In the early 1900s, scientists identified 4 different blood types in humans - A, AB, B, and O - based on the presence of certain proteins called antigens in the blood. This blood typing system, called the ABO system, provided doctors with crucial information about their patients, allowing them to safely perform medical procedures, especially blood transfusions, by matching the blood types of patients and donors.
In the 1920s, scientists recognized that blood types were genetically inherited. A blood typing chart, shown below, was developed to show the relationship between parents and their children.
Scientists realized that they could predict the blood type of a child based on his/her parents' blood types. Conversely, if one of the parents' blood types was unknown, the scientist could use the blood types of the child and the known parent to identify the missing parent's blood type. In this way, scientists used blood typing to determine paternity or maternity of a child. However, because the information from blood typing is limited, it was difficult to definitively identify biological relationships.
For example, if a child had Type A blood and the child's mother had Type AB blood, the child's biological father could have any of the 4 blood types. This means that based on blood typing alone, no man could be excluded as the child's father.
In the end, the power of exclusion (the power of a test to eliminate a certain percentage of the population from being biologically related to an individual) for blood testing is only 30%. Blood typing is not a useful technique for determining paternity.