Molecular Biology/Genetics USMLE Step 1 Practice Question

A 17-year-old high-school student comes to the school health clinic. She has missed her last menstrual period and is worried about being pregnant. In the past history, it is noted that she has sickle cell trait and mild asthma but is otherwise healthy. She has never been pregnant before and is sexually active with one boyfriend. They use barrier contraception on an irregular basis. She reports that her boyfriend once told her that he too had sickle cell trait. She is determined to continue with the pregnancy if the urine pregnancy test turns out to be positive. Her main concern is whether her child will have sickle cell anemia. At this stage, it is appropriate to counsel her that in cases where both parents have sickle cell trait, which of the following represents the most likely risk of their having a baby born with sickle cell anemia?