Before discovering genetic rules, genetic counseling was based on empirical observations. In this process, it was important to recognize that certain diagnoses were more frequent in certain couples’ descendants. The 20th century witnessed revolutionary progress in the science of genetics that coincided with increasing societal demands and therefore became an integral part of modern genetic counseling.

Genetic screening is changing from Mendelian disease ascertainment to predictive testing. We are also learning that the phenotypes of even simple Mendelian disorders are influenced by complex genetic and environmental factors. Moreover, developing knowledge about genotype/phenotype associations and many other aspects of genetic epidemiology will increasingly require referral to clinical geneticists.