Alkaptonuria is a recessive genetic disorder in which the body cannot properly metabolize the amino acid Tyrosine. Its most prominent feature is the discoloration of urine due to an excess of Homogentisic acid. Sir Archibald Edward Garrod first surmised the correlation between enzyme production and genetics when he noticed that children of intermarried couples had alkaptonuria at a much higher rate than those of couples who were not related.
Russell, Peter J. iGenetics: A Molecular Approach. San Francisco: Pearson, 2006.