Grey Locus or Progressive Greying Locus ( Exclusion of Pigment from Hair)  – G  & g
Author – Mary McBryde 2016

We all know how human hair turns grey and eventually white with age. Horses show a similar phenomenon of hair silvering with age but it starts at, or soon after, birth and continues until all the pigment (colour) in the body hair, mane and tail is gone, ie. the horse is snow white. This loss of hair colour is caused by the dominant Greying allele G. The speed of its effects on colour loss varies considerably, but in my experience with the PRE which is dominated by the Greying gene, horses that have 2 Grey genes will usually go white faster than horses with only a single copy of the Grey gene.

All foals which are Gg or GG are born their correct genetic Base Coat Colour, but the colouration will be much more strongly pronounced than in Non-Grey foals. So a non-greying Bay foal will have a light red body, black mane and tail, and pale cream legs with perhaps some black hairs on the joints if it is going to be a very dark bay as an adult ( the black legs appear on the moulting of the foal coat). But a Bay foal + Greying will be born with a much darker red body, black mane and tail, and very often completely black legs. The same occurs in Black Base Coat foals which, if non-greying, are born a light bluish mouse fawn with black mane and tail and pale silver-cream legs, whereas Black + Greying foals will be born much darker, almost black.  Foals which are GG are often born with the effect of silver panda rings around their eyes, where there are already a mass of white hairs mixed with the coloured ones on the upper and lower eyelids and surrounding skin. Foals with Gg tend to be slower to indicate that they are going to go Grey , but one guaranteed sign of the Greying gene is the presence of even a single white hair on the upper eyelid. So far I haven’t met a PRE foal with one or two white hairs on the upper eyelids at a couple of days old that did not eventually go Grey.

It is very important to note down the birth colour of Greying foals as it can be difficult to remember later on exactly what colour they were; even better – keep a photographic record. One observation is that Bay horses with lots black hairs over the upper body (sooty countershading – see next issue) + Greying  will lose all their red pigmented hair colour long before they lose the black pigmented hairs, so that a sooty bay foal may look like a Black going grey by the time it is 2 years old or earlier (very confusing hence the importance of good record keeping). Only clear red bays become that lovely colour of Rose Grey with an almost pink look  for a period of time.

Dappling was once thought to be the perogative of Greying horses but current thinking is that Dappling is caused by a separate Gene – we have all seen dappled bays, and dappled buckskins, even dappled blacks. However the Greying process emphasizes the Dappling effect and produces those beautiful dark circles with light centres on the body for a period of time, but of course these eventually are lost as the Greying process continues towards snow white.

The Greying allele also seems to have some ability to force the expression of duntype primitive markings in some sooty bay and buckskin foals, but  these markings disappear by the time the foal has shed out its babycoat.

It is important to remember that a Greying horse MUST have one Greying parent – the G allele is a dominant and CANNOT be hidden. So two non-grey horses cannot produce a Greying foal no matter how many Greys there are in the pedigree !!  Equally true is that a Greying horse will always produce at least 50% Grey foals  (and 50% non-grey) if it has ONE Grey allele, but if it has TWO Grey alleles it will produce 100% Greying foals.

Note: The Greying gene functions by preventing pigment granules from entering the hair as it grows, it does not stop pigment from being manufactured by the melanocytes. The pigment produced is pushed into the surrounding skin instead, and this makes the skin of a Greying horse darker and darker over time, it also makes their skin thicker and tougher which is why Greying horses are so popular in hot climates where people work their horses very hard. The negative side to this process is that it makes Greying horses more susceptible to melanoma skin cancers, and this is something to always look out for in old Grey PRE. Fortunately equine melanomas are often benign not malignant, and horses with melanomas can live out a normal lifespan.

The publication of the Grey Gene Genetics paper in mid-2008 paved the way for the development of a coat colour test for the dominant Grey allele. The DNA test was created by ‘Animal Genetics’ and in my capacity as the Studbook registrar I worked with Animal Genetics’ UK office to supply them with hair-root samples from various grey PRE horses in the UK, also supplying a breeding history and pedigrees to give an indication of whether the horses were likely to be homozygous or heterozygous for the ‘G‘ allele. The first horse to be confirmed Homozygous for ‘G‘ was the PRE stallion Rioviejo (owned by Sara Pell at that time). Since then the test has become available through other Genetics laboratories too, including VGL in California, and our own PRE specialist lab NBT in Seville, Spain. The test for the Grey allele is a routine component of the ANCCE-LGPRE Service 243 Genetic Coat Colour test suite.

Fleabitten Greys – Now that it is possible to test for homozygous or heterozygous status with ‘G‘, it has become apparent that Flea-bitten Greys seem to be universally ‘Gg‘ – heterozygous, and the horses that go pure white are ‘GG’. However the mechanism for acquiring the ‘Flea-bites’ is unknown at present.

Abstract of Genetics paper published Nature Genetics 2008-06-20
Genetics Of White Horses Unraveled: One Mutation Makes Ordinary Horses Turn Grey, Then White, Very Young

SHADES OF GREY IN PRE HORSES
The genotypes of the horses pictured below are given where known