The most frequently produced effect, so far as known, is the lethal effect ; that is, mutant genes are usually so deleterious in their action that they tend to kill the organism. When they are not positively lethal they are usually retrograde in direction, and hinder rather than help the organism to fulfil the functions of its existence. Such results are to be expected of accidental changes occurring in any complicated organization. A race, there fore, will tend gradually to undergo degeneration in any respect in which selection (natural or artificial) does not persistently weed out the degenerative mutations that continue to occur. It can be only the rare mutations that are helpful which furnish material for evolution. These latter, however, when they show, will tend to multiply.
As a matter of fact, most gene-mutations never show. For most (though not all) mutant genes are recessive to the type from which they arose, and in their cases an obviously mutant in dividual cannot appear unless—usually many generations subse quently to the original gene-mutation—two reproductive cells, each bearing the identical mutant gene, meet in fertilization.
Both conspicuous and inconspicuous, minor and major, funda mental and superficial, character-changes are produced by gene mutation, and either one character or several at once may be altered by a single gene-change. The more far-reaching the
change, the more apt it is to be deleterious rather than advan tageous. Hence geneticists are returning to a view essentially similar to Charles Darwin's (though now much more highly elaborated), namely, that the origin of one species from another usually involves the accumulation of numerous selected small steps of heritable variation.
See ANIMAL BREEDING; BREEDS AND BREEDING; CHROMOSOME; CYTOLOGY ; GENE ; HEREDITY ; PLANT BREEDING.