Chromosome Redundancy and Gene Expression: an Explanation for Trisomy Abnormalities

Abstract

AN extra genetic load in the form of extra chromosomes is a severe impediment to normal differentiation and development in man. There is an especially high association of mental retardation with the extra chromosome state, but a variety of other developmental anomalies are also found¹. Possibly the best known example is Down's syndrome or mongolism in which there are three number 21 chromosomes rather than two, producing severe mental retardation as well as multiple (rather characteristic) somatic abnormalities². Redundancy of the sex chromosomes is also associated with mental retardation, but especially interesting is the fact that a female with extra X chromosomes does not show precocious sexual development. An understanding of the harmful effects of chromosomal redundancy at the fundamental level would, of course, give an insight into some basic aspects of differentiation and development, as well as make it possible to assign specific gene functions to the affected chromosomes.