MUTATION. — This term is commonly used to denote the sudden appearance of a new hereditary character, and the word was introduced by the botanist De Vries. Today mutation is conceived as a variation in the hereditary patrimony: the bearer of the mutated character is called a mutant. Mutation is a rare and abrupt event that occurs prior to the appearance of the corresponding character in the phenotype. Indeed, in large populations, recessive mutations (v. GENETICS; MENDEL AND MENDELISM) often manifest themselves many generations after they have occurred. Mutations are distinguished into: gene mutations, chromosome mutations, and genome mutations.
Gene mutations are transformations in the structure of the gene (v. GENESI). Each gene is conceived as a macromolecule with a not absolutely stable structure. Temperature fluctuations, ionizing radiation, ultraviolet light irradiation, 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 (the impact theory). Probably a single ionization is sufficient to cause structural transformations adequate to change the functions of the gene: each character that a gene can produce as a result of mutation constitutes one of its alleles or allelomorphs.
For example, the gene that determines the red, yellowish, or pinkish-violet eye color in Drosophila melanogaster mutates, transforming into the allele that determines the white eye. There are genes that mutate more frequently than others (labile genes). Among gene mutations, some are incompatible with normal embryonic development or with life: these are called lethal mutations. The alleles that manifest themselves in the wild forms of natural populations are generally dominant (they manifest themselves in first-generation hybrids: Mendel’s first law). The alleles obtained in experimental populations are more often recessive (they do not manifest themselves in first-generation hybrids: Mendel’s first law). Mutations induced by the physical and chemical means mentioned above are the same as those observed in spontaneous mutation, except that they occur more frequently.
Chromosome mutations are breaks in chromosomes, followed or not by reattachment of the broken parts, thus producing hereditary modifications in the structure of the chromosomes themselves. If a break is not followed by reattachment, there is a loss of fragments or deletion of the genome; deletions present for the same genes in both homologous chromosomes (v. CHROMOSOME) constitute a homozygous deletion: its effect is lethal. Chromosome mutations alter the position of genes while leaving their structure intact. They are produced, both naturally and experimentally, by the same causes known for gene mutations.
Genome mutations consist in the loss or addition of entire chromosomes, thus altering the number of genes. The best-known example is polyploidy, which consists in the multiplication of the entire chromosome set (3, 4, 5 times instead of 2 times the haploid set; V. MEIOSIS). The best-known phenotypic effect of polyploidy (which is widespread in plants) is gigantism. The origin of genome mutations is to be sought in anomalies in the distribution of chromosomes during anaphase or in the suppression of mitosis. The factors that determine these mutations are low temperatures or chemical substances (e.g., colchicine). The biological significance of these mutations is very 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 occur in germ cells. The others, which manifest only in regions of the body, are called somatic mutations.