Kinship testing with X-chromosomal markers: mathematical and statistical issues.

Use of X-chromosomal markers for kinship testing is meaningful if the identical-by-descent allele sharing probabilities of at least two individuals involved in the case differ under the different hypotheses made about the composite relationships. In this situation, optimal decision making about one or the other hypothesis should be based upon the likelihood ratio of the genotype data obtained. When more than one X-chromosomal marker is being used, this implies that the patterns of linkage and linkage disequilibrium between the respective loci have to be taken into account. Otherwise, the evidence extracted from the data by means of the likelihood ratio may be misleading. Exact likelihood calculations on complex pedigrees can be performed using available software such as, for example, the "LINKAGE" programmes widely used in genetic epidemiology. The required genetic maps can be created using physical location information available in public databases.