nO Horse Courtesy of Callie Jacobson- Custom Colored Paints

Lethal White Overo (LWO) syndrome, also called overo lethal white syndrome (OLWS), occurs when a horse is homozygous (OO) for the frame overo gene. This genetic disorder causes the intestinal system not to develop properly (involving aganglionosis of the bowel). The foal will die within the first 72 hours after birth when its first meals cannot be digested properly. The lethal white foal will be born almost pure white.

This genetic abnormality is caused by a dinucleotide TC-->AG mutation, which changes isoleucine to lysine of the EDNRB protein. Horses that do not have LWO syndrome can still be carriers of the LWO gene. When they are carriers of this gene, they are said to be heterozygous (nO) for the LWO gene and may pass it on to offspring. The heterozygous LWO gene in a horse occurs when the diploid (one copy from mother and one from father) of the LWO gene contains one frame overo copy and one non-frame overo copy and is often referred to as positive for frame overo.

OO Horse Courtesy of Cara and Jesse Daniels

Since frame overo is a desirable quality and requires one frame overo copy, proper mating must be done to avoid possible loss due to lethal white overo while still achieving a high probability for the frame overo pattern. The way to avoid this problem is to avoid breeding frame overo to frame overo. Identification of frame overo can only be definitively done by genetic testing since the frame overo pattern is not always expressed and can be masked by other genetic traits. The gene has been associated with Paints, Thoroughbreds, Quarter Horses as well as Miniature Horses.

A universal method of describing the genetic testing results has been adopted. The letter "O" symbolizes the DNA mutation of the lethal white foal syndrome (LWFS) overo gene and "n" for the normal, non-overo allele. Therefore, the lethal white foals or Homozygous positive for the mutation are characterized as OO. Heterozygous horses or overos are identified as nO and Homozygous negative or non-overos as nn.

This chart shows the possibilities of offspring when mating two horses.

Breeders breeding two overo horses (heterozygous nO) can expect a 50% chance of producing an overo foal, a 25% chance of producing a lethal white foal OO and a 25% chance of producing a non-overo foal.
Breeders breeding a frame overo horse (heterozygous nO) with a non-overo horse (homozygous negative nn) can expect a 50% chance of producing an overo foal nO and no chance of producing a lethal white foal OO.

There is a rare possibility of there being another mutation within this gene that causes lethal white overo in addition to the one we test for. Even when a horse has been tested nn, it still has the rare possibility of carrying this other mutation that causes lethal white overo.

Lethal White Overo Testing

Animal Genetics offers DNA testing and detection of the gene mutation responsible for Lethal White Overo syndrome.

Cost

$25.00 US per sample.

Collect sample by pulling (not cutting) 20-30 mane or tail hairs with roots attached. It is important that you pull the hairs and confirm that the actual root of the hair is being collected. The root contains the genetic material of your horse that is needed for DNA testing. Therefore, cut hairs do not provide an adequate sample of your horse. Place the collected hairs of each horse in a separate zip-lock bag labeling the bags accordingly with the horses name or identification number. Download and complete a submission form for each sample and send along with payment to Animal Genetics for testing.

Submission Form -

Results

Results are given using the following symbolic notation: