If you read medical literature about Shwachman-Diamond Syndrome, you'll find that many patients with SDS have elevated fetal hemoglobin.
There are 6 different types of hemoglobin (I cannot create the symbols on my computer for them all - mostly Greek letters)
Each one of these different hemoglobins has a different globin chain -- and the chains each are coded by different genes which are "turned off or "turned on" during the different stages of development. During the first 3 mos of embryonic development Hemoglobin Portland is produced(blood cells are produced in the yolk sac at this stage). As the red blood cell making (erythropoiesis) changes to the liver and the spleen-- hemoglobin F appears. When erythropoiesis shifts to the bone marrow during the first year of life, the adult hemoglobins (Hb A and Hb A2) begin to be produced.
Info gathered from several physiology books I have lying around here at home.