How come clinical signs in sickle cell anemia don't appear until the child is a year old?

Fetal hemoglobin: Sickle cell disease results from a mutation in the beta hemoglobin gene. In infants, the beta hemoglobin gene is largely not used by the body (the gene is "off"). Instead, babies use a different gene named gamma hemoglobin. The gamma hemoglobin does not carry the sickle mutation. During the 1st year of life, the body gradually switches from using gamma to beta. Thus, symptoms can be delayed.