Mutable Genes in the Light of Callan's Hypothesis of Serially Repeated Gene Copies

Abstract

MUTABLE genes have been intensively studied in maize by McLintock^1–3 and Brink^4–6 and they occur widely in higher plants⁷. Among many features of mutable genes, all difficult to explain by conventional mutation theory, the following are especially relevant to the present discussion, (a) One original labile gene often gives rise with high frequency to a whole range of more or less stable alleles, quantitatively graded in their expression. (b) In addition, a labile gene can frequently mutate to other unstable alleles, differing from the original in the timing or frequency of mutation or in the kinds of stable derivatives most often produced, (c) The frequency of mutation is frequently drastically affected by changes of temperature (in several examples higher temperature causes reduced mutation frequency), or by genes elsewhere in the genotype; such variables affect the mutable gene specifically in the sense that they have no evident effect on mutation in general. All these features are well shown in the pallida-recurrens mutable system of Antirrhinum majus^8,9. In the following discussion I shall refer to the Antirrhinum mutants because I am familiar with them, although McLintock has documented most of the same effects much more thoroughly in maize.