So apparently these diseases are lost from the population and only recur as rare mutations. There are some dominant genetic diseases that are more common than this. These are diseases that are expressed late in life. Now let's consider heterosis. Remember that in heterosis, when the heterozygote has highest fitness, both alleles are maintained in the population because both are reproduced by the heterozygote.
Some genetic diseases fit the predictions of heterosis.
An example that has been known for many years is sickle-cell anemia. At the sickle-cell gene, SS individuals produce normal red blood cells bur are susceptible to malaria. The heterogyous Ss individuals produce red blood cells that do carry oxygen well but that apparently change to an abnormal shape, and are destroyed by the body, when they are invaded by the parasite that causes malaria. As a result, heterozygotes are resistant to malaria. In areas where malaria is a common disease, it causes high levels of mortality, and being heterozygous at the sickle-cell gene has high fitness.