Autosomal recessive

Phenylketonuria occurs in 4 per 100 000 live births. It results from a defect in phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine, or from enzyme defects in the subsequent metabolic pathway. Phenylpyruvic acid and other abnormal metabolites of phenylalanine are present in the urine after 3 weeks of age, enabling a screening test to be carried out. Carriers of the gene can be detected by a phenylalanine loading test for genetic counselling purposes. Patients have fair colouring, because melanin production is impaired. Untreated cases have severe LD, but if a low-phenylalanine diet is started in infancy and continued until age 10, intellectual development may approach normal.

Galactosaemia results from a defect of galactose 1-phosphate uridyltransferase, the enzyme that converts galactose to glucose. Vomiting, jaundice, and hepato-splenomegaly are present from infancy, and cataracts develop later. It can be detected by finding galactose in the urine. A diet free from milk and other galactose-containing foods ameliorates the symptoms.

Tay-Sachs disease is a degenerative condition of the central nervous system due to an abnormality of lipid metabolism. It occurs mostly in Jewish families. A cherry-red spot appears on the macula after a few months of age and is followed by optic atrophy and blindness. Epileptic fits develop, and patients usually die about the age of 2 years.

Hurler's syndrome (gargoylism) is a disorder of the metabolism of mucopolysaccharides, which accumulate in the brain and other organs, causing hepato-splenomegaly. Aggressive temperament may be associated.

Laurence-Moon-Biedl syndrome comprises LD, retinitis pigmentosa, polydac-tyly, obesity, and hypogonadism.

Congenital hypothyroidism, due to deficiency of one of the enzymes involved in thyroxine synthesis, causes general retardation of development with lethargy and a puffy appearance. Symptoms can be alleviated by lifelong thyroxine replacement therapy.

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