And variations thereof:
Spotting Factor | Pinto | Appaloosa |
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.
 | Frame Overo Patterns
|
| Splashed White Patterns
|
| Sabino Patterns
|
