The Extension is the most basic part of every horse’s genome; it is the foundation to every horse’s colour.  One thing every horse has in common is that they all have mutations at the Extension, from there, horse colours branch out dramatically with dilutes, modifiers, white hair patterns, pinto genes and the LP complex. The Extension locus results in two different colours, black and red. Specifically with the Extension the horse is either red or black, there are no empty sections in the Extension locus. In horses and other animals there are coloured pigments in the hairs that result in the colour we see which line up easily with the Extensions. The pigment responsible for the reds, tans, yellows, etc, is called pheomelanin. The pigment that is responsible in black, blues and rarely a bronze colour is called eumelanin.

Red, i.e.Chestnut

Chestnut is the most basic horse colour of all in my opinion. The red Extension allele is recessive to the black Extension allele; therefore for a horse to be Chestnut it is homozygous at the Extension. A Chestnut horse only has the pheomelanin pigment and will range largely in shades from a yellow to copper to chocolate. This large range of shades has led to great debates, and different names, for what has so far been shown as genetically the same colour. Most notably are people and organizations referring to different shades of Chestnut as Sorrel. This often confuses people into thinking Sorrel and Chestnut are two different colours. Due to only having pheomelanin pigment the horse is entirely even of red pigment over it’s coat, this includes it’s mane and tail (although the mane and tail may be a  different shade then the body). Genetic testing for the red factor (i.e. Chestnut allele) has shown Chestnuts present in domesticated horses as far ago as 3000 BC. A Chestnut horse with no other genes will have black skin and dark brown eyes.

Go to the Chestnut horse album to view pictures of Chestnut horses: 


Black horses consist of only the eumelanin pigment and are therefore even in colour across the entire body (the mane and tail may be a different shade which can be caused by things such as sun bleaching, etc). There are technically no shades to Black; however, many people chose to debate this by breaking it down into many shades such as Jet Black, Raven Black, etc. A better way defining Black horses is into two categories: Fading Black and Non-fading Black. Fading Black horses are horses that are sun bleached. Sun bleaching is where the rays of the sun lighten the horse’s coat. On Black horses the coats turn a tarnished copper in extreme sun bleaching situations. Some horses bleach easier then others and the reason of that is currently unknown.  The black allele is dominant to the red allele on the Extension locus. This means that a Black horse can have two different genomes despite looking the same visually. Homozygous Black will always produce Black offspring where as heterozygous Black when bred under the right conditions can and will produce Chestnut. Black horses are more complicated then just the extension gene; they need the correct alleles with the Agouti locus as well, read the Agouti page for in depth information about this. With genetic testing wild Black horses have been tested to exist as far back 5500 to 4950 BC. A Black horse without any other genes will have black skin and dark brown eyes.

Go to the Black horse album to see pictures of Black horses:

Genetic Testing for the Extensions

The Extension locus is called MC1R. There is genetic testing available to the public for the Extension mutations, determining if horses carries the Red and/or Black alleles. When writing out a genetic code, or genome, of a horse the horse’s Extension is represented with an E or e. Black being the dominant allele it is the upper case E where as Red (Chestnut) being the recessive allele is the lower case e. A Chestnut horse’s genome would be written ee. A homozygous Black horse’s genome would be written EE(and aa as to the Agouti gene) and the herterozygous Black horse’s genome would be written as Ee(aa). However, not all horses that you look at are genetically tested therefore one method used when in this situation is when writing out a genome to an assumed colour the dominant gene is written followed by a – which shows the second allele is unimportant. Therefore, when I walk out into a field and see a random Black horse and I chose to write it’s genome it would be written as E-(aa). As the Chestnut horse is recessive we know that it has two copies of the Red allele therefore there is no guessing to what the second spot in the locus is carrying. This method is also used when creating imaginary genomes when the second allele is unimportant to the dominant allele.