This test specifically discriminates between two forms of the gene, endothelin receptor B (EDNRB); a normal form of the gene and a mutated form of this gene. The mutated form is known to be responsible for Overo-LWFS in Paint Horses, and is also known to be associated with the parental "frame overo" colour/spotting pattern (eg, Metallinos DL, Bowling AT and Rine J (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, 426-431).
An individual contains two copies of a gene, one inherited from its father, the other from its mother. When both copies of the EDNRB gene are the mutated form, LWFS results. However, when an individual has one copy that is normal (also called the "wild-type" of the gene) and one that is the mutated form, then an overo colour pattern results.
The Overo-LWFS test determines the genotype of an individual at this gene, whereas the individual's phenotype (on both LWFS and on coat colour pattern) results from the effects of the gene (or the genotype).
The relationship between an individual's genotype and phenotype is not always straightforward. This is because different mutations in a gene (or different forms of the gene) may have different effects; a phenotype may be produced by more than one gene or be modified by the overall genetic background or make up of the individual; or because the action or effect of a given gene may be dependent on (the individual) encountering particular environmental conditions. As well as this, some genes or their forms are said to be "recessive" (i.e., produce a specific phenotype only when both copies in an individual are of one particular form), whereas others are "dominant" (producing a phenotype when just one copy is of a particular form).
Given the research currently underway in the USA into the genetics of overo spotting, other gene tests may come on-stream in the future to further assist in the definition of Overo.
Overo colour pattern or phenotype
The relationship between LWFS and the overo colour pattern is also known to be not straightforward.
What is clear is that, mostly, LWFS is inherited from parents that both have a "frame overo" colour pattern. The "frame overo" pattern appears to be present in breeds that have Spanish ancestry, and the gene producing this pattern behaves as a dominant, that is, results when an individual receives just one copy of the mutated EDNRB (or overo) gene.
However, the "frame overo" pattern can also result from two non-spotted parents, suggesting that it is the product of a recessive gene. One explanation for this apparent anomaly is that the "frame overo" pattern may be the product of several different genes, or that other genes in the non-spotted parents may modify the expression of the mutated form of the EDNRB gene.
As well as "frame overo" patterns, other overo colour patterns have also been described, eg, calico (with loud to minimal variants, depending on the amount of white present); splashed white; sabino; bald faced and medicine hat. Overo blends, in which two or more different overo patterns are present in the same individual, are also reported.
Given that the overo pattern was originally used by some breeders to mean, "Paint but not tobiano", it is also possible that different spotting patterns were lumped together under the overo "banner" and that mistakes in registering horses as being either overo or some other non-overo pattern has created the condition where the correspondence between genotype and phenotype is less clear.
The American Paint Horse Association has recently published on its web site an excellent "Coat Color Genetics Guide". This has a very clear and comprehensive description of Overo spotting, the overo lethal white syndrome, genetics and pedigree's relationship to this syndrome, and fact versus fiction in some frequently asked questions about this syndrome.
Making sense of your O-LWFS DNA test data
It should now be clear from all of the above that the test results fit into one of the following categories:
1. Those in which the genotype is "O/N" (where "O" signifies overo and "N" normal).
This genotype will be reported in:
(a) horses with the overo colour/spotting patterns. (This is the expected result.)
(b) horses that have a non-overo colour/spotting pattern. (An unexpected result.)
When horses with the "O/N" genotype are mated together, up to 25% of their progeny will be "O/O" or have LWFS.
2. Those in which the genotype is "N/N".
It is very unlikely that these will have an overo-colour spotting pattern. Horses with this genotype will not produce any progeny with LWFS.
Please also be aware that not all white foals will be "O/O" (and, therefore, not suffer from LWFS).
* I FOUND THE FOLLOWING INFORMATION ON THE WEB & THOUGHT IT MAY BE OF INTEREST