The use of DNA amplification for genetic counselling related diagnosis in haemophilia B.

A single base pair variation in the coding sequence of coagulation factor IX produces a protein polymorphism detectable with monoclonal antibodies and a restriction fragment length polymorphism (RFLP). This allows carrier and prenatal diagnoses in 48% of Caucasian families segregating for haemophilia B. However, this RFLP cannot be detected by standard Southern blotting, while the antibody assay may give equivocal results in some females and can only allow prenatal diagnoses on second trimester fetal blood samples. We show that, using the polymerase chain reaction, the polymorphic DNA segment can be amplified and directly tested for the presence of the alternative sequences by a non-radioactive procedure that has the advantage of speed (1-2 days), partial automation and applicability to first trimester diagnoses. We also show that the method gives results on a single drop of dried blood.