EYE COLOUR GENETICS

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Eye colour is generally controlled by three factors: base colour, colour intensity and blue tint. Additional complexity is added by the albino genes (sepia/burmese, mink and point/siamese), dominant white and bicolour. Each of these factors is described under the relevant headings below, followed in each case by a brief explanation of my understanding of the 'Real Life' genetics behind them. This 'RL' information has no direct relevance to Pure Felinity (PF), and is included simply for those who are interested. It is italicised to differentiate it from the surrounding PF detail. Bear in mind that it is just my understanding of the genetics, and may need to be altered as scientists learn more about the way feline eye colour genetics operate.

Kittens will always start off with blue eyes, as they do in RL, with the final colour developing at two months.

RL: The genetics controlling feline eye colour are fairly complicated, and geneticists are still not entirely sure how they operate. Each factor is almost certainly polygenetically controlled (multiple genes causing small effects), and there are at least three related factors at work. The feline iris (coloured part of the eye) has two layers, both of which contain pigment cells known as melanocytes. The inner layer is the epithelium, which contains tightly packed melanocytes that actively produce the pigment, melanin. The melanocytes are so tightly packed that this layer is opaque (cannot be seen through). The outer layer is the stroma, which contains far fewer melanocytes, and is therefore transparent. PF eye genetics are a simplified version of the complex relationship between these structures.

Base Colour
There are three base colours: yellow, orange and brown.

Colour groups with a yellow base colour are: yellow, green, yellow-green and aqua
Colour groups with an orange base colour are: orange, hazel, bronze and grey-green
Colour groups with a brown base colour are: brown, coffee, bronze-olive and grey-brown

RL: The number of melanocytes contained in the epithelium dictates this 'base colour'. No cells would give no colour, few cells gives yellow, moderate numbers give orange and many cells give brown.

Colour Intensity
Each base colour comes in three 'intensities', graded as level 1, level 2 and level 3, with level 1 being the lightest, and level 3 being the darkest.

RL: The intensity of the base colour is controlled by how 'active' the melanocytes in the epithelium are, that is, how much pigment they produce. Inactive cells give pale colours, active cells give deeper colours. PF level 1 is equivalent to fairly inactive cells, level 2 to moderately active cells, and level 3 to very active cells.

Blue Tint
The blue tint also comes in various 'intensities', but starts at 0 and goes up to 3, with 0 being no tint, and 3 being a strong tint.

The level of blue tint also affects what depth of colour blue-eyed (e.g. pointed or white, see below) cats will have, with level 1 tint giving pale blue, level 2 tint giving medium blue and level 3 tint giving deep blue.

RL: The blue tint is given by the stroma, which is basically a transparent lens over the top of the epithelium. If you have ever looked at the edge of a piece of glass (in a fish tank, for example), you will know that the glass looks green or blue, even though it looks clear when you see it straight on. This is due to the way that light is reflected and refracted through the glass. The stroma is like a piece of glass, in which the melanocytes do not produce any pigment, but do interfere with the passage of light. If there are few cells, the light can pass through freely, and there is very little blue tint. If there are a lot of cells, the light is 'bounced' around a lot, resulting in a strong blue colour.

When the three factors are combined, different colours will be produced. For example, combining equal amounts of yellow and blue will give varying shades of green (low levels of each giving pale green, high levels of each giving deep green), combining strong yellow with pale blue will give a more yellowy-green, and combining pale yellow with strong blue will give a more bluey-green, or aqua.

RL: The combination of the yellow, orange or brown 'base' colours of different intensities, given by the epithelium, and the overlaid blue 'tint' given by the stroma, is what gives the final eye colour. A cat with very little, or no, epithelium colour but a strong stroma colour will have very intense blue eyes. A cat with an almost colourless stroma, but a lot of very active pigment cells in the epithelium, will have intense amber or even brown eyes. Between these two extremes is a continuous range of colours.

Most wildcats apparently have hazel eyes, caused by a moderate epithelium colour (orange-ish) and a low-moderate stroma tint. Extremes of any colour tend only to occur where this factor has been selectively bred for. Siamese breeders, for example, have worked over many years, to achieve very low levels of epithelium colour, combined with a very high level of stroma tint. Breeders of British and Persian blacks, on the other hand, have worked to achieve very strong, dark epithelium colour combined with very low stroma tinting.

Albinism (Sepia/Burmese, Mink and Point/Siamese)
The albino genes affect the appearance of the eye colour in the same way as they do the coat colour. In the case of the eyes, they limit the amount of base colour showing. Pointed cats will always have blue eyes, because the pointed gene removes all of the base colour. The level of blue tint remains the same, except for cats with no blue tint, which will go to showing level 1 blue tint (pale blue). The mink gene removes two levels of colour intensity, with colour intensity level 1 or 2 going to 0 (blue eyes), and colour intensity level 3 going to 1 (any colour with a colour intensity of level 1). The sepia gene removes just one level of colour intensity, so colour intensity level 1 goes to 0 (blue eyes), colour intensity level 2 goes to 1 (any colour with a colour intensity of level 1), and colour intensity level 3 goes to 2 (any colour with a colour intensity of level 2).

The reduction in colour intensity level does not affect the underlying eye colour genetics, only the appearance of the eyes. This means that a pointed cat with blue eyes cat can be 'masking' dark brown eyes underneath, and any non-pointed kittens it produces could therefore receive those genes from it.

RL: The albino genes inhibit the ability of all pigment-producing cells to do their jobs. This causes the obvious paling of the coat colour, but also affects the eyes. There is essentially a scale of impact, with Full-Colour (non-albino) at one end, and the (very rare) true albino (no pigment) at the other end. Sepia, mink and points sit between these two extremes. In terms of eye colour, the only factor affected is what we are considering to be 'colour intensity' - the amount of pigment produced by the melanocytes in the epithelium. The sepia gene inhibits this production slightly, the mink 'gene' slightly more, and the pointed gene almost totally.

This means that the underlying genetics are, however, unaffected. If two cats with very dark brown eyes are bred together and produce a mink kitten, the kitten will still receive the genes for very dark brown eyes. This means that he must have received highly active epithelium melanocytes, i.e. melanocytes that should produce a lot of pigment, giving a deep base colour. However, he has then received the mink 'gene', which is more powerful than the gene causing the active melanocytes, and stops the melanocytes from doing their job properly. This causes him to have a much paler epithelium colour. Since his genes are for very active melanocytes, though, if he is bred to another cat with very active melanocytes, any Full Colour (non-albino) kittens they produce will also have very active melanocytes.

Dominant White and 'Bicolour' (Odd Eyes and Blue Eyes in Non-Albino Cats)
Dominant (epistatic) white will sometimes cause the same effect as the pointed gene, removing all base colour, resulting in blue eyes. There is no way to distinguish between the blue eyes caused in this way, and blue eyes caused by points underlying the white. Occasionally, the pigment removal will only affect one of the eyes, resulting in an 'odd-eyed' cat, which has one blue eye and one non-blue eye. The non-blue eye will be whatever colour the underlying genetics dictate that the eyes would be if one wasn't blue. Cats with Harlequin and Van will also sometimes have blue eyes or odd eyes.

All of these events occur randomly, and are not caused by a gene that can be bred for, so an odd-eyed cat is no more likely to produce an odd-eyed kitten than any other similarly-patterned cat. Both blue-eyed and odd-eyed cats will breed according to their underlying eye colour genetics, so an odd-eyed cat with one deep orange eye will breed as a deep orange eyed cat.

RL: Blue eyes and odd eyes in white or bicolour cats is caused by the same gene that causes the white or bicolour. These genes inhibit the ability of melanocytes to produce pigment in much the same way as the albino gene. Due to the more random expression of these genes, however, the impact on eye colour is much more difficult to predict than it is with the albino complex. The operation of these genes on eye colour is understood even less than the rest of eye colour genetics, and there does not appear to be any pattern in the production of kittens with blue and odd eyes. Studies appear to suggest that the percentage of blue-eyed and odd-eyed white kittens born (where no underlying points are present) are the same, at about 15-20% each (leaving 60-70% normal-eyed).


Also in this section:
Eye Colour Genetics (current page)
Understanding the Eye Colour Chart
List of Eye Colours and their Components
Calculating Outcomes
Calculation Examples


Go to:
Genetics Help Home
Eye Colour (current section)
Colours
The basics
Overview of the different genes used
Explanations of the genes
How to work out the results of a breeding
Some examples of breeding calculations
Further help