Rearrangements reorganize the DNA sequences within genomes. Like point mutations, rearrangements are subject to natural selection and thus serve as instruments of evolution.
Deletions remove DNA from a chromosome. Homozygosity for a large deletion is usually lethal, but even heterozygosity for a large deletion can create a deleterious genetic imbalance. Deletions may uncover recessive mutations on the homologous chromosome and are thus useful for gene mapping at the cytological and molecular levels.
Duplications add DNA to a chromosome. The additional copies of genes contained in a duplication are a major source of new genetic functions. Duplications sometimes produce changes in phenotype that allow their detection. Homozygosity or heterozygosity for duplications causes departures from normal gene dosage that are often harmful to the organism. Unequal crossing-over between duplicated regions expands or contracts the copy number of genes and may lead to multigene families.
Inversions alter the order, but not the number, of genes on a chromosome. They may produce novel phenotypes by modifying the activity of genes near the rearrangement breakpoints or by disrupting genes at the breakpoints. Inversion heterozygotes act as crossover suppressors because progeny formed from recombinant gametes are genetically imbalanced.
In reciprocal translocations, parts of two chromosomes trade places without the loss or gain of chromosomal material. By relocating pieces of DNA, translocations may modify the function of genes at or near the translocation breakpoints. Heterozygosity for translocations in the germ line results in semisterility and pseudolinkage.
