Clinical aspects of gene expression.

1. Expression and nonexpressin of genetic information may be viewed in relationship to the biologic structures that express or do not express the genetic information. We suggest defining expressivity as the quality of expression of genetic information in cells, tissues, organs, etc, of individuals, defining penetrance as the quality of expression of genetic information in an individual organism as a whole, and using a new term, phenotrance, to describe the quality of expression of genetic information in generations. 2. Decreased phenotrance may be indicated by "incompletely dominant" inheritance, by conditions for which "dominant" as well as "recessive" inheritance has been reported, or by disorders with sporadic occurrence in most and familial occurrence in some instances. The human conditions with decreased phenotrance that we have studied indicate that there are different types of decreased phenotrance. 3. The mechanisms for decreased phenotrance in man may correspond to certain genetic mechanisms that have been studied in lower organisms, such as delayed mutation, replicating instabilities, controlling elements, extrachromosomal inheritance, and others.