What Is Homozygous
What is Homozygous?
by Wendy Malone
"Homozygous: (Ho-mo-zi-gus) means two copies of the same gene: homo = two of the same thing, zygous = gene (basically). A horse who is homozygous for the tobiano gene means that it has two copies of the tobiano gene, one inherited from EACH parent. A horse must have two tobiano parents to have inherited two tobiano genes." (Reprinted from the Glossary of Terms provided by the Spotted Horse Research Group at the Portland State University.)
The tobiano coat pattern gene in the Paint or Pinto horse is a dominant gene, whereas, the "overo" coat pattern has a recessive gene. Whenever a solid horse is bred for "color" to an overo or heterozygous tobiano (different genes) there is at least 50% chance of a solid foal from the mating. Some Breeders have tried breeding an overo to another overo to increase the possibility of color on the foals, only to find that one of four such matings will produce a "lethal white foal" that will die a painful death within a few hours to a few days as their intestinal tracts are not fully developed.
Knowing this, it is easy to see the value for being able to get color every time, and that is only possible with a homozygous tobiano (TT) since it received a dominant tobiano gene (Big "T") from each tobiano parent. A tobiano colored horse with only one tobiano parent will be heterozygous (Tt). All the genetic combinations from mating a homozygous (100% color producing) tobiano to any mare of any breed will carry one tobiano gene, and since that is the dominant gene, the coat will show that pattern.
The easiest way to understand how this works is to draw the following matrix whereby the parent with a capital "T" has the dominant tobiano gene, and a small "t" does not. If we bred a solid mare "tt" to a heterozygous stallion "Tt" , we would expect the following results. The sire may contribute one "T" to one of the mare's "t's" and we would get the tobiano coat pattern (show "color") 50% of the time, those with the first two potential combinations carrying the dominant tobiano gene (colored horses underlined). The remaining 50% of the possibilities would receive the non-tobiano gene (t), which when combined with the mare's, would give a solid foal the other 50% of the time.
(Heterozygous) Tt x tt (solid or overo)
(Possible foal combinations) Tt Tt tt tt
Had we bred that same solid mare to one of our homozygous stallions, you will get "color" every time since it is genetically impossible to throw a solid foal as we can see from the following mating.
(Homozygous) TT x tt (solid or overo)
(Colored foals the first and every time)
Tt Tt Tt Tt
If we bred one of the heterozygous tobiano offspring from the above mating to a homozygous tobiano we should still get "color" every time, however we now have a 50% chance that the resulting foals will also be homozygous, as we can see from the following chart.
(Homozygous) TT x Tt (Heterozygous)
TT TT Tt Tt
Whenever we breed a homozygous tobiano to another homozygous, all of the resulting foals will also be homozygous as there are no recessive "t" genes available to be passed down from either parent.
TT x TT
TT TT TT TT
So much for the easy part, verifying that a horse is homozygous for any trait has challenged scientists in genetic labs for years because the genes are too small to be seen by a microscope. Recently, geneticists at the Portland State Equine Genetics laboratory and at the University of California Davis were able to isolate the blood type "markers" that indicated the presence or absence of the dominant tobiano gene, since they still can't see the genes to determine the "genotype" of any horse.
It is important to note that just because you have sent in blood samples from the parents and the foal, the lab may not always be able to confirm that the foal is a double gened (TT) candidate for homozygosity. The lab test results may be inconclusive if the parents blood markers are not "informative", which means that the breeder will have to raise the foal and get at least 15 colored foals from breeding solid mates with no solid foals resulting before they could be reasonably sure there would be colored foals every time.
To this day, the proof is still in the foals, as the lab blood tests can only confirm that the horse has the blood markers that are consistent with the homozygous tobiano pattern, but the horse will not be "certified" as homozygous until there are at least 15 colored foals from solid mares. It is for this reason that it is more difficult to certify mares. Because the testing technology is relatively new, there are more discoveries and information appearing daily to help Paint and Pinto horse breeders better understand the process and establish programs to continuously improve their foals.
Since the verification process may take a number of years, many Paint breeders want to breed homozygous stallions to homozygous mare to know that the foals will be homozygous before they even hit the ground. Another way to shorten the certification process is by breeding the stallion to "informative" mare with AA or AB blood markers.
All horses have either an A or B blood type, with the most common being BB, occasionally an AB, and more rarely, an AA blood type can be found. For example, by August of 1997, the lab at Portland State University reported that they had certified 159 Homozygous stallions, one AA in Canada, and one in the USA.
For the equine color labs to certify your horse, they will require a blood sample from each tobiano parent in addition to samples from the foals, They will then be able to "map" the genetic trails and if there are informative AA or AB horses, it will lead to an earlier confirmation of the presence of the homozygous genes.
All the blood tests, parental production histories, and 15 successful breedings will help determine the "genotype" for homozygosity, however, it wont help you if you are standing in a pasture full of tobianos trying to guess which one might be homozygous.
The homozygous "phenotype" (think photo type) is a tobiano with "cat tracks" or "ink spots" in the white areas not necessarily connected to the larger blocks of dark color. The phenotype is the least certain indicator, however, as many heterozygous tobianos have dollar sized spots in their white areas, and there are also homozygous tobianos without them. The presence of these spots on a horse with two tobiano parents would be a good indicator the have a veterinarian draw a blood sample and proceed with the lab certification process.
While these tests help guarantee color every time, they wont be able to predict how much white will be thrown. Breeding to a "certified" homozygous tobiano will certainly take the risk out of breeding for a colored foal, in addition to avoiding a solid "Breeding Stock" foal from an overo stallion and a solid mare or a lethal white which can result from the mating of two overo parents.
For further information, contact Dr. Debbie Duffield at Portland State University Spotted Horse Research Group, 1-800-547-8887, or the Veterinary Genetics Lab, University of California Davis, (916) 752 - 2211