EXPLORE THE RI ADVENT CALENDAR: http://rigb.org.uk/advent Blood transfusion used to be a hit & miss affair, until the discovery of blood groups. The group most people have heard of is the ABO group, controlled by a gene on Chromosome 9 as explained by Nicole Thornton. This gene controls what blood type you are - whether A, B, O or AB. However, there are actually over 30 other blood group systems, most of which are extremely rare. Nicole explains the consequences of receiving blood from the wrong blood group as she chats to Olympia Brown, who learns what blood group she is when giving blood, and how important it is that as many different people donate blood, the first stage in the blood transfusion process. 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