Chetar Arabians ~ EQUINE COLOUR GENETICS ~ White Patterns Introduction ~

As the names suggest, the White Roaning Pattern Genes manifest in a pattern of white hairs interspersed with the dark/base coloured hairs of the horse - The colour genes falling under this catergory are the Rabicano Gene [also known as Arab Roan in some parts of the world] and the Roan Gene [also known as True Roan in various localities].

Rabicano and Roan are differentiated by how extensive the interplay of white and background coloured hairs, as well as the eveness of the resulting pattern of white manifested by the genes. There is [usually] some very distinct differences between these two patterns that enables correct identification.

The Appaloosa Complex is a completely separate, highly distinguishable white spotting pattern.
Appaloosa results in various patterns of small, generally coin sized spots of solid coloured spots superimposed upon a white background area of various size. There are multiple subpatterns that fall into this catergory, though they are all the result of one 'complex' series of genetic mechanisms.

For one or more Overo Genes and the Tobiano Gene to be expressed together is much more common than many people realize.
How often do you see a Tobiano body pattern in concert with white facial markings? That horse has more than one white spotting pattern gene at work - A True Tobiano will have a completely solid coloured head with no facial white whatsoever, even if the body from the throatlatch back is completely white, the head will remain dark! So be attentive when trying to identify white spotting patterns, and make sure you know what to look for - There's reasons that I've included the Primary and Secondary Coat Markers for each gene - and this is one of them! Use the information I've provided.

The fourth white spotting pattern gene, Tobiano, is essentially the "opposite" to the Overo Genes.
Tobiano causes distinctly different white marking manifestations than that of the Overo Genes.
Tobiano is the one and only white pattern gene that does not involve facial white, with a completely dark/solid coloured head being a unique attibute of this gene. Tobiano also results in white markings superimposed upon a base/modified base colour that [almost always] cross the topline between the wither and the tail/dock, in conjunction with a two tone tail colour.

The three Overo Genes are genetically independant of each other, with each resulting in various patterns of white.
The Overo genes cause white markings somewhat similar in appearance, though they are generally visibly distinguishable from each other.
Overo body markings generally occur on the sides and belly but do not cross the topline, and a variety of gene dependant facial markings occur in conjunction with the body white on all three of the Overo Genes.

There are a number of white pattern genes, and just about all horse breeds have at least one or more of the white patterns present.
The patterns falling under this topic can be further catergorized based on the type of pattern manifested by each of the individual genes.
There are two types of pattern manifestation - White Roaning Patterns & White Spotting Patterns.

All of the white pattern genes can occur on any background [base or modified base] colour, singularly or in combination with each other and/or with any other colour gene. A wide array of markings relatively dependent upon expression intensity are manifested by each, with patterns occurring in combination with each other, leaving for some very colourful and uniquely marked individuals.

The White Patterns and their genetic control are very different to that of the basic/background colours. Each pattern of white, be they roan or solidly marked, or even a combination of both, is unique to each individual. While certain patterns of white are the result of the interaction of one locus and as thus is designated the same relevant term of description, each and every white patterned horse is unique unto itself, somewhat like a fingerprint in humans.

Patterns of white are caused by the progressive death and/or "blocking" of the melanosomes, the "packages" that migrate throughout the body during embryonic development that are responsible for the production of the skin and hair shaft pigmentation [see Introduction To Genetics Page for more information]. As the melanosomes are solely responsible for the production of pheomelanin [red pigment] and eumelanin [black pigment], and horses can only produce these two pigments and only in this method, the result of the interrupted migratory path of the pigment producing melanosomes results in a complete lack of pigment in the interrupted areas. The phenotypical result is a white hair shaft with underlying pink skin.

The White Roaning Patterns are the result of single melanosomes failing to reach their designated migration point, while the White Spotting Patterns are the result of a larger migratory path interruption or blockage. Because there are millions of melanosomes that're responsible for a horses' end colour, there are millions upon millions of possible outcomes when the migration path and subsequent destination of the melanosomes path fails - The end result of this being that there are literally an infinite number of different pattern variations from individual to individual that can result from a shared single locus interaction.

Whereas the white patterns are the result of the complete interruption during embryonic development, resulting in partially white haired horses at birth, there is another gene that causes the progressive loss of pigmentation throughout the life of the horse - though the pigment loss is limited to the hairs only and so the genetic mechanism controlling the colour/s is different to the white patterns - More information on the Grey Gene can be found HERE.

Once again, the catergory name is just as suggestive - The White Spotting Pattern Genes manifest in various patterns of solid patches of white superimposed upon the dark/base colour of the horse - The colour genes falling under this catergory are the three Overo Genes - Sabino Overo, Frame Overo and Splashed White Overo, as well as Tobiano and the Appaloosa patterns, also known as the Leopard Complex in various parts of the globe.

As there are multiple white spotting patterns that are the object of various genetic mechanisms, all of these genes can be further grouped into sub-catergories that further help with the easy identification of each of the genes and their resulting patterns. While the pattern groups aren't used to exactly identify each gene [though in some cases it can indicate only the one pattern gene is at work], they can be used in the initial stages of attempting to idenitfy the gene or genes at work by narrowing down the possibilities.

There are three subcatergories of white spotting patterns - The Overo Patterns, Tobiano & The Appaloosa Patterns. Just as with the white roaning patterns, there are some very distinct differences between each of the groups as well as each of the genes falling within each group, that enable anyone who knows what they're looking for to easily identify the gene or genes at play - even when the differences are very subtle, they can still be identified by a keen eye.

As I've said already, once you know what to look for, it gets easier and easier to identify single and combination pattern genes. The first step to identifying which gene or genes are at work is to narrow down the subgroup - And this is how you do it!

Look at the horse you're trying to identify...
Does the horse have larger than normal white facial and leg markings? If so, one of the Overo Genes is most likely.
Does the horse have white body markings that cross the topline between the wither and tail? If so, the horse is Tobiano.
Does it's pattern consist of areas of white that contain coin sized spots of the solid/background colour of the horses? If so, the Appaloosa Complex is at work.
If you answered yes to any of the above questions, then the horse you're looking to idenitfy has a combination pattern - ie. more than one white spotting pattern and so you'll need to look at multiple causatives when identifying such a horse [See below].

Remember when asking yourself these questions that this is only the first stage of idenitifying a white pattern, and that this isn't the be all and end all of finding out what pattern gene/s a horse carries! They're what I use to narrow the genetic possibilities down to subgroup only.
If you want to become good - or even gret - at identifying the different white pattern genes present, look at as many coloured horses as you can, anywhere you can, and practice identifying the genes present - I promise, the more you practice, the easier it gets!!

In comparison, a Roan is distinguished from a Rabicano in that the white interplay of hairs on a Roan is more often that not very even in appearance, and will extend across the entire body with only the head forward of a line between throat and poll, and in some cases the legs, being unaffected by the interplay of white and background coloured hairs. Even the most extensively marked Roan will have a completely solid/background coloured head.

A Rabicano is distinguished from Roan by the limitation of the pattern of white in most cases. All but the most extensively marked Rabicanos will have an uneven pattern of white limited to the flank, tail head and in some cases the abdomen of the horse. An extensively marked Rabicano will carry the uneven white patterning across the entire body including the head.