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(also called chromosomal mutation), any of a number of structural changes in chromosomes. These changes result from abnormal divisions within the chromosomes, generally accompanied by reunification of the resulting segments in combinations other than those existing in the original chromosomes.
Structural changes may occur within a single chromosome or between homologous and nonhomologous chromosomes. Such changes include the loss (deletion) or gain (duplication) of a segment of the chromosome, or the exchange (translocation) of a segment of the chromosome with another segment. In inversion, a chromosome segment is deleted, turned through 180°C, and reinserted at the same position on the chromosome. Deletion and duplication disrupt the gene balance, thus altering the organism’s characters. Inversions change only the sequence of the genes in the chromosome, whereas in translocations the genes of the shifted segment move to another linkage group. Neither inversions nor translocations disrupt the gene balance or alter the organism’s phenotypic characters.
When meiosis occurs among heterozygotes that contain one normal chromosome and one chromosome with an inversion, it is difficult for these chromosomes to come together, and crossing over between them does not take place or is infrequent. Aneuploid gametes often appear, and consequently heterozygotes are less fertile than homozygotes, in which both chromosomes of a given pair either have an inversion or are normal. Since heterozygotes have chromosomes in which translocation occurs, heterozygotes produce many aneuploid gametes and are consequently less fertile than homozygotes.
Chromosomal aberrations may arise spontaneously, but they are more likely to occur under the influence of physical and chemical factors. Minor deletions and duplications may result from irregular crossing over. Chromosomal aberrations play an important role in the evolution of organisms: duplications are the main cause of increases in the number of genes. Inversions and translocations may result in genetically isolated homozygous individuals that are more fertile than heterozygotes. In all chromosomal aberrations, the gene position effect is sometimes observed, in which a gene shifted to a new site on a chromosome exerts a different effect on the organism’s phenotype.
Chromosomal aberrations have a practical application in that they may be induced in order to change the linkage groups of genes to produce economically valuable characters in organisms.
S. M. GERSHENZON