From Mendelian genetics, we predict that for every three tobianos from heterozygous tobiano parents, one is homozygous for tobiano and two are heterozygous. These are not very good odds to help me find a homozygous tobiano. A progeny test provides the best information. The odds that the candidate is homozygous increases with every spotted foal, as long as no solids are produced. The most efficient test to provide the information needed involves breeding to solid mates.
Significant results require at least five foals, preferably more.
In the long run, a progeny test is the only method to prove that a horse is homozygous. I am considering buying a yearling colt. Obviously he has no foals. Genetic markers linked to tobiano may help, but they cannot prove homozygosity since they are not direct tests for the tobiano gene. The tobiano gene is very closely linked to the Albumin and GC genes.