Sickle cell anemia is noted for being phenotypically heterogeneous. Th
is suggests that there are genetic influences that moderate the effect
s of the sickle hemoglobin mutation. In this review I focus on genetic
ally determined modulation of hemoglobin concentration and fetal hemog
lobin levels. Each of these variables has important influences upon si
ckle hemoglobin polymerization and the resulting pathophysiology.