In diagnosis of paternity by means of polymorphic markers, the proportion of men excluded on the basis of the phenotypes of the mother and child is the best index for controlling information. Its expected value, the probability of exclusion of a male chosen at random with respect to a random child-mother couple, calculated from gene frequencies of every genetic system, may be modified by a close relationship between the mother, the real father, and the presumptive father. The father and even more the brother of the mother, if he is the father of the child, diminishes the probability of exclusion of an individual chosen at random in the population, and if he is falsely accused, he has a higher probability of being excluded. On the other hand, the brother of the real father chosen at random in the population has the least chance of being excluded. The two different rules of exclusion are involved in the calculations, the first one being the more reliable.