Alkaptonuria (defective homogentisate oxidase) and albinism (defective tyrosinase) are both inherited as autosomal recessive traits. They are expressed only when an individual is homozygous for the defective allele (aa). Carriers (Aa) are phenotypically normal. Sir Archibald Garrod (1902) first described alkaptonuria as an 'inborn error of metabolism' — the first known example of autosomal recessive inheritance and the biochemical basis of genetic disease. Both involve defects in the tyrosine metabolic pathway.