[pjin]

It would be unethical to maintain a harmful allele of a gene, assuming that the particular allele is unequivocally harmful.

But that is not always the case. The textbook example is sickle cell anemia: although it is very harmful in homozygous individuals, it confers resistance to malaria without negative side effects in heterozygous individuals, which is beneficial.

There's the possibility that a harmful gene today becomes beneficial in the indeterminate future, for a reason that we cannot predict. That is the logic behind genetic diversity in a species, which allows it to cope with new and unpredictable environments by essentially allowing alleles to compete in the "natural marketplace."

If we're going to take control of our genomes and select for ourselves which alleles are harmful or beneficial, we must at least be prepared to preserve genetic diversity, if not in living individuals, then in gene banks or genomic databases.

[bdcravens]

Ditto for Cystic Fibrosis, with regard to cholera resistance.