EXPLORE THE RI ADVENT CALENDAR: http://rigb.org.uk/advent

Of all our chromosomes, chromosome 18 is the least dense in terms of genes, with barely any of its rather paltry 76 million base pairs encoding actual genes. As with the rest of the human genome, a large portion of it is considered "junk DNA", so called because it doesn't appear to have any particular function.

There might just be a reason, however, that these seemingly pointless sequences of DNA have stuck around after billions of years of evolution. As Dr Adam Rutherford explains, what we currently perceive as junk could turn out to have a hidden purpose, which is why it's so important to keep sequencing more and more peoples' genomes.

With thanks to BBSRC: http://bbsrc.ac.uk/

Don't miss the 2013 CHRISTMAS LECTURES on BBC Four, BBC iPlayer and the Ri Channel: http://rigb.org/christmas-lectures

The Ri is on Twitter: http://twitter.com/ri_science
and Facebook: http://www.facebook.com/royalinstitution
Subscribe for the latest science videos: http://richannel.org/newsletter