Frame overo, also sometimes called just “ frame” or "overo" is one of the white patterns present in horses. Overo is a Spanish word, originally meaning "like an egg".
“Overo” is sometimes (still) used as a general term for any white pattern that is not tobiano. Therefore, to avoid any confusion, it is best to use “frame (overo)” when referring to this specific white pattern.
Frame overo can be characterised by white patches on the body and head with a horizontal orientation. The white areas typically don't cross over the back and the legs of the horse, making it seem like the white is “framed” by colour when looking at the horse from the side. The edges of the white patches are defined, sharp and have a more jagged shape, rather than rounded like the white patches of tobiano. Horses with frame overo commonly have a blaze that is wider on the forehead. A coloured upper lip on an otherwise very white head, sometimes called a moustache, is also possible. They may have 1 or 2 blue eyes, even when the skin around them is coloured.
The pattern is extremely variable in expression. Frame overo can be very cryptic; horses may have no white at all, or just a single blue eye, making it hard or impossible to identify the white pattern without genetic testing. Minimal expressions may lead to head markings without any white patches on the body. Horses with a louder expression may have some white patches on their belly and/or neck. The white patches may also cover almost the entire side of the horse, all the way from the neck to the hindquarters.
Frame overo affects any base colour, but the presence of dilutions or grey may make the white pattern harder to see.
Frame Overo - Quarter Horse | ||
When the areas that are normally excluded by frame overo are white, this may be a sign that other white patterns are present as well. Horses with multiple white patterns usually combine the characteristics of the patterns that are present and generally have more white than the individual patterns would cause.
The frame overo (OLW) allele has been mapped to the EDNRB locus. It is incomplete dominant, but lethal when homozygous, resulting in the Overo Lethal White Syndrome (OLWS). More information on OLWS can be found below. Horses that are heterozygous can show no to very loud frame overo characteristics. The causes behind the differences in expression have not yet been discovered. When horses have very expressive frame overo, this may cause deafness in real life. Foals that are homozygous for frame overo are fully “white” but, as mentioned, can't survive.
The resulting phenotype of each genotype in Horse Reality is listed here:
Frame overo can be tested at the Laboratory.
While horses with one copy of OLW are completely healthy, having 2 copies of the OLW-allele is lethal. The EDNRB gene has important functions in both pigmentation and development of the embryo. The gene controls an increase in the number of both melanocytes (pigment cells) and nerve cells. When a horse is heterozygous for the OLW mutation, this results in the frame overo white pattern. However, when homozygous, the multiplication of these cells is incomplete - leaving the foal without melanocytes (and therefore entirely white and may be deaf) and also without nerve cells in the large intestine. This results in underdeveloped intestines: the foal can't move food through the intestines properly and can't poop, leading to colic and eventual death within a few days after birth. This is called the Overo Lethal White Syndrome, or OLWS.
There is no cure for the syndrome, and humanely euthanising is the only option for the foals. Therefore, it is very important to be careful and cautious when breeding. When 2 horses that both have a copy of OLW are bred together, there is a 25% chance that the foal will be OLW/OLW and will have to be euthanised as a result of OLWS:
Dam | |||
Sire | OLW | n | |
OLW | OLW/OLW Lethal |
OLW/n Frame overo |
|
n | OLW/n Frame overo |
n/n No frame overo |
In the game, you will receive a notification when a foal is born with OLWS, which mentions that the foal had to be humanely euthanised:
Especially since the expression of frame overo can be so minimal, genetic testing is very important, even if a horse has no visual frame overo characteristics. When a horse is known to be frame overo, this should be considered when looking for a horse to breed it with. The safest option is to avoid any frame overo x frame overo breedings.
Another thing worth noting is that both frame overo x frame overo and frame overo x non-frame overo breedings, result in the same chance of getting a foal with the frame overo pattern. In both cases, there will be a 50% chance of an OLW/n foal - but with breeding to a non-frame overo, one avoids the chance of getting a foal that will have to be euthanised.
Frame overo is only present in American breeds, or horses descending from these lineages. The following table lists all breeds that can currently have the frame overo (OLW) allele in-game.
Breeds |
Akhal-Teke Horse |
Arabian Horse |
Brabant Horse |
Brumby Horse |
Camargue Horse |
Cleveland Bay |
Exmoor Horse |
Finnhorse |
Fjord Horse |
Friesian Horse |
Haflinger Horse |
Icelandic Horse |
Irish Cob Horse |
Kladruber Horse |
Knabstrupper |
Lipizzaner Horse |
Lusitano |
Mongolian Horse |
Mustang Horse |
Namib Desert Horse |
Noriker Horse |
Norman Cob |
Oldenburg Horse |
Pantaneiro Horse |
Pura Raza Española |
Quarter Horse |
Shetland Pony |
Shire Horse |
Suffolk Punch |
Thoroughbred |
Trakehner Horse |
Welsh Pony |