Secondary effects of aneuploidy

In earlier discussions of the effects of aneuploidy (Weil and Epstein, 1979; Epstein et al., 1981), the effects were divided into two categories – primary and secondary. The term “primary” was used to refer to the gene dosage effects discussed in the previous chapter, and “secondary” to denote all of the effects consequent to the primary ones (Fig. 5.1). The major reason for drawing this distinction was to focus attention on the fact that the primary gene dosage effects do not in themselves explain the deleterious outcomes of aneuploid states and that it is necessary to consider the functional or secondary consequences of these quantitative alterations on the synthesis of gene products. The significance of an aneuploidy-produced increase or decrease in the synthesis of a gene product must therefore be understood in terms of the function of the product and the alterations in this function which result from a change in the quantity of the gene product. In this context, the term “function” is being used in a very general sense and refers to whatever role the gene product has in the development, structure, or metabolism of the organism.

In addition to the distinction between primary and secondary effects of aneuploidy, a distinction was also made between two categories of secondary effects – direct and indirect (Fig. 5.1).