Sub-Aortic stenosis (SAS) is a type of heart disease that is usually fatal. It is very different than what is termed a heart murmur. It sometimes shows up on a standard veterinary examination with a stethoscope when the dog is a puppy, and sometimes is not diagnosed until much later (well over a year of age). A Doppler EKG is the usual diagnostic tool used to pin point SAS. It is a progressive disease, untreatable and inoperable in dogs. Although surgery has been attempted at several veterinary schools, the results have been unsatisfactory.
Sub-Aortic Stenosis, also known as sub-valvular aortic stenosis, is a polygenic dominant disease, although some of the data is equivocal regarding whether it is incomplete penetrance or modifying factors. Data on a study of Newfoundlands clearly showed that the disease was dominant, most probably polygenic, and uncertain what the other factors were.
Currently, research is being done on a significant incidence in Bouviers, where the pedigrees indicate that it is polygenic dominant.
The carrier modes, although "somewhat" similar between polygenic-dominant and recessive--in that a series of genes (multiple individual alleles in recessive, multiple single dominant loci in polygenic dominant) are required--is different in polygenic dominant inheritance in that one set of genes gets transmitted as carrier or affected genes, and the other may get transmitted as clear (whereas in recessive, both are carriers if bred to a clear, or are affected if bred to another affected or carrier).
In polygenic dominant, if one of the parents of the litter were not individually affected (which is possible in a sub-clinical affected status), the BOTH of the lines are probably carriers. The disease shows up when you have a combination of the critical genes loci brought in from both sides of the lines, where if the lines were separated, the disease would not be expressed. That is one of the unique factors of polygenic dominant expression.....you need a series of gene loci ALL to be there for the disease to be expressed.
References to read: Ettinger, Veterinary Internal Medicine, SECTION VIII, Chapter 74, "The Cardiovascular System/Congenital Heart Disease"; R.L. Pyle & D.F. Patterson, et.al., American Heart Journal, Sept 1976, Vol 92, No 3, pp. 324-334).(Source: David J. Sheckler, DVM, Saratoga, CA)