Genes To, O, Sb and others

White patching refers to a horse whose coat consists of clearly demarcated white patches between patches of colour, frequently referred to as pinto horses (from the Spanish expression for painted). The white patches are larger than those considered white markings and extend to other parts of the body. 

The white patches are a result of incomplete migration of melanocytes during embryonic development. As the pigment melanin in hair and skin can only be produced by melanocytes, areas without melanocytes have pink skin and white (unpigmented) hair. If the depigmentation affects the eyes, they are blue.

Genes To, O, Sb and others

Pinto colouring is a result of genetic factors; there are several genes involved and depending on the gene, the pattern of white patches may vary. The exact size and dimensions of any white patches is entirely random, not genetically defined and is therefore highly individual. Pinto genes can be combined with any basic colour and any subsequent genetic modification, therefore not only bay, black and chestnut with white patches is possible, but also for instance dun, grey, palomino or even cream with white patches are possible. Grey or cream with white patches are hard to identify – in greys with a pinto factor the colour of the skin remains a giveaway; the skin of the grey patches is dark; that of the white patches is pink.

Colour terms

The terminology used to describe horses with white patches depends on which part of the world the horse is from:

• Piebald: a black horse with white patches as it is known in the UK.
• Skewbald: a horse of any other colour with white patches as it is known in the UK. 
• Coloured horse: a horse of any colour with patches or spots, occasionally also used to refer to any horse not of a basic colour or grey (e.g. palomino) in the UK.
• Pinto: in the US, the basic colour is described and the expression pinto added, e.g. Buckskin pinto.
• Paint: a horse with white patches in any basic colour that must originate from a Quarter horse.

Tobiano factor (To)

The tobiano factor is a dominantly inherited gene that creates white patches with a more or less vertical alignment. The limbs are predominantly white and the white patches usually extend across the back at least once. The head is frequently coloured with white markings of varying size. The horse may have blue eyes, but less frequently so than with other pinto factors.

Overo factor (O)

There are thought to be multiple genes which can result in the typical overo appearance of highly irregular (puzzle-like) patches of white that extend in a more horizontal orientation along the side and the underside of the horse on any basic colour. Limbs can be dark or white depending on type, and the head is often white with blue eyes.There are several typical patterns.

Frame overo

The white patches are mainly on the side of body and neck with the basic colour on the legs and along the back. The gene responsible for this type of overo is associated with lethal white syndrome (LWS) in which homozygous foals (foals with two overo genes) are white all over and have a bowel disorder leading to death within 72 hours of birth.

Splashed overo

The white patches are predominantly on the legs, belly, underside of neck and head (the horse looks like it had been dipped in white). The patches are clearly demarcated, any basic colour is possible. The gene responsible is not associated with the lethal white syndrome but can be associated with deafness.

Sabino/sabino overo

This type of patching is typically caused by the sabino gene which was originally classified as a part of the overo complex as it can look similar to other pinto genes such as splashed overo. The white patches extend up the limbs to elbow / stifle, the horse often has belly marks and broad white markings on the face. The most typical feature is extensive roaning (mixing in of white hair) of the rest of the body, particularly in proximity to the patches. Horses with two sabino genes can be completely white with just a small patch of colour on the poll region. A very similar type of patching is also created by other, unknown genes, for instance in the Shire and Clydesdale heavy draft horses.

Sabino/sabino overo

With overo genetics, horses with one overo gene may only have minimal white patches (sometimes no more than "normal" white markings), whereas horses with two overo genes generally have large amounts of white, or may be entirely white (LWS). In horses with tobiano genetics, the extent of patching does not depend on the number of genes – both one and two genes can have large or small patches of white. As the genetics of overo and tobiano are completely different, a horse may have both genes, in which case features of both factors may be visible.