Elements of Genetics pp.110-112 (1969)
C. D. Darlington and K. Mather

Somatic Mutation

Instances are known, in Antirrhinum and elsewhere, of two allelomorphic genes or two homologous chromosomes mutating in the cell simultaneously to the same new allelomorphs. Such events suggest a chemical determinacy in the mutation even if they arise in two steps. These instances are, however, very rare and it is a remarkable fact, or even principle, that allelomorphs are (with these few exceptions) independent in their mutations. The original form of any change or mutation in the body of a zygote must therefore be a heterozygote. And the original form of any heterozygote must be a mutation. Only later can the heterozygote appear as we characteristically find it, that is as a cross between two homozygotes, as a hybrid in the restricted pre-mendelian sense.

It is also worth noting that most mutations occur somatically, that is in the nuclei of a growing body. Hence the new type of cell is not merely heterozygous. It is also incomplete so far as the body is concerned: it is a patch. In plants where the germinal tissue is not separated early in development a change may or may not affect the germ layer. In any case only one layer is changed and hence gives rise to a chimaera with mixed tissues: a layer of mutant lies inside or outside a layer of the unchanged type.

Chimaeras can be produced artificially as graft-hybrids or by any treatment which alters the chromosomes. Most old vegetatively propagated plants such as pelargoniums and potatoes have become chimaeras owing to somatic mutation at some time in their history. When they are propagated from root-cuttings (or from disbudded tubers) shoots grow out of the concealed inner layers which reveal the chimerical structure and the ancient mutations of these plants. The elucidation of these properties by Baur, Bateson, Winkler and Crane has enabled us to understand that natural changes in the arrangement of tissues lead to the repeated appearance of "sports" in chimaeras which themselves have arisen from a single original mutation (cf. Fig. 22).

FIG. 22.—Leaves and fruits of 2X Solanum sisymbrifolium (s), 6x S. nigrum (n) and the graft-hybrid or chimaera with one layer of s over n which sorts out to give pure nigrum in parts of its leaves (after Jørgensen and Crane, 1927).