Rare Variants and Risk

Everyone walks around carrying roughly 100 broken genes — copies that no longer work. Most cause no trouble because you also carry a healthy backup copy on your other chromosome, which quietly does the job. Problems mostly appear when a child inherits a broken copy from both parents. That is why couples who are close relatives face higher odds.

Some gene variants help and hurt at the same time. The sickle cell variant is the classic case. One copy gives strong protection against malaria, a major killer in many regions. But inherit two copies and you get sickle cell disease, which is severe. Evolution kept the variant common in malaria zones because, on balance, it saved more lives than it cost.