Homozygous Tobiano Test--The tobiano pattern
of coat spotting in horses and ponies appears to be inherited as a simple
dominant gene. (Bowling,A.T., Journal of Heredity 78:248-250,1987). A spotted
foal can be produced by a tobiano x non-tobiano mating, but never from a
non-tobiano mating. Tobiano foals with only one spotted parent will not
breed true for tobiano because they have received a
"non-tobiano" gene from their solid parent that they will pass on, at
random, to half of their progeny. Matings between tobiano parents can produce
true breeding (homozygous) horses. Breeders that can recognize
homozygotes may use them to design subsequent matings so that every foal has the
Tobiano (toe-bee-AH-no)--Tobiano horses generally have white feet and
lower legs, and the white spotting on the body usually crosses the topline
somewhere between the ears and tail. The white areas tend to have sharp,
definite edges, and also are vertical in character. As a rule, the heads
on tobiano horses are very conservatively marked, and they almost always have
Homozygous Tobiano--A tobiano horse that is
homozygous will ALWAYS throw a spotted coat pattern or a
tobiano coat pattern in their offspring. Even if the other parent has a solid,
overo, or tobiano coat pattern, the homozygous tobiano gene will be dominant in
the offspring's coat color. The tobiano gene is an inherited dominant
No direct genetic test for tobiano spotting, such as a DNA test,
is available. To determine if a horse is homozygous, you must evaluate the
- Pedigree: are both parents
- Test breeding: no solid color
offspring from solid color mates
- Phenotype: tobiano with secondary
body spotting (called ink spots, paw prints, bear paws or cat
tracks). Ink spots are small colored spots that occur in the white
patches. They are usually small and round, can occur in groups, and are
therefore distinct from the other colored patches on most paint-type
horses. Ink spots are the expected background body color of the horse on
which they occur.
- Studbook data: the horse
has no solid color offspring.
- Genetic marker analysis: research has
shown that it may be possible to follow the inheritance of the tobiano
chromosome in pedigrees through analysis of records of parents and offspring
for linked genes (ALB and GC).
What is Homozygous Tobiano?
What is the Homozygous Black Gene?
Black and red are the two basic pigment
colors in the horse. The ability of a horse to produce these pigments is an
inherited trait, with red being recessive to black. A diagnostic DNA test
is now available that can identify black horses that carry the red
recessive gene thereby providing the information necessary to determine what
color offspring a horse can throw. The absence of the red factor presents
itself as "E". Red horses are homozygous for the red factor, symbolized as
ee, and black horses with one copy of the red factor are heterozygous Ee, and
black horses without the red factor are homozygous for dominant
alternative to the red factor, symbolized as EE. The horse that is homozygous
for the black gene (EE) cannot have red foals (chestnut or sorrel) regardless of
the color of the mate. The basic color of the horse will be black or bay,
but depending on genes at the other color loci, the horse may be buckskin, zebra
dun, grulla, perlino, gray, white, or any of these colors with the white
hair patterns tobiano, overo, paint, roan or appaloosa.
The test can be performed by either blood
or hair root samples.
Sources for above material
- Equine Color
Genetics by D. Phillip Sponenberg, DVM, PhD
Laboratories, Inc., Woodland, California
Genetics Laboratory, School of Veterinary Medicine, University of
California, Davis, California
For more in-depth information online on the Homozygous
& Black gene & other equine colors and markings, go to our Breeding for Color page.