Palko's Medical Laboratory Procedures 3rd Edition by Danielle Wilken, Phyllis Cox

Exercise 48

In the lab where Sonja works, an African-American husband and wife recently had their blood tested and were surprised to learn that each is heterozygous for sickle-cell hemoglobin. Because neither ever has had any symptoms of sickle-cell anemia nor known any close relatives with sickle cell anemia, they cannot understand how they could have a child with this life-threatening disease. What is the explanation for this?