As previously noted, there are well over 100 mutations within the CF gene. In CF, there are mutations common to certain ethnic groups. For example, PKU can be due to an abnormality in the phenylalanine hydroxylase gene or the tetrahydrobiopterin gene. Family cooperation is critical. For susceptibility genes, the presence of the gene does not necessarily lead to the disease or illness.
Linkage analysis is often the procedure used to identify individuals who carry susceptibility genes.
At the cell level, familial breast cancer is technically homozygous recessive. Although testing is available, most genetic and public health associations recommend limiting the test to high risk families. The financial considerations in individual or family studies can be a hurdle for the laboratory and the patient. The social and ethical dilemmas, and the limited public health resources are unresolved issues. Thus, DNA gene testing, although possible, may not always be doable.