Gene-environment interactions and the complexity of human genetic diseases.

In the assessment of mortality and morbidity risk, the ability of family history and genetic test results to predict the age of occurrence, severity, and long-term prognosis of 'genetic' diseases is important. An increasing number of gene-gene and gene-environment interactions have been demonstrated in a number of monogenic Mendelian diseases. These interactions can significantly modify the clinical presentation (disease phenotype) of diseases previously regarded purely as 'genetic.' As a result, 'genetic' diseases can be positioned in a continuum between classic Mendelian and complex disease where the extremes, pure genetic or solely non-genetic, do not exist. The position of any given disease in this continuum is defined by three components: the major gene(s) contributing to the phenotype, the variability added by modifier genes and the significance of environmental factors influencing the phenotype. As the predictive value of genetic test results can be significantly influenced by additional genetic and environmental risk factors, a better understanding of these factors may influence the quantification of mortality and morbidity risk.