the sequence of a chromosome can be represented by ABCD@EFGH, where the @ symbol represents the centromere. Which of the following chromosomes represents an inversion in this chromosome
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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.