G, A, T and C, the four letters of the genetic code -- representing the four bases that make up every strand of DNA -- have become famous in the past decade. As scientists unlock the DNA codes of everything from bacteria to humans, the scale of the sequences -- humans have 3 billion pairs of these bases in their genome -- can make them seem abstract. But, for me, seeing this sequence brought genetics into focus.

The letters I was looking at came from my own DNA, sequenced by Mark Jobling, a geneticist at the University of Leicester in the U.K. for the 20th anniversary of the invention of DNA fingerprinting by the university's Alec Jeffreys. More specifically, the DNA was from my mitochondria: the powerhouses in the cells that convert glucose into usable energy so that I can function. The mitochondria were bequeathed to me by my mother, who got hers from her mother, and so on.

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