The mud-loving steeplechaser Russe Blanc (pictured), winner of the Betfred Classic Chase at Warwick in January, is unusual because he is officially registered as a “white” thoroughbred.

But the very basic first Pony Club lesson tells us that there is no such thing as a white horse. White horses are grey, aren’t they?

One of the most spectacular advances in equine science in the past 20 years has been in the field of genomics. We now know about inherited disorders and genetic aberrations that give rise to specific diseases.

We understand how domesticated horses are related to their original prehistoric ancestor and to the other six surviving species in the genus Equus, animals such as asses and zebras.

Scientists have also unravelled some of the genetic basis for horse coat colour, identifying specific genes controlling some of factors that determine the colour of skin and hair.

Most of the genetic material is found within the nucleus of each cell where the horse has 32 pairs of chromosomes, which are paired chains of DNA molecules. A “gene” is a colloquial term for a short segment or zone on a chromosome that determines a specific characteristic or function in the animal.

Each gene consists of two parts. These may be the same or they may be different, and one may override the other in its effect on the horse.

Going grey

We know that the base colour of a horse’s coat is controlled by one pair of genes, the “extension” or E genes, which can be either black or red/brown.

If a horse inherits two black E genes, his skin will be very dark and his coat dark brown or black. If he inherits two red/brown E genes, he will have brown skin and a lighter red/brown coat. The two extremes are a true black or dark bay, and a chestnut.

But a second gene, the A gene, also affects the way in which the base colour is distributed. This gene tends to push the black colour to the points of the horse — the ears and legs, the mane and the tail. So, if a horse has black and red E genes plus the A gene, he will turn out to be bay.

The gene for grey is different because it does not affect the inherited colour of the horse. Greys are born as bays or chestnuts, or even blacks, but the grey gene causes premature whitening of the hair. Exactly the same process occurs in people, who go grey and white with age, but grey horses have a gene that speeds this up considerably.

Coat colour is further complicated by at least 10 other genes called the “dilution” genes and white-spotting and depigmentation genes. One of these is the cream or cremello (C) gene, which gives rise to pale, pink skin and blue or pink eyes. Another is the white (W) gene. This is unique and gives rise to a variety of spotty or blotchy skin patterns and pale hair colours.

While horses of some breeds with double white genes have serious health issues, this is not the case with thoroughbreds.

A one-off wonder

So what about Russe Blanc?

If you look carefully at photographs of him, you’ll see that he is not a grey because his skin is not dark. When he is wet you can see through his coat to his skin, which is covered in small blotches of pigment.

His colour is very unusual in thoroughbreds. It would be possible to analyse his genetic make-up to see what coat colour genes he carries and whether he has a unique mutation. He may have a mutation of the W gene, or he may carry a novel gene within the white spotting and depigmentation category.

Whatever, he is a very good horse. As he is a gelding, however, he won’t be passing on his colour genes to any progeny.

Ref: Horse & Hound; 3 March 2016