Tobiano Paint Pattern

Wow. On my last post I wondered where the time went. That was 4 days ago... Irony.

Here is my small Tobiano contribution. I really encourage anyone that may be reading this to visit the aphatobianohorse blog that is linked on the right.

The Tobiano Pattern

This pattern is known to be dominant and is a quite common pattern to see.
The pattern itself, generally, is pretty consistent. Most tobianos have many of these traits:

- a 'war shield' or large spot of color over the chest and shoulders and on each flank; sometimes only one flank spot, and rarely, none.

-facial markings like a solid horse: star, stripe, snip (One of the fun ways to figure out if a horse is tobiano is to picture them with a cooler blanket on up to their poll. If they look like a normal colored horse with 4 whites, they are probably tobiano). Rarely a tobiano will have Overo type markings on it’s face.

-mixed white and colored manes and tails or one of the two.

-spots usually have even, clean, crisp, non-jagged edges.

-spots are arranged vertically with white extending over the topline.
four white legs (few have one solid leg, and even less have 2 solid legs)

-The pattern can be an almost solid horse with 4 white legs and a few white mane hairs to almost solid white with a colored head.

-skin underlying the white spots is pink and under the colored areas it is black, though pigmented skin surrounding a color spot may be overlain by white hair which will cause a cast to the border - like a halo

-"ink spots" in the white patches

-eyes are usually brown but one or both may be blue or partially blue

Tobiano Genetics

The pattern genetics are fairly straight forward. The spotting is due to a dominant gene. So, every tobiano horse has a parent with the tobiano gene. Sometimes, there is a tobiano horse that comes from 2 seemingly solid horses. In reality, at least one of the parents was a minimally marked tobiano that was registered as solid, due to not enough white on the body.

There are 2 terms that are handy to know when discussing equine genetics, Heterozygous and homozygous. Heterozygous means that the horse has only one copy (out of 2) of the gene in question. Homozygous means that the horse received one copy of the gene from each parent, so it has 2 copies. A homozygous tobiano horse will always produce a tobiano foal.

The tobiano gene has two alleles (possible forms in which a gene for a specific trait can occur). The dominant allele, TO, produces the tobiano pattern and the recessive allele, to, is non-tobiano. A horse that is homozygous for tobiano is symbolized as TOTO.
There is a ton of genetic information I could try to delve into here, but that is the basics.
A test has been developed to determine zygosity of tobianos.

Here is some info from UC Davis VGL (website linked at right):

Determining if a Horse is Homozygous for Tobiano

There is no direct DNA diagnostic test available for the tobiano gene, but progeny testing provides the next best information. A horse that produces any non-tobiano colored foals is not homozygous for tobiano. A horse bred to at least 10 solid colored mates that has only tobiano foals is statistically 99.9% certain to be homozygous.

Many horses that are homozygous for tobiano have a secondary spotting pattern of small clustered dark spots with roan edges breaking through in the white areas of the body. These spots, sometimes called "ink spots" or "paw prints" are often, but not always, associated with homozygous tobianos.

As most owners do not want to wait for progeny information from a very young horse and secondary spotting is not absolutely associated with tobiano homozygosity, a test is available that can help predict the likelihood that a horse is homozygous for tobiano.

Tobiano Zygosity Testing

There is no direct test at this time for the tobiano gene, however there are closely linked markers that can be used to determine if a horse carries one or two copies of the tobiano chromosome. Previously, the blood markers, Albumin (AL) and Group-specific component (GC) were used for this linkage analysis. VGL is now using hair samples to test for six closely linked DNA markers, thus eliminating the need for blood samples.

Requirements for Tobiano Zygosity Analysis

The minimum requirement for a horse to be homozygous for tobiano, is that it must have two tobiano parents.

The new tobiano analysis requires the following information for complete analysis:

Pedigree: three-generation pedigree with color and patterns marked

Photos of the subject horse: good quality photos and both sides preferred. Digital photos are accepted.