For example, changing a 1 to a 0 could make a letter g become a letter b .

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We are concurrently sending the attached letter to G.

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Thanks to the work of the appropriately named (and blue-eyed) Danish geneticist Hans Eiberg and his colleagues, we now know that the chief mutation that causes blue eyes is a single letter change, from A to G, at the 26, 039, 213rd position on chromosome 15, within a gene called HERC2.

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The codons were written in a four-letter alphabet, A, C, G, T, that abbreviated the names of the chemicals of which DNA is made.

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