The gene for hemoglobin has two alleles, A and S. Normal adult hemoglobin is produced by the allele A. That molecule has a shape that allows it to bind with oxygen molecules. There is another hemoglobin allele, Allele S. Produces abnormal hemoglobin molecules, which causes red blood cells to have an abnormal shape (as in the image on the right (2) below) (options in the comments)