Paint Horses:
About Homozygous

By Donna O'Neill

The Paint
Horse

Return to
Features
Page

About Homozygous - In Laymen's Terms.

In recent years there have been exciting development in the paint horse world of predicting, accurately, if a given horse will carry the tobiano color pattern. This predicting is accomplished by a series of blood-tests and color pattern linkages. Linkage is establishing a recognizable chain of coat patterns, each having an identifiable color code. This may sound more complicated than it really is. It is done through basic genetics, using dominant and recessive genes. It has been established that the tobiano gene is dominant over all other, non-tobiano, coat patterns genes. Thus whenever the tobiano gene is present, it will be expressed visibly, even in its Hetrozygous (single-gened) form.

With this in mind, we can understand how a homozygous,Double-gened horse will always give its offspring a tobiano gene, simply because it only has a tobiano gene to give. This gene, in turn, must always produce a tobiano coat pattern because it is always dominant over all other coat patterns. A series of blood tests are used to identify markers connected to these genes. (the genes themselves are too small to be seen) and then can be determined to be linked to either the tobiano or non-tobiano markers. A short series of letters are used to identify these markers. They are then used to evaluate the probable connection to the coat pattern. Once this is accomplished, one can sometimes piece together the puzzle. Of course, not every horse's blood markers will reveal these answers. Horses such as this are referred to as being uninformative for the tobiano marker linkage.

When attempting to produce a homozygous horse it is always necessary to have two tobiano parents (and in some cases, a toby-overo can pass on it's tobiano marker) because each horse must contribute one tobiano gene in order for the offspring to posses two tobiano genes, and thus be homozygous.

In order to demonstrate how this formula works, I will give you an example of a common case scenario. We stand a black and white stallion here named Dan Dee Chief. He has been determined to be a homozygous tobiano. He is genetically unable to produce anything except tobiano (and possibly a toby-overo) offspring no matter what he is bred to. This horse has been determined to be homozygous through an extensive breeding record, as well as the marker analysis that we have been discussing.

We have bred this stallion to a tobiano mare who has one tobiano parent and one Quarter-horse (Non-tobiano) parent. When this mare was tested to see what her blood-markers were, it showed an AB marker linkage. We were able to link her A marker to her Quarter horse dam and her B marker to her Tobiano sire. This was done with additional blood tests on both parent horses, and with this knowledge ,we can check the blood on the foal when it arrives and ascertain whether the mare contributed her solid A marker or her tobiano B marker. Since the sire of the foal is homozygous for the tobiano gene and has BB markers, he can only give a B tobiano marker. Thus, if the foal carries a BB marker linkage, then we know for certain that the foal has inherited one tobiano gene from each parent which would make the foal homozygous, if on the other hand, the mare is shown to have put in her A marker, then we will know that the foal cannot be homozygous for the tobiano gene. You will, however, be able to distinguish the tobiano gene from the non-tobiano gene, which would make the foal informative and it then can be used in a homozygous breeding program in the future.

The ability to control color patterns in horses could be a wonderful tool in producing and maintaining quality and athletic ability in the paint horse blood-lines today. The color aspect would no longer be a throw of the dice and the breeder could concentrate on the more important areas of conformation and disposition. If used properly, this could be invaluable in helping the Paint breeder accomplish his long coveted goal of producing a good, colored foal every time...