Experimental Variation

Many abnormal types of recombination occasionally occur, owing to such disturbances of chromosome-behaviour as the loss or reduplication of a chromosome or section of a chromosome, or the reduplication of one or more entire sets of chromo somes. When such an occurrence alters the proportions existing between the different kinds of genes (by changing the numbers of some and not of others) there generally results a combination of various (usually detrimental) abnormalities at once. At other times, a chromosome may become broken in two, or two chromo somes unite to form one, or a piece broken off one chromosome and united to another. The genes thereby become rearranged, but the characteristics of the individual probably remain little affected, since they depend rather on the kind and numbers of different genes present than on their arrangement. X-rays, cold and other influences are known to induce such disturbances of chromosome-behaviour.

Mutations in the Genes.

It has been found in fruit-flies (Drosophile) that any given gene, existing in a fly at the present day, has probably remained constant in its composition for sev eral thousand years. This stability is not passive, for, as every organism grows and reproduces through a process of repeated cell growth and cell-division, each gene must repeatedly reproduce itself, and in so doing it must each time construct a daughter-gene that possesses exactly its own peculiar structure. Occasionally, however, something goes wrong, and either the mother-gene be comes altered in its composition or else the daughter-gene is not formed exactly in the image of the old gene. As a result of this mutation there arises a new type of gene, having a different effect upon the organism, and this mutant gene is then capable of re duplicating its own new type—a process which it usually performs with as constant accuracy as that which the old gene had ex hibited. It is this peculiarity of the gene—its retention of the power of self-reduplication despite the occurrence of alternations (mutations) in its composition—which makes heritable variations possible, and hence organic evolution.

There is evidence that, when one gene in a cell mutates, the thousands of others, even including the similar or identical gene derived from the other parent, remain unaffected and constant.

The mutation, then, may be regarded as an "accident" of sub microscopic dimensions, dependent on the occurrence of certain "chance" configurations of atoms, electrons and energy-quanta within or near to the gene in question. It is not strange, in view of this, that it has not yet been proved possible, by particular ex ternal conditions, to dictate the occurrence of specified types of gene-mutations in preference to others.

Though the kind of mutation which shall occur has not been brought under control, experiments with a number of different organisms have shown that heavy treatment with X-rays will make the mutational accidents in general occur far more fre quently than otherwise, so that gene-mutations of varied kinds may be produced at a rate in some cases over a hundred times higher than that at which they previously were found to occur. This raises the conjecture, as yet untested, that possibly the 7-rays (related to X-rays) which originate in minerals of the earth's crust have played an essential role in the causation of the varied "natural" mutations through which evolution has come about. However, there is evidence that temperature, genetic com position, and perhaps other factors, also affect the general fre quency of gene-mutations. No particular stage in the life-cycle is known to be especially favourable for the occurrence of gene mutations ; certainly they may happen either in embryos or in adults, and in somatic as well as in germinal tissue, though of course only mutations in germ-cells can be inherited.

Though most gene-mutations seem, in the sense above ex plained, to be accidents, some genes appear to be less stable than others, and a few are definitely known to be exceedingly mutable ("ever-sporting"). In the latter cases, which may represent changes of a different sort from most gene-mutations, the sta bility of the mutable gene may be much affected by specific con ditions. These effective conditions differ in different cases. Some are determined by the nature of the tissue and developmental stage in question, others by the external environment, and still others by the genetic composition.

A given gene has the capability of mutating in various ways, so as to form a number of different kinds of genes ("multiple allelomorphs"). Usually the effects of these differing changes in a given gene resemble each other strongly, and often, though not always, the discernible differences between them lie chiefly in the degree of change produced. There are also numerous cases on record of mutations that seem quite identical in kind with other mutations which occurred independently in homologous genes of other individuals belonging to the same or a related species. Not infrequently, too, different kinds of genes (non-homologous) will, on mutating, produce similar or indistinguishable final effects on the characteristics of the organism. In this connection it is found that certain character-changes can be produced by mutations in a greater number of different genes, and hence tend to occur more commonly, than others.