The rate of change of hereditary factors in Drosophila

Abstract

A knowledge of the rate at which hereditary changes of various sorts occur is the necessary groundwork for an adequate understanding of evolution. The wide recognition given to this fact is attested to by the vast amount of literature on the subject of “variation,” but, with our new exact knowledge of the Mendelian and chromosomal method of inheritance of the so-called “variations,” it is evident that this literature has very little bearing on the real question of how often changes in the hereditary factors, i.e., mutations, actually occur: for the breeding procedures used in the experiments there considered were not of the type necessary for ferretting out the new mutant factors as they arise, and for distinguishing between them and the apparent variations caused by the sorting out of old mutant factors into new combinations. There is, to be sure, enough work to show that the real mutations are “rare”—whatever that term may mean; but, so far as an approximate quantitative determination of the rate of factor change is concerned, it is not possible, from the published work, to determine even its general order of magnitude. Some special scheme of crossing is required for this purpose.

In the present series of experiments with Drosophila, the X chromosome was chosen as the most convenient one for the detection of mutation, since every hereditary factor in either of the X chromosomes of the female fly stand revealed in the characters of one half of her male offspring, no matter what their father was. Thus, if the female has a new mutated factor in one of her X chromosomes, even though she does not usually show that factor herself, and even though her mate does not contain it, nevertheless one half of her sons are bound to show it and the mutation will thus be recognized.