The Extension Locus – E  & e
The original wild type gene for horses is labelled ‘E’. It is responsible for the normal production (or extension) of black pigment (eumelanin) and red pigment (phaeomelanin) throughout the coat. The exact distribution of the black and red pigments across the body hair and points (mane, tail, and lower legs) is controlled by the Agouti locus. The most commonly seen colour pattern being Bay, which is a stronger pigmented (more intensely coloured) version of ancestral wild-type colouration in horses. The ‘E’ allele is dominant so it will always control pigment production whether the horse has one copy or two copies of the E allele.

The alternative allele at this locus is called ‘e’. It is a recessive mutation which occurred at least 6 or 7,000 years ago in domesticated horses when the original gene ‘E’  was mis-copied to become ‘e’ . The ‘e’ allele prevents the formation of any black pigment in the horse’s coat hair, but allows pigment to develop normally in the skin and eyes. Horses with a pair of  recessive ‘e/e’ alleles will have an all red coat and points which is often referred to as a (Chestnut Base Coat). The horse which is ‘e/e’ will be red all over regardless of what pigment pattern the Agouti locus genes are programmed to produce, so both Bay and Black horses that are ‘e/e’ will look identically RED ALL OVER.

Two very Dark Chestnut PRE Mares

The opposite end of the range, a Light Chestnut PRE Mare

Chestnuts can be found in a variety of shades of red ranging from dark liver chestnut to light orange chestnut. Manes and tails may be much lighter than the body hair due to an unidentified gene known as ‘Flaxen’, or may be much darker than the body looking almost black (but not black), or the same colour as the body. These pigment variations of red cannot yet be explained by simple genetic schemes although some group of researchers somewhere are probably trying to do so right now.

Chestnut with very Dark mane & tail – PRE Filly

When two red horses are bred together (e/e x e/e), the offspring are also red (e/e). If the offspring has black pigment E or is grey G then the parentage as given is most likely incorrect and needs checking!

GENETIC TEST:- The ‘e’ allele can now be identified in a horse via a genetic test using a hair sample. This allows you to determine whether a horse has no copy, one copy, or 2 copies.

Genetic Laboratories that provide Coat Colour DNA analysis (NB:-other Laboratories are available around the world)

1.The ANCCE-LGPRE Genetics Laboratory (NBT, Seville) will test for the Extension Locus alleles at the same time as they test for 6 other colour genes if you apply to the BAPSH Registry Office for LGPRE Service 243 using the ANCCE International Service Application form. Find the current Fee for this Service on the Registry Service Fees page.

2. Animal Genetics UK, based in Cornwall, UK and Florida, USA – www.animalgenetics.eu/equine

3. UC Davis Veterinary Genetics Laboratory, California, USA – www.vgl.ucdavis.edu/services/horse

Note that some of the Genetics Laboratories may provide Colour Gene Test results which refer to ‘e’ as Red Factor and ‘E’ as Black Factor, which gives the rather misleading impression that horses with the ‘E’ allele are all solid black in coat colour whereas most of them are Bay in colour with red and black pigment. A better term for ‘E’ would be Black/Red Factor to indicate that both pigments can be manufactured via this dominant allele.