Interviewee: Matt Ridley. Mutations in the HGD gene can cause the rare disorder alkaptonuria. The gene usually produces an enzyme that breaks down a substance called homogentisic acid. When this enzyme doesn't function properly, the homogentisic acid builds up and causes symptoms of alkaptonuria that include arthritis, black urine and reddish earwax. (DNAi Location: Genome > Tour > Genome spots > Chromosome 3: alkaptonuria > A chromosome 3 story)

On chromosome 3 there's a gene with a special role in the history of genetics. It's a gene associated with a disease called alkaptonuria, which turns urine black and earwax red. In 1902, Archibald Garrod noticed that this gene runs in families. He suspected that what was wrong was that there was something missing from the biochemistry of the people with this disease. In 1995, the gene was discovered, and it turned out that he was right. What's missing is an enzyme, which is made by a single gene. That's what genes are; on the whole, they're recipes for single proteins, such as enzymes.

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