subaortic stenosis, or sas, is a genetic ailment that causes a narrowing of the passage of blood between the heart and the aorta. this leads to heart problems and sometimes sudden death. it affects larger breeds such as the newfoundland dog and the golden retriever. in some dogs, such as collies, the blue merle or harlequin coloring is actually the heterozygote of a partially recessive gene preventing proper development of the nervous system; therefore, if two such dogs are mated, on the average one quarter of the puppies will have severe genetic defects in their nervous systems and sensory organs ranging from deafness to fatal flaws.