Color Genetics in Cats
When you take away all the fancy modifications/additions/dilutions/ect of cat genetics, there are only 2 genes for color. That’s right – 2 – nothing more and certainly nothing less.
What are those 2 colors? Black or Red.
Inside of every cell in every living creature, there is a substance called DNA. DNA is basically the blueprint by which all parts and bits of an organism is made. The DNA for every organism is different – just as each human has unique fingerprints, however, there are parts of the DNA “blueprint” that are shared.
DNA is incredibly long and complex, and it is broken into smaller pieces within the cells of animals in order to be more functional. Those pieces are called “Chromosomes.” There are two copies of the DNA in each cell. There is only one except to this rule, that is the pairing of the “sex” chromosomes. That pairing is either XX for females of XY for males.
Each parent donates one copy of DNA to the offspring. Since females ONLY have X chromosomes, they can only pass on X chromosomes. The males can either pass on X or Y, so it is the father that determines the sex of the offspring. There are other chromosomes other than X and Y, but they are identical in both male and female.
The location of the gene for black or red is on the X chromosome. Since male cats only have one copy of the X chromosome, they can either be black or red – but not both. Females, on the other hand, can have two copies of black, two copies of red OR one copy of each – which is the foundation for what one might call the “3rd” Basic color – Tortoiseshell.
DNA has two copies of each gene within the cell. Sometimes, both copies of the gene are exactly the same. However, when they are not, often times one gene is capable of suppressing the other and stopping it from “working.” A gene that suppresses another gene is called a “Dominant” gene. A gene that is suppressed is called “Recessive.” In order for a recessive gene to be “seen,” it must be present on both copies of the DNA (that is, all recessive, no dominant gene present)
In the case of Black and Red, it happens that they are both Dominant, so in female cats, you see something called “co-dominance” when they have a copy of each gene (some of the cat is black, some of the cat is red.)
Another pair of terms that are linked together are “Homozygous” and “Heterozygous.” “Homozygous” means – both copies of the gene are the same. Heterozygous means that the two copies are different.
How you can go from black, red, or tortoiseshell to all the varied colors we see in British cats? This can be most simply explained by “modifier genes.”
There are genes on other chromosomes – not the X or Y-chromosomes – that can change or modify the black or red color.
4. Pointed (Himalayan)
None of these genes are located on the X or Y-chromosomes, so they are not linked to the sex (gender) of the cat. So males and females both have two copies of each “modifying” color gene.
A modifier gene is recessive, it means there must be TWO copies of the gene for it to actually take effect. That means both parents must have the gene, or carry it. IE: you cannot get a pointed kitten out of a parent that doesn’t carry colorpoint.
Dilute: The dilute gene is a recessive gene. So if at the “dilute” loci, there is a copy of the “dilute” gene on both copies of the DNA, a black cat becomes blue, a red cat becomes cream, and a tortoiseshell becomes a blue-cream. If there is only ONE copy of the dilute gene, the color of the cat DOES NOT CHANGE, but the cat can be called a “dilute carrier,” and that cat can produce dilute offspring IF bred to a another dilute cat or dilute carrier. If a dilute cat is bred to another dilute cat, ALL the offspring will be dilute.
Tabby: Tabby is a dominant gene, so a cat only needs one copy of this gene in order to “show” tabby. With the tabby gene, a black cat becomes a brown tabby, a red cat becomes a red tabby and a tortie becomes a brown-patched tabby. If a cat has two copies of the Tabby gene (homozygous tabby), then ALL offspring will have at least one copy of the Tabby gene, therefore, all offspring will be tabbies of some sort.
Bicolor: Bicolor is also a dominant gene, so again; only one copy of this gene is needed in order to be expressed. With the bicolor gene, a black cat becomes black & white, a red cat becomes red & white, and a tortoiseshell becomes a calico. If the cat has two copies of the bicolor gene, often the cat is mostly white, otherwise known as a “Van Bicolor.” These cats will always have all bicolor offspring, no matter what they are bred to.
Pointed: The pointed (or Himalayan) gene is recessive, so a cat must have 2 copies of this gene in order to be pointed. Two copies of the pointed gene will make a black cat become a seal point, a red cat will become a flame point, and a tortoiseshell becomes a tortie point. If a pointed cat is bred to another pointed cat, all the offspring will be pointed.
Chocolate: Chocolate is a very special gene that can only modify the Black gene. It has no effect on the red gene. It is also a recessive gene. It must be present on both copies of the DNA in order to be visible. Two copies of the chocolate gene will make a black cat become a chocolate, a red cat will not change color – it will still be red – and a tortoiseshell cat becomes a chocolate tortie (the black parts are now chocolate, but the red is not changed). The chocolate gene, when it interacts with the dilute gene, creates “Lilac.” It is important to note that a red cat can have two copies of the chocolate gene – these cats are referred to as “Red chocolates.” That simply means that all their offspring will carry the chocolate gene, or show it if they are bred to another chocolate carrier or visual chocolate.
White: White is a very special category unto itself. In a way, in the world of Dominant genes, the white gene is the ultimate king of the hill. It trumps all other genes for color, no matter if they are dominant or recessive. So with the white gene, the black, red, and tortoiseshell cats all become the same color – white. If a cat has one copy of the white gene, it will be white, and about 50% of its offspring, when bred to non-whites, will be white. A cat that has two copies of the white gene is referred to as a “homozygous white” and 100% of its offspring will be white – regardless of what color it is bred to. The white is a “masking” gene, meaning a cat can have a color in its DNA, but that color is suppressed (the same way a normal dominant gene suppresses a recessive.) White isn’t really a color of cat, since it’s really just the result of the suppression of color.
Shaded/Smoke/Shell: Smoke is a dominant gene, so it cannot be "carried" in the DNA or hidden from sight (unless the cat has a copy of the White gene.) One copy of the smoke gene will turn a black cat into a black smoke, a red cat into a red smoke, and tortoiseshell into a tortie-smoke. A cat with two copies of the smoke gene will always produce smoke offspring. The smoke gene is one that can cause some confusion due to a concept called "variable penetrance." Basically, that means that some cats with the smoke gene have a mostly colored hair shaft with a white undercoat. Some of them will be about 50/50 on the hair shaft in respect to color/white. These cats are called "shaded." Finally, some cats will have a mostly white hair shaft with color just on the tip - these are called "shell," and they tend to be very rare. Additionally, when a red cat is "shaded" or "shell" it is called a "cameo" or "shell cameo." The dilute version of this, cream shaded or cream shell, is called "cream cameo" and "cream shell cameo."
Silver/Golden: Of all the colors or divisions of British, the Silver and Golden is probably the least understood by those who do not work with these colors – and perhaps even by those who do! These unique and beautiful colors are a combination of genes working together to produce an overall effect. The first gene is actually the “Tabby” gene, which is often called “agouti.” The 2nd gene is called the “inhibitor” gene, which is actually the same gene as the smoke gene. The final gene, or set or genes, actually, is what sets the silvers and goldens apart and makes them so unique. These are termed “wide banded polygenes.” All three of these genes are dominant, so they cannot be carried or hidden unless the cat also has a copy of the White gene. As with the chocolate gene, the polygenes do not have any effect on the red color (other than eye color, and this isn't quite understood). The polygene can have “variable penetrance” just as smoke gene can, which is what causes the range of silver colors from shaded silver (which has a fairly heavy layer of black, easily seen) to the chinchilla silvers (which have just a dusting of black, they appear to be a white cat with just a dusting of color.) A golden is produced by a cat that has a Tabby gene and the polygene, but no inhibitor (or smoke) gene.