“Double Merle" is defined as a dog having any M Locus allele combination which can delete pigment to white and therefore has the ability to cause vision and/or hearing impairments - this includes m/Mh as well.
Using the 7 alleles set out in the "langevin et al" research paper there are 28 possible Merle allele combinations. Thirteen of these combinations have the ability to delete pigment to white and are considered to be "Double Merle".
These combinations can be compared to a sliding scale - the longer the allele combination then typically the more pigment can be deleted. Example - Mc+/M and Ma/M do not typically delete as much pigment as M/M, M/Mh or Mh/Mh
When a dog has a vision or hearing impairment due to being "Double Merle", this is associated with the combined Merle pigmentation trait only, which is not an “inherited disease”. The vision or hearing impairment is not a dominant or recessive trait but directly linked to the combination of Merle alleles. In this case the impairment is a secondary issue due to Merle "genetics" and not "inherited" from both or either parent – it is the Merle allele inherited from each parent that when combined is the cause.
This impairment cannot be passed to future generations, as it is not "hereditary", the only thing his offspring will inherit is either Ma+ - 260 or Ma+ 264. Any possible vision or hearing impairments in his offspring depends solely on the Merle allele inherited from the dam. If the dam is for instance m/m or Mc/Mc then there is no possibility of any resulting pups being “Double Merles” and therefore no vision or hearing impairments related to Merle...
If he is bred to an m/Ma female then any resulting Ma/Ma+ (264) could have pigment deleted to white and therefore a risk for impairment. This is not new or recent information, this is basic genetics 101.
This was known long before any Merle testing was available. Dr. Strains also covers this topic in several of his papers... This is the difference between "Genetic" and "Hereditary".
Personally, I believe one of the reason people are scared of merle is because breeding of merle requires research and responsibility. Some breeders do neither and assume others don’t take the time and effort either, which is a shame because all breeding should be done with careful thought and research.