The white cells of the blood are a source of DNA , whose analysis reveals our genetic makeup. Blood samples from different human population groups can therefore give us valuable information about genetic variation within and between different human populations, and so about the population interrelationships. A detailed analysis of the DNA variation observed in a carefully sourced collection of blood samples from all over the UK has enabled the creation of a “genetic map” of the UK that helps to tell us where the British people came from. We can even detect differences within the UK between people from Devon and Cornwall. By extending the study to Irish populations we can build up a picture of the origins of the populations of the UK and Ireland as they have developed since the end of the last ice age. Recent studies of DNA from ancient burial ground skeletons have, however, suggested intriguing differences from the modern populations. These may be explained by the burial sources of these ancient DNAs not being typical of the overall population that existed in Great Britain and Ireland at that time. Walter Bodmer is a human geneticist and cancer researcher. Formerly professor of genetics at Stanford and Oxford Universities and then Director of the Imperial Cancer Research Fund (now CRUK ), he now leads the Cancer and Immunogenetics Laboratory at the Weatherall Institute of Molecular Medicine in Oxford University, where he is an Emeritus Professor. His major current research interests are in the fundamental genetics and biology of colorectal cancer and their potential clinical applications, and in the characterization, analysis, and population distribution of genetic diversity in human populations.