Spotting and white markings
Melanocytes are rather special cells. First they take their embryonic origin from the same part of the embryo and at the same time as the central nervous system (the brain & spinal cord). After the brain and spinal cord are formed these special cells that are left over at the edge of the area of central nervous system formation are called Neural Crest Cells.
These special Neural Crest cells then migrate throughout the body to form the melanocytes of the skin, the adrenal glands, the dentine of the teeth, some of the bones of the base of the skull and the voice box, the cornea of the eye, special sensory cells of the ear and special components of the involuntary nervous system in the viscera.
The genes producing unpigmented white patches on the body do so by interfering either with the total number of neural crest cells produced or with their ability to migrate. In the developing embryo some structures have a stronger attraction for these migrating cells than others and in a sense have a 'priority' on them if they are in short supply. The skin has the lowest priority and this competition for limited numbers of neural crest cells accounts for some of the commonest patterns of distribution of white markings on domestic mammals.
If one of these patches of white skin is taken and transplanted into a dark colored spot on the animal the resulting transplant will remain white since it has no pigment cells. Injuries that damage the melanocytes in an area can also result in white patches due to failure of these cells to regenerate. In riding horses, it is common to see white spots around the withers and back area.
The major series of genes affecting the distribution of melanocytes in mammals are commonly called the "S" or spotting series - alleles that affect the distribution of pigment bearing cells. The "S" series is responsible for blazes and stockings.
TYPICAL WHITE MARKINGS
In horses, however, there are other genes that produce spotting of different kinds, the pinto, piebald or "broken" pattern, including Tobiano, Overo, (including Frame, Sabino, and Splashed White); and the Appaloosa pattern.
And variations thereof:
Minimal Expression - T1 Through Extreme Expression - T6
The Tobiano Pattern can manifest itself on any coat color
|Minimal expression - T1||T2|
|T5||Maximum expression - T6|
Note similarity to overo pattern in T1 and T6
And variations thereof:
There are probably at least three variations of the "overo" pattern caused by different genes or gene combinations. Many breeders are participating in research, so hopefully one day, there will be an easier explanation of the patterns and the genes that produce them.
From our research there seems to be the following variations of overos:
These patterns can also be combined.
The Overo Pattern can manifest itself on any coat color
|Minimal expression- O1||O2|
|Note cream patches on leg and behind ears, not totally white||The only homozygous LWO foal at Unicorner, we did not suspect the dam was an overo|
A big thanks to Lisa Davis of LTD Miniatures for her help!
Spl 1 Minimal expression
SP 6 - Maximum expression