We already know that some of the gene edits scientists are considering for clinical use have secondary effects. For instance, editing an embryo’s CCR5 gene might make the resulting human resistant to HIV but more susceptible to the West Nile virus. Correcting the two mutated copies of the beta-globin gene in people who suffer from sickle cell disease would rid them of the illness but also deprive them of the mutation’s protection against malaria. These are far from the only gene edits that have both positive and negative effects.
