Our understanding of the Lethal White Overo Gene (LWOG)

A secondary goal of our program at Hays Paint Horses is to produce overo-marked horses that are negative for the lethal white overo gene (LWOG). Testing shows the overo pattern markings are related to a gene-string that may cause the birth of a solid white foal. These “white” foals die within hours of birth as a result of an incomplete intestinal tract. This genetic combination is only possible when both parents (stallion and dam) pass this lethal white overo gene to the offspring. As more information becomes available, it is clear that a high percentage of overo-marked horses carry the gene responsible.  However, a small percentage (and it’s our goal to increase that percentage through selective breeding!) of regular overo marked horses do not carry this gene.  Our stallion(s) have been found to be Negative for the LWOG by DNA testing.  This means that our stallion(s) cannot pass the gene and his offspring cannot die from this defect even when the mare has and passes the gene to her offspring (remember, the lethal white foal only results if both stallion and dam pass the gene to the foal).  We have successfully produced and procured several overo marked regular registry paint broodmares and horses – all Negative for the LWOG.  By breeding to stallions without the gene, Hays Paint Horses avoids the LWOG problem even though the likelihood of ‘painted’ foals is decreased (prevalence of breeding stock offspring is increased when one parent does not carry the gene).  But a healthy and well-bred horse (even without spots) is much preferable to having a white foal that must be euthanized.  And, hopefully in our small way, we may add to the viability of the Overo-marked APHA bloodlines by breeding for color without taking the risk of losing foals to the lethal gene.