Frame overo

What it looks like

A typical frame horse has irregular white patches along the sides, framed by colored areas. The back often stays colored, and the face may have extensive white. Other horses have very little white, or another pattern hides the frame appearance. A solid-looking horse can still carry the variant, so visible white cannot reliably tell you whether it is present.

Sources for this explanation [1] · [3]

What happens biologically

EDNRB is a gene whose instructions help certain cells develop during embryonic life. These include pigment cells and nerve cells that coordinate intestinal movement. When a foal inherits two copies of the frame-associated variant, essential nerve-cell groups, called ganglia, are missing from part of its intestine. This is intestinal aganglionosis: food cannot move normally, creating a fatal functional obstruction.

Affected foals can be born alive, usually with an entirely or almost entirely white coat, and die within the first days of life. This is not the same situation as an embryo that cannot survive pregnancy. The white hair itself is not the cause of death; the same DNA change affects pigmentation and intestinal development.

Sources for this explanation [2]

Inheritance and DNA testing

For this test, O names the frame-associated variant and N means that variant was not detected. O/N means one O copy; O/O means two copies; N/N means no O copy. One copy can affect the coat without causing the fatal intestinal syndrome. Two copies cause the disease.

If both parents are DNA-confirmed O/N, each can pass either O or N. Each conception therefore has a 25% chance of O/O, the affected foal; a 50% chance of O/N, a one-copy foal; and a 25% chance of N/N, a foal without this variant. With a tested O/N parent and a tested N/N parent, O/O cannot occur for this variant. These probabilities apply anew to each conception. Both parents need appropriate testing; the registry term overo is not a genetic result.

Sources for this explanation [1]

Confusions, limits and health

Overo is also used for patterns with different genetic causes, and a white-born foal does not automatically have this syndrome. Conversely, a newborn with colic or failure to pass meconium, its first droppings, needs urgent veterinary assessment regardless of color. Tobiano or splashed white does not cancel the risk from two EDNRB frame copies. Interpret the specific DNA result together with clinical findings.

Sources for this explanation [2] · [3]

Where the explanations come from

Laboratory resources explain which variants are tested and how to read their results. Research papers describe the evidence, discoveries and remaining uncertainties. Each article links its explanations to the relevant sources. Published titles stay in their original language so you can find the exact work.

1. 1 · Laboratory explanation

   UC Davis VGL — Lethal White Overo

2. 2 · Research paper

   Santschi et al. (1998) — Endothelin receptor B polymorphism associated with lethal white foal syndrome in horses

3. 3 · Research paper

   Santschi et al. (2001) — Incidence of the endothelin receptor B mutation that causes lethal white foal syndrome in white-patterned horses