Molecular diagnosis of the fragile X [Fra (X)] syndrome: calculation of risks based on flanking DNA markers in small phase-unknown families.

We studied two small, two-generation families with the fragile X [Fra (X)] syndrome. The absolute phase of the DNA markers in relation to the disease in the mother was not known in either family. We present the derivation of risks for these families using flanking markers, taking into account the uncertainty regarding maternal phase. Since the use of flanking markers fails to yield useful counseling data in the 1/3 to 1/7 of all cases where a single recombination event occurs between the two flanking markers, we calculate the probability that this method is likely to be successful or unsuccessful when prenatal diagnosis is attempted using linked RFLPs.