(i) Sickle cell anaemia : It is an autosomal hereditary disorder due to a mutation of single nitrogen base. It results in 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. 

(ii) Alkaptonuria: This was one of the first metabolic diseases described by Garrod in 1908. It is an inherited metabolic disorder produced due to deficiency of an oxidase enzyme required for breakdown of homogentisic acid (acid called alcapton, hence, alkaptonuria is also written as alcaptonuria.) Lack of the enzyme is due to the absence of the normal form of gene that controls the synthesis of the enzyme. Hence, homogentisic acid then accumulates in the tissues and is also excreted in the urine. The most commonly affected tissues are cartilages, capsules of joints, ligaments and tendons. The urine of these patients if allowed to stand for some hours in air, turns black due to oxidation of homogentisic acid.