Lethal white is a genetic defect in foals of matings of some overo mares to some overo stallions. The nervous system to the bowel of the fetus does not develop normally resulting in impaction colic and death shortly after birth. Experience suggests that the incidence of this defective gene is quite high. Currently it appears to be a simple recessive mode of inheritance. What that means is that if a horse carries one of the genes he shows no signs of illness, but if he inherits a pair of the defective genes, one from the mare and one from the stallion, he will contract the problem. It is important to realize that even if the breeding animal only carries one of the genes he can perpetuate the problem in the breed. Passing on this gene to other healthy appearing horses. Recently the University of Minnesota Veterinary Genetics Lab has developed a test to detect the unapparent carriers. This is the first step toward eliminating this heart breaking disease in the breed.

In lethal white foals the nervous system to the terminal portion of the bowel does not develop. The result is there is no motility in that portion of the effected bowel and so an impaction builds up. The first signs of problems in that these babies are white and there is no meconium. Twelve to 24 hours after birth the foal begins to colic. Eventually the bowel becomes devitalized and toxins leak into the blood and death from shock occurs.

This cause for the defective nervous system is carrier horses with a mutation in one of the paired endothelium receptor B (EDNRB) genes. The result is a substitution at the 118 position of lysine for isoleucine at residue 118 on the EDNRB protein. The carriers while having one defective gene also has one normal gene, which protects them from the disease. In order for a newborn foal to have the disease requires that he inherit two defective genes, one from each parent.

There are no numbers that I have seen on how many overo paints carry this gene but color and pattern are strongly associated with incidences. In one large survey frame and frame-blend overos passed the gene 96% of the time. In this study calico overos with large amounts of white possessed the gene 100% of the time. Interesting is that the mutation is in the tobiano population with an incidence of around 10%.