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Locus Information: O locus

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)”.

General Information:

  • Frame is most common in American breeds such as Quarter Horses, Paint Horses, Appaloosas, Miniature Horses, Tennesse Walker Horses and American Saddlebred (“Glenknoll’s Moretosea”-bloodline).
  • Also found in Thoroughbreds in “Patchy Lassy/Nite Spot (Pesty Axe x Torchy’s Rainbow)” and “Blue Gazi” bloodlines and Canadian Sports Horses in the “Outrageous Fortune” bloodline.
  • Great phenotypic variablity → horses with the frame mutation may show the “typical” pattern, or no white at all.  For this reason, all horses that may carry frame should be tested before breeding, even if they do not show a white pattern.
  • The white pattern normally has a horizontal orientation (in contrast to the Tobiano pattern) and defined edges. The white rarely crosses over the back.
  • Horses often have a blaze that is wider on the forehead.  White around the eye may be associated with the eye being blue.
  • The term “Frame” comes from the appearance of the implies that the white pattern rarely crosses a horse’s back.
  • Horses with two copies of this variant are affected by OLWS and die within few days after birth.

Possible Genotypes:

Genotype: Effects:
n/n No Frame Overo mutation.
O/n The horse has one copy of the O variant → pattern with defined edges and big horizontal patches that rarely cross the back. Horses commonly have a white blaze that is wider on the forehead.
O/O The horse has two copies of the O variant → causes OLWS and leads to the death of the homozygous foal within few days after birth.


Test information: This test detects a mutation in the EDNRB gene.

Metallinos, DL; Bowling AT; Rine J (June 1998). “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 (6): 426–31 Abstract

Further information is available at Online Mendelian Inheritance in Animals.

A Frame Overo pattern with the visible horizontal orientation of white patches and colored back. This horse may also have another white spotting variant, because frame usually does not give this much face white, or socks.

Test #: H205

Samples: Blood, Hair

Price: 35 € (net)

Time required: 10-14 days


The stallion “Silvery Moon xx” has the Frame Overo pattern as well as two copies of the W20 variant. W20 enhances the amount of white in the pattern. Photo courtesy of Haupt- und Landgestüt Marbach.

See Silvery Moon’s xx genetic testing results (with kind permission from Haupt- und Landgestüt Marbach):



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