The popular perception is that evolution “sorts out” humankind’s genetic imperfections -- alas, not so. Humans continue to be born with genetic predispositions to disease that shorten or that drastically affect the quality of their lives. In some instances, those deleterious genes actually have benefits, but it is also possible that natural selection has yet to weed them out.
A deleterious gene is one that virtually all reasonable individuals “would judge consistently to cause very premature death or serious health problems that drastically compromise the capacity” of afflicted individuals to carry out normal or near-normal life plans. So wrote medical ethicist and philosopher Leonard M. Fleck in his essay, "Just Genetics: A Problem Agenda," which appeared in the collection "Justice and the Human Genome Project."
Examples of deleterious genes include those for Huntington’s disease, cystic fibrosis, Tay-Sach’s disease, sickle-cell anemia and a predisposition toward coronary artery disease.
In Ethnic Populations
Deleterious alleles (variants of a gene) are usually recessive, thus, will not propagate if only one parent carries the variant. But in close populations or ethnically homogenous ones, the likelihood is higher of both parents carrying that allele, hence the incidence of sickle-cell anemia among those of African descent and Tay-Sachs disease among Ashkenazi Jews.
How and Why They Propagate
Deleterious genes are generally recessive alleles, yet the traits persist in populations despite natural selection.
One theory holds that deleterious traits may be maintained by a mutation that keeps arising in a population (e.g., neurofibromatosis, which causes tumors of the nervous system). Natural selection may actively weed out the trait; still, new mutations continue to arise.
A second theory is that a genetic disorder that presents later in life does so only after parents pass on those genes (e.g., that for Huntington’s disease, the neurodegenerative disorder). Natural selection generally weeds out traits that either offer no reproductive advantage or that inhibit reproduction, but is "less selective" against traits that present themselves after prime reproductive years.
A third is that some deleterious genes carry a heterozygote advantage. For example, carrying two copies of the gene for sickle-cell anemia can be deathly, but a single copy confers resistance to malaria, an advantage to sub-Saharan Africans.
A fourth theory is simply that natural selection has yet to remove the gene, particularly if that gene once held an advantage. For example, the gene that causes cystic fibrosis is theorized to have provided resistance to cholera.