We have seen that deleterious traits can remain in the population if they are recessive and if they also provide heterozygous advantage. But we also know of dominant genetic diseases. How do these conditions appear and remain in the population? Dominant traits initially appear as do all other genetic variations: by mutation. In fact, some dominant genetic diseases show a high rate of new mutations; that is, they are not inherited from the parents but appear anew. For example, Marfan syndrome is a dominant genetic disease caused by a mutation in a connective-tissue protein called fibrillin. The gene encoding this protein is about 200,000 base pairs long, quite a large gene. Because most of this protein's structure is important for its proper functioning, the large gene that codes for it provides a large target for mutations. It is estimated that approximately 25 percent of Marfan cases are due to new mutations.
Scientists are identifying more dominant mutations as time goes by, but the great majority of these mutations are extremely rare. Some mutations are probably not recognized as dominant because the individual with the dominant mutation does not survive to pass it on. The first dominant mutation found in humans, brachydactyly, was recognized as such because of one large family affected with it. Similarly, other rare genetic diseases are only recognized when a large family with a particular phenotype is recognized and pedigree analysis is performed.
Yet we find that some dominant diseases are more common than one might expect from a rare mutation and a few families passing on this trait. This is the case for Marfan syndrome, Huntington's chorea, and familial early-onset Alzheimer's disease. The three diseases share a common feature: individuals with the disease traits do not express the disease phenotypes until they are past their reproductive age. That is, by the time the phenotype appears, individuals may have already passed on their defective genes to the next generation. Thus, selection does not have a chance to act because affected individuals reproduce before the disease is apparent. Selection can only act on the pheno-type, and selection manifests itself in individuals' success or failure in passing on their genes.
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