Determining informativity of marker typing for genetic counseling in a pedigree

For the situation of a Mendelian disease linked to a genetic marker, a new method is described that allows evaluating for genetic counseling the information potentially available from the linked marker before the marker data are actually obtained, that is, prior to drawing blood for marker typing. For a consultand in a family pedigree, the method determines the risk distribution (small families) or an approximation to it (larger families) and calculates the probability that the risk will deviate beyond certain limits from its a priori value, which exists without marker data, for example, that the risk will be smaller than 0.10 or larger than 0.90. The method was applied here to a pedigree of 15 individuals for which analytical calculations would be difficult to carry out.