EXPLORE THE RI ADVENT CALENDAR: http://rigb.org.uk/advent The creation of our sex cells -- sperm and eggs -- is an intricate process which can spell the difference between life and death. We typically inherit 23 pairs of chromosomes from our parents, but when mistakes occur in the process of meiosis (the formation of sperm and egg cells) some sex cells can end up with two copies of the same chromosome. The chromosome that this usually occurs with is chromosome 21. When that sex cell then fuses with another to form an embryo, that embryo inherits 3 copies of chromosome 21 (one from one parent, and two from another) meaning that it has 47 chromosomes instead of 46. As chromosome 21 is one of the shortest chromosomes the embryo can survive this duplication, but the result is a condition called Down's Syndrome, which causes developmental and physical disabilities. TV presenter and biologist Dr Yan Wong explains how this process occurs. With thanks to BBSRC: http://bbsrc.ac.uk/ Don't miss the 2013 CHRISTMAS LECTURES on BBC Four and BBC iPlayer: http://rigb.org/christmas-lectures The Ri is on Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for the latest science videos: http://bit.ly/RiNewsletter