Genetically determined markers on the surface of cellular blood elements (red and white blood cells, platelets). Blood groups have medical, legal, and anthropologic importance. In medicine, the matching of ABO and Rh groups of recipients and donors before blood transfusion is of paramount importance; other blood groups also can be implicated in incompatibility. Markers on white cells (histocompatibility antigens) are shared by a number of body tissue cells; these markers are important to the survival of transplanted organs and bone marrow. In law, the recognition of identity between bloodstains found at the scene of a crime and those on clothing of a suspect has resulted in many convictions, and blood typing has served to resolve paternity disputes. From an anthropologic standpoint, some blood groups are unique to specific populations and can be a reflection of tribal origin or migration patterns. Blood groups are also valuable markers in gene linkage analysis, and their study has contributed enormously to the mapping of the human genome. See also: Blood; Human genome