X-linked diseases and carrier detection

Advances in molecular biology applied to the location of genes have generated a real evolution for the screening of women carrying X-related diseases. It is however imperative to first try to define the status of these women using classical methods: bayesian calculations taking into account genealogical data and direct screening which is partially reliable because of the inactivation of an X chromosome in women. The new genetic engineering techniques enable to locate the gene in an affected patient and follow its transmission in the families with the use of tracers linked to the gene of the disease. The difficulties of these studies are due to two phenomena. First, the risk of allele recombination because of a crossing over during the mitosis. This risk must be computed and specified. The second phenomenon is related to variations of information in the families which may either completely prevent identification of the carriers, or give less reliable results. The increasing number of molecular probes should enable to resolve this problem.