Andrew, sickle cell is a positive survival trait in malarial areas. You really need to understand the basics of biology before you can discuss biology with a biologist.
You're not a biologist, you just play one on FR. I have investigated the sickle trait. It involves low O2 levels. What do you know about it?
Consequently, sickle cell haemoglobin (HbS) replaces normal adult HbA in the red blood cells. This apparently trivial alteration to the b-globin gene alters the quaternary structure of haemoglobin which, in turn, has a profound influence on the physiology and well-being of an individual.
Haemoglobin S is less efficient at carrying oxygen than normal haemoglobin. It is also much less soluble and begins to crystallise as the oxygen concentration falls, as it does in the capillaries of the tissues.