Finding contributing loci, however, is extremely difficult. Once a gene is found, tests for susceptibility alleles at that locus will have low predictive value until several of the other inherited and environmental contributing factors are discovered and tests and other methods are devised to detect their presence. Mendelian disorders are characterized by complete or high penetrance of alleles at single gene loci. When the genotype capable of resulting in disease is present, the disease will almost always appear although its severity may vary. Clinical laboratories can offer predictive genetic tests without having to demonstrate their clinical validity or utility.
Situations do arise when offering genetic testing to a patient is appropriate.
In healthy relatives with a positive result, the benefits of early monitoring or prophylaxis have not been fully established. Genetic testing also gives parents the opportunity, through prenatal diagnosis and other strategies, to avoid the conception or birth of another affected child. Not all patients or families will want genetic testing for this purpose. Nevertheless, physicians have an obligation to make their at-risk patients aware of the options. Primary care physicians might be spurred into action more readily by a revolution if every patient was eligible for screening.