(i) Albinism : It is caused by the absence of the enzyme tyrosinase which is essential for the synthesis of the pigment from dihydroxyphenyalanine. The gene for albinism (a) does not produce the enzyme tyrosinase but its normal allele (A) does. Thus, only homozygous individual (aa) is affected by this disease. Albinos (individuals with albinism) lack dark pigment melanin in the skin, hair and iris. Although albinos have poor vision yet they lead normal life.
(ii) Sickle cell anaemia is an autosomal hereditary disorder due to a mutation of single nitrogen base. It results the formation of an abnormal haemoglobin called haemoglobin S (Hbs). In this, only one amino acid-6th amino acid of β-chain glutamic acid is replaced by valine. The erythrocytes become sickle shaped under oxygen deficiency as during strenous exercise and at high altitudes. They cannot pass through narrow capillaries. They clog blood capillaries. Blood circulation and oxygen supply is disturbed. Spleen and brain get damaged. Patient feels acute weakness. Homozygotes (Hbs / Hbs) usually die before reaching maturity.
