Sickle-cells anemia is a genetic disease that can be identified by screening a blood sample for abnormal hemoglobin. A person with an HbS gene from one parent and a normal HbA gene from another parent (a heterozygote) is said to have the sickle-cells trait. About 40% of the hemoglobin in these individuals is HbS, but there are generally no ill effects. A person with HbS genes from both parents (a homozygote) is said to have sickle-cells disease, and all of his or her hemoglobin is HbS. Laboratory screening tests to separate HbA and HbS proteins can easily determime whether prospective parents carry the sickle-cell trait.