Colour Coding!

by Amanda Bliss

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It’s that time of year again. Yes, the time when breeders are biding their time between stud seasons, doing their best to hide from the weather, and waiting (and waiting…and waiting…) for the pitter-patter of little hooves to begin. This downtime is often put to use daydreaming about what that little foal is going to look like when it finally arrives. With foetal sexing available via ultrasound, a few breeders already have a good idea of what to expect in the gender department, but colour is often a bit of a mystery. It needn’t be confusing, however, and often with only a little bit of detective work, breeders can figure out the chances of obtaining a specific colour or coat pattern.
For those unfamiliar with basic genetic concepts, a few definitions follow this article to aid in understanding, as well as links to online resources and genetic testing, and images of horses with their accompanying colour genotypes.

Decoding the Coat

Extension—the black & red basics!
Genetically speaking, each horse has a base coat colour of black or red built on very limited number of genetic options on the extension gene. This is often referred to as the “e-series”, and the genes are denoted by either E for black-based horses, or a red-based e. Every horse has two alleles at this location, which means it will have room for two of these “e-series” genes, one from each parent. The black E is dominant to the red e, so a horse can have two copies of E or one E and one e, and have the same phenotype—black-based, EE or Ee. A chestnut horse always has two copies of the red gene—ee. Stallions advertised as being homozygous black will only produce foals with a black or bay base-coat, never chestnuts, though this base colour may be modified by other genes such as grey.

Agouti—to bay, or not to bay
The agouti modifier takes a black-based horse, and restricts the black pigment to the points—legs, mane and tail. Bay horses are always black-based (EE or Ee), with an agouti modifier. This modifier is dominant, so if a foal receives it from one parent, it will always appear bay. The agouti modifier is usually noted as A. The genotype of a bay horse is always either AA or Aa. A black-based horse without the agouti modifier (aa) will be black. However, a chestnut horse could have any A-series genotype, because it has no black pigment to restrict. It is impossible to make an assumption about whether a chestnut horse will pass on an agouti gene simply by looking at the horse—you will need to either have a genetic test performed for the agouti modifier, or examine the horse’s parentage.

Tobiano—see a pattern?
Tobiano seems to be the most common white spotting pattern which breeders aim for in the U.K. It is a dominant coat pattern which is expressed on top of the base coat, and is indicated here as T. A horse which appears to be tobiano will have the genotype TT or Tt, and a solid horse will be tt. Stallions advertised as being homozygous tobiano are TT and will always pass on the tobiano pattern to their offspring. Heterozygous tobiano stallions will pass on the pattern 50% of the time when bred to solid mares.

Cream Dilution—the gold standard
The cream dilution is also a dominant modifier, however it differs in that you are able to tell from the phenotype whether a horse is homozygous or heterozygous for cream, or CR. The cream dilution dilutes any red pigment in its heterozygous form, has minimal effect on black pigment. The homozygous form has a dilution effect on all pigment. A heterozygous (CRcr) horse will be palomino (chestnut), buckskin (bay), or smoky black (black). A homozygous cream horse (CRCR) will be cremello (chestnut), perlino (bay), or smoky cream (black), and differences may be very difficult to detect.

Grey—the un-colour
Grey is not technically a colour, it is another modifier which progressively strips the pigment from a horse’s coat, and will eventually hide the horse’s original colour phenotype. Like agouti, the grey modifier is dominant, so if your foal receives a copy from at least one parent, it can’t hide, and the foal will go grey. Unlike agouti, grey has an effect on all base colours, so it cannot hide behind a red coat! The grey modifier allele is denoted G, and a horse which appears grey will be either GG or Gg. A non-grey horse is always gg.

These are just some of the more common colours. Genetic tests are also available for other modifiers and patterns, including sabino, silver, champagne, pearl, frame overo, dun (not to be confused with buckskin—the cream dilution of bay) and roan. Some of these are multigenic, or a result of different genes acting together, however some laboratories have developed tests for the most commonly involved gene, or other genetic markers. A new test is being developed for black and tan, which may cause some to revise the genotypes they originally assumed their horses had.

Applying the code

So how do you apply all of this genetic information to figure out the odds of your foal being a specific colour? The simplest way is using an online colour calculator, but these work best if you understand what they are calculating. That’s where the Punnett Square comes in! The following example shows the mating of two black-based horses, each of which has a chestnut parent—meaning they are both heterozygous, or Ee. The genotype of each parent is placed outside of the box, and where they intersect, a potential offspring genotype is the result.

In this instance, there are three possible genotypes which could result, EE, Ee, or ee, but only two phenotypes—black or red. There is a 75% chance of the offspring being black-based, and 25% chance of red (chestnut). By genotype, there is a 25% chance of EE (homozygous black), 50% chance of Ee (heterozygous black), and 25% chance of ee (homozygous red). This test can be performed with each “chunk” of colour that you wish to know about, and then assembled together for a full phenotype, stringing together the information for each gene.

As a second example, we can check the chances of getting a homozygous tobiano foal. The sire has been tested as homozygous tobiano, and we know that the dam is heterozygous, because she has one solid parent, so the cross can be denoted TT x Tt.

For this cross, 100% of the foals will exhibit the tobiano spotting pattern, however 50% will be homozygous (TT) and 50% will be heterozygous (Tt). In order to determine which genotype the foal has inherited, genetic testing will be required.

Hopefully, this article has taken a little bit of the mystery out of coat colour genetics, and will prompt readers to put pen to paper to calculate odds and learn more about colours. Of course, as fascinating as coat colour can be, one should not forget the old adage which dictates that a good horse is never a bad colour!

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