Most mutations are clearly either very small, involving <103 nucleotides, or big, involving many genes or whole chromosomes – chromosomal abnormalities are rearrangements (deletion, duplication, inversion, or translocation), or altered numbers of chromosomes (aneuploidy and polyploidy). Here, “chromosomal abnormality” means a mutation affecting two or more genes. This chapter deals with the causes and consequences of chromosome mutations in eukarya.
Chromosomal rearrangements arise more rarely than do micromutations. Furthermore, having two identical copies of a chromosome rearrangement in a diploid genome is often lethal. Deletions and duplications are extremely rare in natural populations, but, surprisingly, inversions and translocations are relatively common in many species of plants and animals. The evolutionary basis of this seeming paradox is, in a nutshell, that diploid individuals having one rearranged copy and one normal copy of a chromosome often have a reproductive advantage (e.g., higher survival or fertility). Such a reproductive advantage leads to an increase in the population frequency of the chromosomal rearrangement (Chapter 39).
Rearrangements usually result from an exchange following two or more double-strand breaks in physically close chromosomes. Ionizing radiation, transposons, or oxidation by free radicals can induce double-strand breaks.
A chromosomal deletion is the loss of a segment of the chromosome (Figure 21.1). Large deletions, sometimes called chromosomal deficiencies, result from two double-strand breaks followed by loss of the segment between the breaks and a rejoining of the outside pieces.