It is known that A to A matings of MRI scanned Cavaliers have produced Off-springs with SM
Bet, this point is noted by Alison and me
but as also stated, this is very rare and in all the research studies so far, NO clear dogs have been produced in matings of dogs that were believed to be asymptomatic (and therefore assumed by their breeders to be clear but later scanned and found to have SM). Whereas clear dogs have produced almost no dogs with symptomatic SM and puppies on scans so far are either clear or have very mild status.
Inheritance is also not understood for MVD (sadly a situation set to continue as breeders refused to give enough heart info for the genome study for BOTH these conditions to have been analysed -- surely a crime in itself for the breed!!). Yet you yourself have argued strongly and passionately for years that dogs should be cardiac tested for the same reasons that people argue dogs need to be scanned -- every indication is the condition is strongly hereditary in the breed, the evidence is that breeding clearer dogs produces clearer offspring than breeding affected dogs, all else being equal. And there isn't a geneticist or researcher that believes the condition is not strongly hereditary, that I know of, in any country where research is being done -- dogs that are symptomatic tend to start to produce dogs that are also symtpomatic, as can be seen in several well known cases.
Given that breeders live with many dogs that many times are kept in kennels or separate dog quarters, the chance of observing symptoms also is low as many symptoms are not that pronounced until and unless the dog reaches severe status -- so I have no doubt that well intentioned breeders who do not scan are breeding symptomatic dogs that they think are not symptomatic. I have a deaf dog that came from a very health focused breeder who -- despite the fact that the dog was one of their personal favourites -- never realised she cannot hear. If this is the case with something as seemingly obvious as total deafness, I've no doubt it is very easy to miss the morning and nighttime scratching that is a common initial symptom, and may only happen after dogs are kennelled or off to the dog room, or before the breeder sees them in the morning. For example, I have absolutely no doubt my Leo would have been used at stud had I not bought him (this was the intention) because unless someone had him sleeping in the same room as themselves, they'd have missed all his growing symptoms between age 2-5, his prime breeding years, when he would scratch during the night or very first thing in the morning, never on walks though, no initial air scratching, no yelping, nothing.
I have spoken to Sarah Blott who definitely feels the condition is hereditary -- her goal is hopefully to discover the mechanism, eg to not understand IF but understand HOW. The problem is that breeders continue to not do scans, or work with her. More data is needed for her project to have any chance of success. Some actually feel there is already enough scan info out there to develop breeding recommendations based on DNA testing alone but the problem is that breeders have not submitted enough of the scan information that is already out there, and that is forcing breeders who do care, to continue to have to pay for scans.
Breeders and clubs could do much towards progressing research if there were some strong club and breeder support for the existing research programmes much less the work that will need to be done in future. I do know that several geneticists are not as hopeful as Sarah for the breed's future so really breeders should be fully backing her work as their best hope for having a breed to show in the years to come.