Sickle-cell disease

- Sickle-cell disease affects one out of 400 African-Americans​ - It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells​ - In homozygous individuals, all hemoglobin is abnormal (sickle-cell)​ - Symptoms include physical weakness, pain, organ damage, and even paralysis​ - Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms​ - About one out of ten African-Americans has sickle-cell trait, an unusually high frequency ​ - Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common​