Introduction And History Of Merle
There have been many questions regarding Merle since Dr. LA Clark first idenfied the mutation in 2006. Since Merle affects so many breeds I am sharing this excerpt from my book in order to make this important information more accessible for everyone in all breeds.
MERLE - a SINE insertion (Short Interspersed Element) on the SILV gene.
Think of a piece of foreign DNA (viral-like) that has "inserted" itself into the chromosomes and has changed the number of base pairs by adding a piece of DNA. This SINE insertion lives off the host much like a parasite and can change the way cells function.
Both SINEs and LINEs (Long Interspersed Element) are common in canines and all mammals, including ourselves. They can be a form of evolution, sometimes destructive and sometimes a useful incorporation into the genome. Most SINE insertions occur in areas of the genome where they cause no damage or change to the host. However, some occasionally insert in regions where they can disrupt and alter expression.
There is some thought that it is these SINE and LINE insertions in canines that have helped to "create" (with our outside influence) our different dog breeds.
In the case of the Merle mutation, a SINE insertion impairs the ability of cells to produce normal pigment - a defective or faulty design.
Another example of a SINE insertion within the canine genome is the mutation for Tan Trim on the A Locus as previously mentioned. SINE insertions can also cause diseases like Narcolepsy in Dobermans and may be responsible for some cancers.
The Merle SINE mutation consists of 3 parts - a head, a body and a tail (poly-A tail). The tail contains a long string of repeating base pairs. Think of the poly-A tail as a section of a lariat-like thread of DNA that "catches and ties" into the dog's DNA, the head then attaches itself - "bites down" - so the whole invader can work its way into the DNA.
This is not very scientific language, but gives a good visual of the process.
The original Merle test only identified the body of the Merle mutation and the assumption was made that any reasonable length of tail produced some kind of Merle pattern; the longer the tail then the more Merle. No distinction was originally made between the tail length (base pairs) and Merle phenotype.
When Merle testing was first available any length of Merle was reported as a full "M" Merle. Numerous Catahoula owners had their dogs tested and were receiving results that their solid looking dog was in fact "M/M".