When Ava visited her Uncle Eli in the hospital, he appeared tired and pale. He complained of sharp pains in his bones. "T've got sickle cell disease," Uncle Eli explained, short of breath. "I just hope it doesn't run in your side of the family."

That evening, Ava searched the Internet for information about her uncle's disease. She saw photos of red blood cells shaped like the letter C - a far cry from the healthy, round blood cells most individuals have. Ava learned that these sickle-shaped cells are rigid and sticky. In blood vessels, they form clumps that can block blood flow and even cause organ damage. "Am I at risk?" Ava wondered. To find out, she would need to investigate her family history - and discover the genetic combinations that make up her genotype. As you read this chapter, look for clues that would help Ava predict whether she might carry the sickle cell trait. Then, solve the mystery.