What Is Homozygous Tobiano?

WHAT IS TOBIANO?

The term 'tobiano' refers to the dominant gene that produces the tobiano spotting pattern.

Since it is a dominant gene, at least one parent must be tobiano.

WHAT IS HOMOZYGOUS?

Homozygous is a term that means "two of the same." A horse can be homozygous for a wide variety of characteristics - the term is not limited to coat color. For example, when a person talks about his/her "homozygous stallion," and doesn't specify what the stallion is homozygous for, it could mean just about anything. That stallion could be homozygous for big ears, stifle problems, or the red factor gene. If a stallion owner says that his horse is "homozygous tobiano," then the term homozygous is describing the tobiano gene, meaning that the horse has two tobiano genes and will always produce tobiano (spotted) offspring.

In order for a horse to be homozygous for the tobiano gene, both sire and dam MUST be tobiano. It makes no difference how many spotted horses you can count on the pedigree; if one parent is solid, the horse CANNOT be homozygous tobiano. A blood test is available to determine if a horse is homozygous tobiano.

WHAT ABOUT BREEDING TO A STALLION THAT IS NOT HOMOZYGOUS TOBIANO?

A tobiano horse that is not homozygous for the tobiano gene is called "heterozygous tobiano." A heterozygous tobiano will pass its tobiano gene on to its offspring 50% of the time.

The most important thing to keep in mind is that using the color of a stallion's offspring to predict what percentage chance you have to get a specific color gene is just plain silly. First of all, that stallion must have sired AT LEAST 50 foals to even get a representative sample - and that's still a very, very small sample and inconclusive if some of the mares are solid and some of the mares are spotted. The color genetics of the mares the stallion is bred to makes a tremendous difference in the color of the foals. For example, a tobiano stallion may have produced 60 tobiano foals and 20 solid foals, but what color were the mares? A majority of the mares bred to a tobiano stallion are tobiano. Any heterozygous tobiano stallion bred to 300+ solid mares will only produce 50% spotted foals - it may be 53% or 46%, but never as high as 85%. Any heterozygous tobiano stallion bred to 300+ heterozygous tobiano mares will produce 75% tobiano offspring.

A stallion that is heterozygous for the tobiano gene will produce tobiano offspring 50% of the time when bred to mares who do not carry the gene. A stallion that is homozygous for the tobiano gene will produce tobiano offspring 100% of the time when bred to mares who do not carry the gene. Figures like 70% or 85% CANNOT refer to a single horse's ability to produce offspring with a specific gene.

HOW DO I BREED FOR A HOMOZYGOUS TOBIANO FOAL?

As mentioned earlier, both sire and dam must be tobiano. If sire and dam are both heterozygous tobiano, you have a 25% chance for a homozygous tobiano foal. If one parent is homozygous tobiano and one is heterozygous tobiano, you have a 50% chance for a homozygous foal. If both parents are homozygous tobiano, you have a 100% chance for a homozygous foal.

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