allele commonly exhibit the phenotype with white patches centered in the body and neck, framed by colored areas. However, the pattern can be suppressed or combined with other patterns. Researchers estimate about 20 percent of frame overos are “unapparent,” mak- ing visual identification difficult. But determining whether a horse carries the frame overo mutation is vital for breeders because foals receiving two copies of the allele, one from each parent, are born affected with Overo Lethal White Syndrome. All white or nearly all white, these non-viable foals might appear normal at birth, but they show signs of colic within 24 hours due to intestinal tract abnormalities. No treatment is available, and foals are usually euthanized once the diagnosis is confirmed. It is important to note, however, that not all white foals are afflicted with OLWS, so veterinary diagnosis is recommended.
The frame overo mutation has been found in tobiano, tovero and solid Paints as well as Quarter Horses, and there is a 25 percent chance of producing an OLWS-affected foal when crossing two carriers. APHA recommends testing of any horse prior to breeding to determine if the horse is a carrier of the lethal white gene, symbolized as O in laboratory reports.
This mare and foal display fairly typical frame overo coat patterns—large white markings on the neck, shoulders and barrel, framed by dark hairs.
Sabino 1
Several genes cause patterns commonly described as sabino, but researchers have developed a test for only one specific pattern caused by the Sabino 1 gene, SB1. It produces a white spotting pat- tern characterized by white patches with irregular borders on the face, lower legs or belly, and interspersed with white hairs on the midsection, either as spots or roaning. The white areas lack pigment, both in the hair and the skin.
SB1 has a semi-dominant mode of inheritance, which means that heterozygotes do not look identi- cal to homozygotes. Although the presence of one SB1 allele is visible, its effect is not as strong as when the horse is homozygous. In those cases, the horse is often com- pletely white or nearly all white. Testing for Sabino 1 is useful because this pattern could easily be confused with frame overo, roaning and rabicano. If a horse carries multiple patterns of white,
Royal Country Grip and her sire, SM Country Snowman (left), express sabino-like patterns, but it’s impossible to know if they’re caused by the Sabino 1 gene without genetic testing.
without testing it might be impossible to tell if he carries Sabino 1. It can also identify minimal Sabino 1 carriers, who when crossed with another Sabino 1 car- rier could produce a viable white or nearly all-white foal that could be mistaken for one afflicted with OLWS. The Sabino 1 test can only identify horses with the SB1 allele. The genetic causes of other types of sabinos are still unknown. Your horse could be a sabino, but not a Sabino 1 carrier.
GENETICS DECODED
Gene: a unit of heredity transferred from parent to offspring that determines a particular characteristic
Allele: variations of a gene, like Tobiano and non-Tobiano
Chromosome: strings of genes— horses have 32 pairs of chromosomes
Genotype: the genetic makeup of an individual
Phenotype: the external appearance of an individual
Homozygous: having two identical alleles for a specific trait
Heterozygous: having two different alleles for a specific trait
PAINT HORSE RACING JULY 2013 23
JESSICA HEIN
APHA FILE PHOTO
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