Sickle-cell disease
# Sickle-cell disease is a genetic disease reported from negroes due to a molecular mutation of gene HbA on chromosome 11 which produces the β chain of adult haemoglobin.
# The mutated gene HbS produces sickle-cell haemoglobin.
# The sixth amino acid in β chain of normal haemoglobin is glutamic acid.
# In sickle-cell haemoglobin this amino acid is replaced by valine.
# The children homozygous (HbSHbS) produce rigid chains.
# When oxygen level of the blood drops below certain level, RBCs undergo sickling.
# Such cells do not transport oxygen efficiently; they are removed by spleen causing severe anaemia.
# Individuals with the HbAHbA genotype are normal, those with the HbSHbS genotype have sickle-cell disease, and those with the HbAHbS genotypes have the sickle-cell trait.
# Two individuals with sickle-cell trait can produce children with all three phenotypes.
# Individuals of sickle-cell trait are immune to malaria.