Mutation

MUTATION. - This term is commonly used to indicate the sudden appearance of a new hereditary character, and as such the word was introduced by the botanist De Vries. Today mutation is conceived as a variation of the hereditary patrimony: the carrier of the mutated character is called a mutant. Mutation is a rare and sudden event, which 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, chromosomal mutations, genomic mutations.

Gene mutations are transformations of the structure of the gene (v. GENE). Every gene is conceived as a macromolecule endowed with a structure that is not absolutely stable. Shifts in temperature, ionizing radiation, ultraviolet light radiation and chemical substances (mutagenic agents, such as mustard gas) determine ionizations in atoms of the gene molecule. In the case of X-rays, the ionization is produced by a collision between a secondary electron and an atom (hit theory). Probably a single ionization is enough to determine sufficient structural transformations to change the functions of the gene: every character that - following a mutation - a gene can produce constitutes one of its alleles or allelomorphs.

For example, the gene that determines the red, yellowish or violet-pink pigmented eye of Drosophila melanogaster mutates, transforming into the allele that determines the white eye. There exist genes that mutate more frequently than others (labile genes). Among gene mutations, some are incompatible with normal embryonic development or with life: they are called lethal. The alleles that manifest themselves in the wild forms of natural populations are generally dominant (they manifest themselves in the hybrids of the first generation: Mendel's first law). The alleles obtained in experimental populations are more often recessive (they do not manifest themselves in the hybrids of the first generation: Mendel's first law). The mutations induced with the physical and chemical means indicated above are the same as those observed for spontaneous mutation, only they are more frequent.

Chromosomal mutations are breaks in the chromosomes, followed or not by reattachments of the broken parts, and thus produce hereditary modifications of 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. Chromosomal mutations alter the position of the genes, leaving their structure intact. They are produced, naturally and experimentally, by the same causes known for gene mutations.

Genomic mutations consist in the loss or the addition of entire 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, 5 times rather than 2 times the haploid complement; v. MEIOSIS). The best known phenotypic effect of polyploidy (quite widespread in plants) is gigantism. The origin of genomic mutations must be sought in anomalies of the distribution of chromosomes during anaphase or in the suppression of mitosis. The factors that determine these mutations are low temperatures or chemical substances (colchicine, for example). 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 have occurred in the germ cells. The others, manifesting only in regions of the body, are called somatic mutations.

Claudio Barigazzi