Frame Overo coat pattern / OLWS (EDNRB-related) - Horse

What does this test analyse?

This genetic test detects the EDNRB variant c.353_354delinsAG in horses. The trait is known as Frame Overo, Overo, OLWS, Overo Lethal White Syndrome, Lethal White Overo, Lethal White Foal Syndrome and, in some contexts, as a form of congenital megacolon caused by impaired enteric nerve development.

With one copy, the variant causes the frame-overo pattern or may be present in horses with little or no obvious white pattern. With two copies, the same variant causes OLWS: foals are usually born almost white, lack sufficient functional nerve cells in part of the intestine and rapidly develop severe colic and intestinal dysfunction. This is a life-threatening combination that should be actively avoided in breeding.

Practical value of this test

• The test identifies frame-overo carriers, even when the white pattern is minimal, atypical or masked by other coat-colour genes.
• Breeders can avoid carrier x carrier matings and greatly reduce the risk of OLWS foals.
• The result supports colour planning, registration, sales information and safe stallion or mare selection in lines with overo or Paint background.
• In a horse with white patterning, the test helps distinguish EDNRB frame overo from other white-pattern loci.
• For foal planning, the test is especially valuable because the homozygous result is not merely a colour outcome, but causes a severe lethal disorder.

Inheritance and result

Frame Overo acts as an incompletely dominant coat-colour variant: N/O can cause the frame-overo pattern and can pass the variant on. The lethal syndrome occurs with O/O. An N/N horse does not carry the tested EDNRB variant. Because white-pattern expression can be variable, DNA testing is more reliable than appearance alone.