UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
Lethal White Overo

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Horse breeding programs specializing in overo have particular challenges compared with programs for other white patterns such as tobiano. Not only is there the possibility of producing a solid dark foal without the overo pattern but there is also the risk of producing an all-white foal that dies of complications from intestinal tract abnormalities (ileocolonic aganglionosis). As far as we are aware, overo horses themselves have no specific health risks. While breeding evidence shows that some overos are heterozygous for a gene that is lethal in the homozygous condition, it has not been easy to identify which horses have the overo gene that is associated with the lethal white foal syndrome. Occasionally even solid-colored horses without obvious body spotting patterns have been reported to produce lethal white foals. Clearly the spotting pattern classified as overo is phenotypically and genetically heterogeneous.

Breeders can test horses for this mutation to avoid producing lethal white foals and to identify new pedigree sources of the overo gene that may be useful in their breeding programs. The gene appears to be associated with horses often characterized as "frame-overos" in Paints and Thoroughbreds, but is also present in some tobiano/overos, some solid-colored (breeding stock Paint) offspring from overo matings, some tobianos and Quarter Horses without obvious evidence of the overo pattern. The gene has also been identified in an overo Miniature Horse.

Using the letter "O" to symbolize the DNA sequence of the lethal white (LW) overo gene and "N" for the sequence of the non-overo, then the lethal white foals can be symbolized as OO, their overo parents as NO and non-overos as NN.

Breeding predictions between LW overos (NO x NO):

N 25% NN solid 25% NO overo
O 25% NO overo 25% OO lethal

Breeding predictions between LW overo and solid (NO x NN): No possibility of lethal white foals.

N 50% NN solid 50% NO overo

We know of no other mutations that are associated with lethal white overo horses. However, owners requesting the diagnostic test should understand that there is the rare possibility that two NN horses could have a lethal white foal due if both the sire and dam carry a mutation at a site other than the one detected by this test.

Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL