though, it can be difficult to identify whether the cat is a Silver Shaded with ghostmarkings, or an unclear Silver Classic Tabby.
Unlike purefelinity, the gene, or set of genes, responsible for the Wideband 'effect' in 'real life', also has some effect on Self/Solid and Smoke cats, particularly in Longhairs. This is believed to be responsible for the Smoke/Cameo/Shell in Chinchillas, where the Wideband 'effect' causes the Smoke gene to be very variable in its expression. To a lesser extent, the same is true of Self/Solid cats, in which the Wideband effect can tend to push the pigment towards the tips of the hair, resulting in a paler undercoat. This is known as an 'un-sound' coat, and is a common outcome in Self/Solid offspring of Shaded or Chinchilla parents.
(Rf/rf) Rufusing (Rufused/Golden)
This gene causes a rusty tinge on the coat that is undesirable in most breeds and is therefore penalised. However, it also causes the colour 'Gold' when combined with the Inhibitor/Silver gene, and is not penalised in this case.
As with 'Silver' cats, 'Gold' cats come in 'Gold Smoke', 'Gold Chinchilla', 'Gold Shaded' and 'Gold Tabby', and the genetics are the same, except that the cat must also be homozygous for Rufusing (rfrf). A 'Gold' cat will have a warm golden-cream colour anywhere that a 'Silver' cat has Silver, making it a warmer coat than the Silver, but also making it more difficult to identify the main colour, particularly in the case of Chinchillas.
RL: Like many other genes, whether or not a 'rufusing' gene exists in 'real life' is a subject of a much debate and disagreement. Again, it is likely that 'rufusing' is, infact, a series of polygenes affecting the level of coat pigmentation, rather than a single gene.
In some breeds, 'rufusing' has been actively bred for, in order to strengthen the colour of the undercoat, for example. In Abyssinians, a highly rufused cat is the ideal, since that gives a richer, warmer tone to the colour. It also tends to level out the difference between 'ground' and 'marking' colour, resulting in the appearance of fewer markings; something that is also desired in the Abyssinians. In most breeds, 'rufusing' causes the colour to appear dirty, while on White coats it results in a rusty tinge that is known as 'tarnishing'. Both of these effects would be marked down by a show judge, and cats with this issue are therefore usually neutered and sold as pets.
Unlike purefelinity, where 'rufusing' combined with Inhibitor/Silver causes the desirable Golden colour, in 'real life', it causes tarnishing on Silver coats. This is equally undesirable as it is on White coats. 'Real life' Golden is caused when the Wideband effect/gene is present without the Inhibitor/Silver gene. This is something that isn't possible on purefelinity, because the PF Wideband gene only operates on Silver coats. In 'real life', however, non-Silver cats come in Shaded (Golden Shaded) and Chinchilla (Golden/Golden Chinchilla), just as they come in both Tabby and Silver Tabby. A Golden Persian is simply a Chinchilla Persian without the Inhibitor gene. Tabbies, Shadeds and Goldens have the same issues with it being difficult to tell them apart as Silvers do, and in some breeds the differences can be a constant source of disagreement amoung groups of judges and breeders.
(C/cb/cs) Albino (Sepia/Point/Mink)
This gene causes the coat to have dark 'points' and a light body colour. There are three types of albino colouring: Pointed, Burmese and Mink, and all three are equally recessive.
The normal, dominant allele is 'C' and gives solid, even colour across the body (no darker points). There are two mutated alleles of this gene, causing a lightening of the warm areas (torso) in relation to the cold areas (face/mask, ears, legs and tail), which remain the same colour as in a solid colour cat.
The Sepia 'cb' (Burmese) allele, when homozygous (cbcb) allows the warmer areas to develop colour, but not as dark as that of the points. A genetically black Burmese (Sable Burmese) will have very dark points (basically the same as a Pointed cat), and a warm brown body.
The Pointed 'cs' (Siamese) allele, when homozygous (cscs) only allows the cold areas of the body to develop a colour, leading to an almost white body, with 'normal' colour points. A genetically black Siamese (Seal Point) will have very dark, seal brown points and an almost white body.
If a cat receives one of each of these genes, it is a Mink 'cbcs' (Tonkinese), and is a mixture of the two. The points will be the same colour as a Pointed or Burmese-coloured cat, but the body colour is somewhere in the middle. A genetically black Tonkinese (Natural Mink) will have the same dark points as a pointed cat, but a lighter chocolate-brown body.
Because a Mink has one of each gene, a mating between two Mink cats can produce any of the three (statistically ¼ Pointed, ¼ Burmese and ½ Mink).
RL: In 'real life' pointed cats are always born white (a litter at 2 days old), and the colour begins to develop on the points when the kitten is a few days old (one of the kittens at 3 weeks), continuing until the cat is fully grown (the litter at 10 weeks). The allele remains 'active', however, so if a pointed cat is kept outside in the cold, the 'white' part of its coat will become darker, returning to the original lighter shade when the coat warms up again. Siamese cats can also become 'shaded' with darker colour on their bodies as they get older and they become less efficient at heating themselves. Burmese-coloured kittens are born lighter (although still coloured) and gradually develop their darker colour as they grow. Tonkinese/Mink kittens are somewhere in between, as usual!
The 'real life' Albino series actually extends beyond the Sepia-Mink-Point series into true Albinos with no colour at all. This form of Albinism is extremely rare in cats (and therefore doesn't feature on purefelinity), and is recessive to all other forms. It is sometimes suggested that there are five Albino forms - Sepia, Mink, Point, Blue-eyed Albino and Re-eyed Albino. However, I (Player #7418) believe that the blue/red-eyed variation is simply the difference between different levels of 'Stroma Tint' in the eyes, rather than a further step on the Albino 'scale'. If this is true, Blue-eyed Albinos would be identical to Red-eyed Albinos except that their stromas have different reflective/refractive properties, resulting in the eyes of the former showing up blue.
RL: Non-agouti (Solid) is a 'mutation' of the dominant 'Agouti' allele, which suppresses the 'attacking' of the melanin, enabling the hair shaft to maintain the same colour along its whole length. This gene can work to varying degrees, however, and is not always entirely effective. Where the non-agouti allele fails to work fully, the result is Tabby 'Ghostmarkings' - faint Tabby markings that are often found on immature cats (where they will usually fade with time) but also on mature cats (where they are almost always penalised). Some Smoke kittens will also show Ghostmarkings, but again these usually disappear before adulthood. The main exception to this is the Smoke form of the Egyptian Mau, which has been selectively bred to exhibit fairly strong ghostmarkings throughout its life.
There are very, very few completely 'Solid' Red or Cream cats, because the 'non-agouti' allele is very ineffective on 'phaeomelanin' (Red pigmentation). Breeders have succeeded in creating 'Solid' Red cats by selectively breeding for cats with rufusing (which tends to 'wash out' the contrast between the 'marking' and 'ground' colour), and other similar genes. As a result of this, most 'Solid' red cats show some level of Tabby 'Ghostmarkings' (this Red Burmese is a good example), giving away their Agouti heritage.
If the cat is heterozygous for Van (S1s), heterozygous for White Spot (Ss) or homozygous for White Spot (SS) it will be a Bicolour, and about 1/3 of the coat will be white (usually the feet, nose, chest and stomach).
If the cat has both White Spot and Van (SS1) it will be a Harlequin and over 2/3 of the coat will be white with between one and several spots of colour on the body (remember, though, that it is technically the white that is the 'spot'!).
If the cat is homozygous for Van (S1S1) it will be a Van, where the colour is restricted to the head and tail with the rest of the body being white.
RL: The White Spot allele (S) hampers the migration of 'melanocytes' in the same way as the Dominant White gene (see below). However, this gene only hampers the migration in limited areas of the coat, causing 'white spots'. It is still not entirely understood what causes the difference between Harlequin and Van, although there is considerable evidence for other unknown polygenes, or modifier genes, that affect the 'spotting' caused by the main 'White Spot' gene. On pf, the S1 gene represents some of these polygenes.
The White Spot gene (or genes!) doesnt just affect the migration of the melanocytes in such a way as to cause white areas. It also affects the migration more generally. This is most obviously seen on Tortie & White cats, where the Tortie will be in large patches, rather than mingled fairly evenly throughout the coat, as would be the case in a Tortie without White Spotting. Melanocytes begin their migration around the body at the site of the 'nural crest', or top of the head. From here, they spread out across the body to give it colour. The White Spot gene seems to make these melanocytes lazy, so that they make an incomplete migration. This is why it is common for cats with the White Spot gene to have a white tummy, but a coloured back - you never see a cat with a coloured tummy and a White back!
This gene causes the cat to have white feet, known as 'Mittens or Gloves'. A cat that is homozygous for Mittens (i.e. should show them) but has White Spot, will not 'show' mittens - you never get a 'Black mackerel tabby and white bicolour and white mittens', for example.
RL: It is not known whether this gene is related to the White Spot gene or not, but many suspect that there are unknown polygenes that link the two, and that Mittens may therefore simply be a very minor expression of the White Spot gene. There is also some suggestion (particularly among Birman breeders), that this gene may infact be dominant, but with such variable expression that it will sometimes not show at all. This is hotly contested!
(W/w) Dominant White (Solid White)
This gene is known as 'epistatic', meaning that it is dominant over all other colours, and causes a solid White coat. The 'true' colour of the cat remains 'underneath' the White (known as 'masking'), and will therefore be passed on to the kittens as normal (although White kittens will also mask their 'true' colour). This can result in matings between 'pure' cats producing mixed breeds if the 'masked' colour is incorrect for the breed (for example a White Siamese that hides non-Pointed).
RL: A solid white coat can be caused by any of three factors: true Albino (white coat with red eyes), Complete White Spot (White Spot gene covering the whole coat) or Dominant White. On pf, the only one of these that can occur is Dominant White, so it is the only one that I will look at here. It is a genetic mutation that over-rides all other pigmentation genes, and causes a complete absence of melanin in the hair.
The absence of melanin is caused by the same sort of migratory 'laziness' introduced by the White Spot gene (see above). Unfortunately, in both cases, this 'laziness' affects other types of cells as well as the melanocytes. When the hearing cells fail to migrate properly, this causes deafness in one, or both ears, and Dominant White and White Spot cats are threfore more prone to deafness than other colours. Doubling up on these genes seems to increase the risk, so a 'real life' White cat should never be bred with one exhibiting White Spot.
Genetics Help Home
Overview of the different genes used
Explanations of the genes (current page)
How to work out the results of a breeding
Some examples of breeding calculations