Final answer:
Sickle-cell anemia symptoms do not appear before the age of 6 months due to the presence of fetal hemoglobin.
Step-by-step explanation:
The symptoms of sickle-cell anemia do not appear before the age of 6 months due to the presence of fetal hemoglobin. Fetal hemoglobin helps to prevent the sickle-shaped red blood cells from forming, so infants with sickle-cell anemia do not show symptoms until their fetal hemoglobin levels decrease.
Fetal hemoglobin is the main oxygen transport protein in the fetus during the last 7 months of development in the womb and in the newborn until these levels are replaced by adult hemoglobin . Consequently, infants are usually protected from the effects of sickle cell anemia at birth and for some months afterward because fetal hemoglobin inhibits the sickling of red blood cells. As the level of fetal hemoglobin decreases after birth to be replaced by adult hemoglobin, the protective effect fades, and symptoms associated with sickle-cell anemia, such as pain crises and anemia due to hemolysis, can begin to emerge.