A method for determining the inheritance of a particular gene in a family. It is used in the diagnosis of genetic diseases, such as cystic fibrosis and Huntington’s disease. Molecular markers, such as single nucleotide polymorphisms or restriction fragment length polymorphisms (RFLPs) situated in or near the locus of interest, are identified using DNA probes, and suitable markers selected. These can then be traced through members of the family and used to detect the presence or absence of the disease locus prenatally in future at-risk pregnancies.