Ann T. Bowling, PhD, Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis, California 95616-8744

For tobiano breeders, the blood type may contain an additional information component, but (as always) final interpretation should be left to an expert. There is no direct test for the tobiano gene, but genes linked to tobiano have been identified. Nearly all tobiano Paint horses (more than 90%) will have factor B in the AL system and factor S in the GC system, although they may also have an A in the AL system and an F in the GC system. Those tobiano horses with both a tobiano dam and a tobiano sire have a 25% chance to be true breeding for tobiano. Most such horses will have in the AL system only factor B and in the GC system only S. Exceptions are known and analysis of family data is an important part of the process to provide evidence for predicting whether a young horse will be true breeding for tobiano. Definitive evidence can be obtained when the horse has sufficient offspring to meet accepted levels of statistical assurance.

If both parents are known to be heterozygous for tobiano, and both are ALB-AB and GC-FS, then their ALB-B and GC-S tobiano offspring is highly likely to be homozygous for tobiano (greater than 90% chance).