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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 pattern.                                                                                                            
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 dark eyes.
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 gene.
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 following:
  • Pedigree: are both parents tobiano
  • 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?
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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 are:

  •      Equine Color Genetics by D. Phillip Sponenberg, DVM, PhD
  •      Stormont Laboratories, Inc., Woodland, California
  •      Veterinary 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.