No one knows why African-Americans tend to have a higher risk for hypertension. One controversial hypothesis, known as the Middle Passage theory, is that black people who survived the harrowing trip across the Atlantic Ocean from Africa in the holds of ships did so because their bodies were better able to preserve salt, allowing them to avoid death by dehydration. Therefore, the theory goes, some African-Americans have a higher prevalence of “salt-sensitive hypertension”: Their blood pressure increases by unusually high amounts in response to salt.  Isolation of the MYH9 gene does not necessarily refute the theory