Sickle cell anemia is prevalent among African Americans. Children who inherit only one copy will not. The error in the hemoglobin gene results from a genetic mutation that occurred many thousands of years ago in people in parts of Africa, the Mediterranean basin, the Middle East, and India. A deadly form of malaria was very common at that time, and malaria epidemics caused the death of great numbers of people. A third hurdle is multigene disorders .
Most genetic disorders involve more than one gene.
Most diseases involve the interaction of several genes and the environment . Many people who develop cancer not only inherit the disease gene for their disorder, they may also have not inherited particular tumor suppressor genes. Diet, exercise, smoking, and other environmental factors may have contributed to their disease. Studies of identical twins show that individuals with the same genetic makeup do not develop the same diseases and disorders. This is irrefutable evidence of the role environment plays in gene expression.