A third type of chromosome rearrangement is a chromosome inversion, in which a chromosome segment is inverted — turned 180 degrees (see Figure 9.5c). If a chromosome originally had segments AB*CDEFG, then chromosome AB*CFEDG represents an inversion that includes segments DEF. For an inversion to take place, the chromosome must break in two places. Inversions that do not include the centromere, such as AB*CFEDG, are termed paracentric inversions (para meaning "next to"), whereas inversions that include the centromere, such as ADC* BEFG, are termed pericentric inversions (peri meaning "around").
Individuals with inversions have neither lost nor gained any genetic material; just the gene order has been altered. Nevertheless, these mutations often have pronounced phe-notypic effects. An inversion may break a gene into two parts, with one part moving to a new location and destroying the function of that gene. Even when the chromosome breaks are between genes, phenotypic effects may arise from the inverted gene order in an inversion. Many genes are regulated in a position-dependent manner; if their positions are altered by an inversion, they may be expressed at inappropriate times or in inappropriate tissues. This outcome is referred to as a position effect.
When an individual is homozygous for a particular inversion, no special problems arise in meiosis, and the two homologous chromosomes can pair and separate normally. When an individual is heterozygous for an inversion, however, the gene order of the two homologs differs, and the homologous sequences can align and pair only if
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