Nearly six years after the sequence of the human genome was sketched out, one might assume that researchers had worked out what all that DNA means. But a new investigation has left them wondering just how similar one person’s genome is to another’s. Geneticists have generally assumed that your string of DNA ‘letters’ is 99.9% identical to that of your neighbour’s, with differences in the odd individual letter. These differences make each person genetically unique — influencing everything from appearance and personality to susceptibility to disease.
The differences in question – made up of stretches of DNA that span tens to hundreds of thousands of chemical letters — are called ‘copy-number variants’, or CNVs. Within a given stretch of DNA, one person may carry one copy of a DNA segment, another may have two, three or more. The region might be completely absent from a third person’s genome. And sometimes the segments are shuffled up in different ways. The new study, led by Hurles and Stephen Scherer of the Hospital for Sick Children in Toronto, Canada, and their colleagues is the most detailed attempt to find how CNVs are scattered across the whole human genome. To do this, they compared genome chunks from 270 people of European, African or Asian ancestry.
According to the team’s back-of-the-envelope calculations, one person’s DNA is probably 99.5% similar to their neighbour’s. Or a bit less.