MUTATION. — This term commonly denotes the abrupt appearance of a new hereditary character, and it was introduced in this sense by the botanist De Vries. Today mutation is conceived as a variation in the hereditary constitution; the carrier of the mutated character is called a mutant. Mutation is a rare and abrupt event, which occurs before the corresponding character appears in the phenotype. Indeed, in large populations, recessive mutations (v. GENETICA; MENDEL E MENDELISMO) often become manifest many generations after they have occurred. Mutations are distinguished as: genic, chromosomal, and genomic.
Genic mutations are transformations in the structure of the gene (v. GENE). Each gene is conceived as a macromolecule endowed with a structure that is not absolutely stable. Sudden changes in temperature, ionizing radiation, irradiation with ultraviolet light, and chemical substances (mutagens, such as mustard gas) cause ionization in atoms of the gene molecule. In the case of X-rays, ionization is produced by a collision between a secondary electron and an atom (collision theory). Probably a single ionization is sufficient to cause structural transformations adequate to change the functions of the gene: every character that a gene can produce as a result of mutation constitutes one of its alleles or allelomorphs.
For example, the gene determining the eye pigment—red, yellowish, or purplish pink—in Drosophila melanogaster mutates, transforming into the allele that determines the white eye. Some genes mutate more frequently than others (labile genes). Among genic mutations, some are incompatible with normal embryonic development or with life: they are called lethal. The alleles that appear in the wild forms of natural populations are generally dominant (they appear in the hybrids of the first generation: Mendel’s first law). The alleles obtained in experimental populations are more often recessive (they do not appear in the hybrids of the first generation: Mendel’s first law). The mutations induced by the physical and chemical means indicated above are the same as those observed in spontaneous mutation; they are merely more frequent.
Chromosomal mutations are breaks in chromosomes, followed or not by the rejoining of the broken parts, and thus produce hereditary modifications in the structure of the chromosomes themselves. If a break is not followed by rejoining, there is a loss of fragments, or deletion, of the genome; deletions affecting the same genes in both homologous chromosomes (v. CROMOSOMA) constitute a homozygous deletion: its effect is lethal. Chromosomal mutations alter the position of genes while leaving their structure intact. They are produced, naturally and experimentally, by the same causes known for genic mutations.
Genomic mutations consist in the loss or addition of whole chromosomes, and thus alter the number of genes. The best known is polyploidism or polyploidy,
which consists in the multiplication of the entire chromosomal complement (3, 4, or 5 times rather than 2 times the haploid complement; V. MEIOSI). The best-known phenotypic effect of polyploidism (very widespread in plants) is gigantism. The origin of genomic mutations is to be sought in abnormalities in the distribution of chromosomes during anaphase or in the suppression of mitosis. The factors that produce these mutations are low temperatures or chemical substances (colchicine, for example). The biological significance of these mutations is quite different from that of the first two categories.
Mutations occur in all cells: those of greatest genetic interest are those that are transmitted because they have occurred in germ cells. The others, which appear only in regions of the body, are called somatic mutations.