Mary Smith's sister has type I PKU, and Mary is planning to marry Bill Jones with no history of the disease. It will help to draw a pedigree. A more accurate carrier frequency based on RFLP analysis of the general female population has been performed on peripheral blood samples. The hemophilia A gene is refractory to new mutations. The hemophilia A gene is a hotspot for spontaneous mutations.
Affected sons can receive the defective allele from their fathers as well as carrier mothers.
The disease is sex-influenced. The disease is incompletely penetrant. Carrier testing is not available. Risk to a couple in which the woman has a sibling affected with the disorder. Husband's family history is negative.