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Homozygous Paint Horses

This is a homozygous sorrel tobiano stallion. He is an APHA Reserve World Champion and has his Superior Award in Halter. His sire and dam are both APHA Superior Halter horses.

Homozygous Tobianos:  A true homozygous Paint horse is always a tobiano.  The homozygous horse has two identical tobiano genes and must be sired by a tobiano, and have a tobiano for a mother.  The true homozygous tobiano must have secondary body spotting called "ink spots" and "paw prints" and must never have produced any solid colored foals from solid colored mates. Homozygous Tobianos will always pass a tobiano gene to an offspring (genes TT).  The majority of homozygous tobiano stallions standing today, sire 100% tobiano paint colored foals from the mares they breed.  A few horses that are listed as homozygous, have sired the occasional solid breeding stock foal, but this is rare. 

This is a classically marked black homozygous tobiano stallion.  This horse shows all of the characteristic traits of homozygous horses like blue zones around the paw prints on his hip and withers and ink spots.

Ink Spots, Paw Print Markings:  These unique and distinctive markings are usually located in the 'white' hair areas on a tobiano and are small dabs of spots or color (1-3 inches) surrounded with prominent "blue zones" (white hairs on black pigmented skin that give the hair a blue appearance) surrounding the 'spot'. Usually there will be groups of prints that look like dog or cat tracks. The black homozygous tobiano shown above has nice examples of these markings and there is a larger photo on the "Overo-Tobiano-Tovero" web page.  These patterns tend to show up on homozygous individuals, who have two tobiano genes, but also occur on heterozygous tobiano horses who only have one tobiano gene.

Heterozygous Tobianos:  A tobiano foal who has one tobiano parent and one quarter horse parent can only have one tobiano gene, regardless of its markings and therefore, cannot possibly be homozygous, nor can it consistently sire colored foals.  When genes at the same location are not the same, and the parent can pass either gene (Tt), the horse is considered to be heterozygous.

What does "Informative" and "Uniformative" mean?
In a horse classified as  "informative", the blood markers of the individual can be used as indicators to test their offspring for homozygous tobiano status.  In an "uninformative" horse, the blood markers of this individual do not confirm the tobiano status of their foals, so the ability of this horse to consistently sire or produce colored foals, cannot be determined

The Blood Markers:  All equine parentage was originally verified through blood typing.  Although researchers have never been able to identify a "color" gene, they were able to trace blood markers passed from the parent to the offspring for tobiano Paint horses.  Studies were conducted on tobiano mares and stallions, and their resultant offspring. These studies led to the discovery of blood markers that are 'linked' to the tobiano gene. The prevalent theory is that the genes producing blood type are very close in location to the tobiano gene. Subsequently, when the gene for a blood type is passed along, the tobiano gene located next to it will be passed along to the foal, too. This process has been labeled  'linkage', and it makes the identification of tobiano status possible in foals from informative parents. Test breeding on solid mares is used to confirm the status of individuals thought to be homozygous, as it is possible for these genes to be 'unhooked' and not passed together.

Interesting Examples:  A stallion can be designated as "informative" if his blood markers can definitively prove he is carrying the tobiano gene.  If a stallion has two tobiano parents, but has sired, or produced any solid foals, then this horse has only one tobiano gene and cannot be homozygous without two tobiano genes.  A tobiano with one tobiano parent and one quarter horse parent can still be informative and have the A/B informative designation, depending on the makeup of it's blood markers.  It cannot, however, be a homozygous tobiano, since both parents are not tobiano horses.

A tobiano stallion, for example, that has two tobiano parents, whose blood markers are inconclusive to show tobiano status is considered to be "uninformative". However, if this horse is later bred to 15 solid mares and produces 15 tobiano foals then he would be considered homozygous and informative. I won't go into what A/B means.  You'll have to look that up for yourself.  It gets pretty lengthy and complex after this, and is simply more information than I need to know at this point in time.  Ingesting the material I summarized here, took a lot of study and research to simplify the concepts! 

There are a lot of great charts and other information and details about genetics that I haven't included here as they are pretty lengthy, but you get the idea.  Once the blood markers of all tobiano horses are established, if an owner has studiously ingested the reams of information available, and how this all relates to the production of informative, homozygous offspring, it is possible to scientifically breed and produce an informative, homozygous Paint horse every time.  Wow!  After reading all of the material available, and studying all the charts and myriad of genetic combinations, I was really impressed! 

Hope this information has been helpful. 
Contact me
if you have any questions or would like to discuss this issue further.  Remember that the goal of all responsible breeders is to make the decisions that result in a colored horse who has all of the desirable conformation traits of our breed.  Color should not be the priority on your breeding choices list, unless you're just looking for a back-yard pet.   

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Ronny & Michelle Stallings

2422 Dr. Sanders Road

Aubrey, Texas  76227

(940) 365-2860