How does the presence of abnormal hb leads to sickling of rbc?

Change in shape: Sickle cell anemia is caused by a single code letter change in the dna. This in turn alters one of the Amino Acids in the hemoglobin protein. Valine sits in the position where glutamic acid should be. The valine makes the hemoglobin molecules stick together, especially under conditions of low oxygen, forming long fibers that distort the shape of the red blood cells, and this brings on an attack.