, Sickle-cell anaemia is an autosome linked recessive trait (eliminating option A). It is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the \beta globin chain of haemoglobin; option B states the reverse (Valine by Glutamic acid), making it incorrect. The disorder arises due to a change in a single base pair of DNA (G-A-G to G-U-G), known as a point mutation .