Genome variations are differences in the sequence of DNA from one person to the next. Just as you can look at two people and tell that they are different, you could, with the proper chemicals and laboratory equipment, look at the genomes of two people and tell that they are different, too. In fact, people are unique in large part because their genomes are unique. The more closely related two people are, the more similar their genomes. Scientists estimate that the genomes of non-related people—any two people plucked at random off the street—differ at about 1 in every 1,200 to 1,500 DNA bases, or "letters." Whether that's a little or a lot of variation depends on your perspective. There are more than three million differences between your genome and anyone else's. On the other hand, we are all 99.9 percent the same, DNA-wise. (By contrast, we are only about 99 percent the same as our closest relatives, chimpanzees.)

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