Its nastiest toxin is actually part of a virus which settled in it millions of years ago, and Dr Heidelberg and his colleagues speculate that most of the second chromosome was also picked up from somewhere else along Vibrio's evolutionary way.

ECONOMIST: The cholera genome

Second, some of the places along the chromosome at which crossing over happens most frequently differ between the sexes.

ECONOMIST: Human genetics

Using the second swabs to test for additional markers on the Y (male) chromosome, can help narrow a large pool of close matches - potentially thousands of profiles - to a more manageable number.

BBC: DNA crime-fighting in UK 'lagging behind', experts say