How common is the inherited persistence of fetal hemoglobin?

Difficult to measure: This is difficult to measure because in most people with inherited persistence of fetal hemoglobin are entirely asymptomatic. However, population studies have shown that about 10% of the population has been seen to have some persistence of fetal hemoglobin but in varying levels (from 0.8% of cells containing the fetal hemoglobin up to about 10% fetal hemoglobin levels).

Not common: I do not know the exact percentages and I am not a pediatric hematologist, but it is not common.