Frame Overo (Coat colour pattern Horse)
With Frame Overo Spotting, the horse has a distinct, horizontal orientation of white patches and a coloured back. The pattern is very variable in appearance, sometimes with very little or no white. Often there is a blaze with more white in the forehead region. Blue eyes and a coloured upper lip are also possible. The term "frame" indicates that the white patches usually do not exceed the back when viewed from the side.
Horses with one copy (O/n) show the spotting. Horses with two copies of the mutation (O/O) are affected by OLWS (Overo Lethal White Syndrome) and die within a few days after birth due to constipation colic.
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- Great variability in the appearance. Sometimes very little or no white.
- White patches show a horizontal orientation (in contrast to the Tobiano piebald pattern) and defined, sharp borders.
- Often horses have a blaze with more white in the forehead region. Blue eyes and coloured upper lip are also possible.
- The term "frame" indicates that the white patches usually do not exceed the back when viewed from the side → frame piebald.
- Very common in American breeds such as Quarter Horses, Paint Horses, Appaloosas, Miniature Horses, Tennesse Walker Horses and American Saddlebred ("Glenknoll's Moretosea" bloodline).
- Also in Thoroughbreds from the line of "Patchy Lassy/Nite Spot (Pesty Axe x Torchy's Rainbow)" and "Blue Gazi" and Canadian Sports Horses from the line of "Outrageous Fortune".
Overo Lethal White Syndrome (OLWS)
Horses with two copies of the mutation (O/O) are affected by OLWS (Overo Lethal White Syndrome) and die within a few days after birth due to constipation colic.
The breeding of two Frame Overo Horses (O/n x O/n) should be avoided, as there is a 25% probability that the offspring will be affected (OLWS O/O).
Animals with suspected Frame Overo should be genetically tested before breeding.
Genotype and Lab Report
Inheritance: incomplete autosomal dominant
→ Animals with only one copy of the variant (O/n) will show a less pronounced phenotype than horses with two copies of the variant (O/O). In this case it means that homozygous foals (O/O) will die soon after birth because of the “Overo Lethal White Syndrome (OLWS)”.
n/n = No Frame Overo mutation
The horse does not show the spotting.
O/n = One copy of the Overo variant
Pattern with defined edges and big horizontal white patches that rarely cross the back. Horses commonly have a white blaze that is wider on the forehead.
O/O = Two copies of the O variant
Causes OLWS and leads to the death of the homozygous foal within few days after birth.
Metallinos, D.L., Bowling, A.T., Rine, J.: A missense mutation in the endothelin-B receptor gene is associated with lethal white foal syndrome - an equine version of Hirschsprung-disease Mammalian Genome 9:426-431, 1998. Pubmed reference: 9585428.
Further information is available at Online Mendelian Inheritance in Animals.