following information was copied
from the Veterinary
Genetics Laboratory, School of
Veterinary Medicine, University of California, Davis, CA. 95616-8744
YOUR TOBIANO HAVE POTENTIAL
BE TRUE BREEDING FOR THE TOBIANO PATTERN?
The tobiano pattern of coat spotting in horses and ponies appears to be
inherited as a simple dominant gene. A spotted foal can be produced by
a tobiano x non-tobiano mating, but never from a non-tobiano x
non-tobiano mating. Tobiano foals with
only one spotted parent will not
breed true for tobiano because they have received a "Not-tobiano" gene
from their solid parent that they will pass on, at random, to half of
Matings between tobiano parents can produce true breeding (homozygous)
tobiano horses. Breeders that can recognize homozygotes may use them to
design subsequent matings so that EVERY
foal has the tobiano pattern.
direct genetic test for tobiano spotting, such as a DNA test, is
available. Owners of tobiano horses interested to know whether their
animals are homozygous for the tobiano spotting gene (TOTO) need to
assemble several kinds of information. The following types of
information taken together can be used to provide evidence for true
breeding (homozygosity) for tobiano:
both parents tobiano
tobiano with secondary body spotting ("ink spots," "paw prints," "cat
breeding: no solid color offspring from solid color mates
data: No solid color offspring
marker analysis: research at UC Davis has shown 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).
DETERMINING IF A TOBIANO PAINT
IS HOMOZYGOUS FOR THE PATTERN GENE
FREQUENTLY ASKED QUESTIONS
What is the chance that a tobiano, with two tobiano parents, is
homozygous for tobiano?
From Mendelian genetics, we predict that for every three tobianos from
heterozygous tobiano parents, one is homozygous for tobiano and two are
These are not very good odds to help me find a homozygous tobiano. Is
there other information available to help me determine if a horse is
A progeny test provides the best information. Any solid foal
(regardless of the color of the other parent), provides evidence that
the candidate homozygous tobiano is NOT homozygous. The odds that the
candidate is homozygous increases with every spotted foal, as long as
no solids are produced. The most efficient test to provide the
information needed involves breeding to solid mates. Significant
results require at least five foals, preferably more. In the long run,
a progeny test is the only method to prove that a horse is homozygous.
I am considering buying a yearling colt. Obviously he has no foals. Are
there any tests that can tell me if he is homozygous for tobiano?
Genetic markers linked to tobiano may help, but they cannot prove
homozygosity since they are not direct tests for the tobiano gene. The
tobiano gene is very closely linked to the Albumin and GC genes. Most
often the tobiano gene is marked by the "B" variant of Albumin and the
"S" variant of GC, although exceptions do occur. These variants also
appear in solid color horses, but the combination is significantly less
frequent than in tobianos. A tobiano with tobiano parents that also has
only the markers ALB-B and GC-S is five times more likely to be
homozygous for tobiano than to be heterozygous.
marker information from parents as well as offspring is available, the
odds of homozygosity for tobiano offspring (by marker analysis alone)
may change (up or down). If either parent is heterozygous for tobiano
and homozygous for ALB-B and GC-S, then the marker testing is
inconclusive. The horse could be homozygous, but the markers now
provide only the information that the horse is equally likely to be
homozygous or heterozygous.
both parents are known to be heterozygous for tobiano, and both are
ALB-AB and GC-FS, then their ALB-B and GC-S tobiano offspring is highly
likely to be homozygous for tobiano (greater than 90% chance).
a direct test for tobiano homozygosity, some buyers and/or mare owners
are appreciative of any information that increases the odds for
predicting that a tobiano from tobiano parents is homozygous. Others
find this information too complicated and are prepared to take their
chances or wait for progeny test information.
Homozygous "EE" Black Horse
is the Homozygous Black Gene?
red coat color in horses is inherited as a trait recessive to black. A
blood test for the red factor, based on DNA analysis, provides the
information to determine what color offspring a horse can throw. The
absence of the red factor presents itself as "E".
= no red factor detected. The horse can be assumed to be homozygous for
black pigment (EE). It 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.