[Evidence in cases of disputed descent from the viewpoint of the blood group expert].

The purpose of official guidelines is to ensure that when blood group serology is used to establish or contest kinship--usually a biologic child-father relationship--the prevailing conditions are both optimal and as similar as possible. Thus, they are concerned with "general conditions of evidence". Beyond this aspect there are the specific conditions of evidence pertaining to each individual case. They are based above all on the serotype of the child (and that of its mother), including, in deficiency cases, the number and relationship of relatives consulted; they affect both the exclusion expectation and the positive proof. With a new computer program it is now possible to calculate mean W values including upper and lower tail probabilities even in complex cases. From these one can determine whether or not the probability value (W) obtained in a specific case is "typical" for the alleged relationship.