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A rearrangement in which a chromosomal segment is rotated 180 degrees. The symbol used is In. Inversions in which the rotated segment includes the centromere are called pericentric inversions; those in which the rotated segment is located completely on one chromosomal arm and do not include the centromere are called paracentric inversions. Inversions can occur when two double-strand breaks release a chromosomal region that inverts before religating to flanking DNA. Pericentric (includes centromere; "peri" means "around, about") ABC - cen - DEFGH AD - cen – CBEFGH Paracentric (does not include centromere; "para" means "beside, beyond") cen - ABCDEFGH cen – ADCBEFGH Even though gene order is changed in an inversion, many inversions do not cause abnormal phenotypes. Many inversions can be made homozygous, and inversions can be detected in haploid organisms. However, if the breakpoint of an inversion is within an essential gene, individuals homozygous for the inversion will not survive. Unusual phenotypes can also be observed if the inversion places a gene or group of genes in a new regulatory environment. In inversion heterozygotes, the number of recombinant progeny is reduced. Why? During meiosis, the homologous chromosomes in inversion heterozygotes form an inversion loop to maximize pairing. Recombination within the inversion loop leads to abnormal chromatids (whether the inversion is pericentric or paracentric). Thus, even though crossovers can occur, the abnormal recombinant gametes can rarely give rise to viable progeny upon fertilization. We see a preponderance of nonrecombinant progeny. Let's look in in more detail at crossovers in the two inversion types. In pericentric inversion heterozygotes, a cross-over in the inversion loop will lead to recombinant gametes bearing a duplicated region and a deleted region. Zygotes that form from these gametes will probably not survive because of the abnormal dosage of some of the genes within the inversion. In paracentric inversion heterozygotes, a crossover in the inversion loop will lead to recombinant chromosomes that have altered gene dosage and centromere number (one acentric and one dicentric). The acentric chromosome is lost and the dicentric chromosome breaks randomly during meiosis. Upon fertilization, the recombinant gametes cannot support survival of the zygote. Only nonrecombinant progeny survive. SEE FIGURE 13.13. Thus the diagnostic features of inversions are (1) inversion loops, (2) reduction of RF, (3) reduced fertility, and (4) inverted arrangement of chromosomal landmarks.
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