Mutable Genes in Maize and Callan's Chromosome Model of Serially Repeated Gene Copies

Abstract

IN a recent discussion of mutable genes, Fincham¹ proposed that the mechanism underlying the instability in higher plants, especially the diversity in the pallidarecurrens mutable system of Antirrhinum majus, could be explained by using Callan's chromosome model² in which a cistron consists of serial (tandem) repetitive copies of DNA sequences. According to Fincham, specific features of unstable genes usually include: (a) the occurrence of frequent stable alleles, quantitatively graded in expression; (b) the mutation to other unstable alleles, differing from the original in the timing or frequency of mutation, or in the kinds of stable alleles most often produced; and (c) the general genotypic and environmental effect on the specific mutation of a labile gene. Fincham marshals evidence from mutable gene studies which suggests that a serial repetition of DNA sequences is a property of the genes of higher plants and, according to Callan's model, a cistron consists of a number of copies joined end to end and arranged as intercorrectible “master” and “slave copies”.