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While many believe evolution purges genetic defects, humans still inherit variants that predispose them to serious disease. Some of these harmful genes even offer short‑term advantages, and in many cases natural selection has not yet eliminated them.

Definition

“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.” — Leonard M. Fleck, *Just Genetics: A Problem Agenda*, in *Justice and the Human Genome Project*.

Examples

• Huntington’s disease
• Cystic fibrosis
• Tay‑Sachs disease
• Sickle‑cell anemia
• Predisposition to coronary artery disease

In Ethnic Populations

Most deleterious alleles are recessive and therefore remain silent unless both parents carry the variant. In genetically homogeneous or closely related communities, the chance that both parents are carriers rises, explaining the higher prevalence of sickle‑cell anemia among people of African descent and Tay‑Sachs disease in Ashkenazi Jews.

How and Why They Persist

Several mechanisms help deleterious genes survive natural selection:

• Ongoing mutation – New copies of a harmful allele continually arise (e.g., neurofibromatosis).
• Late‑onset disease – Conditions that manifest after reproductive age (e.g., Huntington’s) escape early selection pressure.
• Heterozygote advantage – One copy can confer benefits, such as malaria resistance in sickle‑cell carriers.
• Historical advantage – Alleles that once provided protection (e.g., cystic fibrosis conferring cholera resistance) persist until selection can eliminate them.