Introduction

Abstract

The 23 pairs of human chromosomes contain more than 50 000 different gene pairs or loci, certain genes specifying the sequence of amino acids in proteins and others controlling the rates or times of initiation of protein synthesis (Stanbury et al., 1978). The structure of each gene is subject to ionation or mutation, the immediate effect of which is an alteration in the quality or quantity of a specific protein. The original concept of inborn errors of metabolism, of ‘one gene-one enzyme’ (Garrod, 1908, 1909; Harris, 1963) has been superseded by the modern concept of inherited metabolic diseases of one cistron — one polypeptide chain, the cistron being a functional unit of DNA controlling the synthesis of one polypeptide chain, many of which may go together to form a composite enzyme protein. At the time of writing, about 180 diseases are known in which a disorder of intermediary metabolism occurs as a result of an inherited single enzyme defect (Raine, 1972, 1974a; Watts et al., 1975; McKusick, 1978; Brock, 1972; Stanbury et al., 1978). Most of these diseases are of autosomal recessive inheritance, and, although the individual incidence of the homozygous or carrier state is low, the overall occurrence of inherited metabolic diseases is much higher. The morbidity and mortality of these diseases are high and, with severe mental and physical retardation often occurring in surviving cases of some disorders, the rarity of the individual disorders is no longer a valid reason for failing to consider them (Rosenberg, 1974).