A chromosome mutation is a change in the structure or number of whole chromosomes, as opposed to a gene (point) mutation, which changes the base sequence of a single gene. Chromosome mutations can happen spontaneously, often during crossing over in meiosis, and their rate is increased by mutagens such as ionising radiation.Changes in structure occur when chromosomes break and rejoin incorrectly:
• Deletion: a section of chromosome is lost, along with the genes on it.
• Duplication: a section is repeated, so the genes on it are present in extra copies.
• Inversion: a section breaks off, turns through 180° and rejoins, so its genes are in reverse order.
• Translocation: a section of one chromosome breaks off and joins a different, non-homologous chromosome.For example, a chromosome with genes in the order ABCDEFG would become ABCFG after deletion of D and E, ABCDEDEFG after duplication of D and E, and ABEDCFG after inversion of C, D and E.
Changes in number involve gaining or losing single chromosomes (aneuploidy, usually caused by non-disjunction) or whole sets of chromosomes (polyploidy, common in plants such as wheat).Chromosome mutations are often more harmful than point mutations because they affect many genes at once, and large deletions are usually lethal. However, duplications can be a source of new genes in evolution, because the extra copy can mutate and take on a new function while the original copy keeps working. A chromosome mutation in a cell that forms gametes can be passed on to offspring.
• Deletion: a section of chromosome is lost, along with the genes on it.
• Duplication: a section is repeated, so the genes on it are present in extra copies.
• Inversion: a section breaks off, turns through 180° and rejoins, so its genes are in reverse order.
• Translocation: a section of one chromosome breaks off and joins a different, non-homologous chromosome.For example, a chromosome with genes in the order ABCDEFG would become ABCFG after deletion of D and E, ABCDEDEFG after duplication of D and E, and ABEDCFG after inversion of C, D and E.
Changes in number involve gaining or losing single chromosomes (aneuploidy, usually caused by non-disjunction) or whole sets of chromosomes (polyploidy, common in plants such as wheat).Chromosome mutations are often more harmful than point mutations because they affect many genes at once, and large deletions are usually lethal. However, duplications can be a source of new genes in evolution, because the extra copy can mutate and take on a new function while the original copy keeps working. A chromosome mutation in a cell that forms gametes can be passed on to offspring.