Chapter

Inversions

R. J. McKinlay Gardner, Grant R. Sutherland and Lisa G. Shaffer

in Chromosome Abnormalities and Genetic Counseling

Fourth edition

Published on behalf of Oxford University Press

Published in print November 2011 | ISBN: 9780195375336
Published online October 2012 | e-ISBN: 9780199975174 | DOI: http://dx.doi.org/10.1093/med/9780195375336.003.0009

Series: Oxford Monographs on Medical Genetics

Inversions

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INVERSIONS ARE INTRACHROMOSOMAL structural rearrangements. The commonest is the simple (or single) inversion. If the inversion coexists with another rearrangement in the same chromosome, it is a complex inversion. The simple inversion comprises a two-break event involving just one chromosome. The intercalary segment rotates 180°, reinserts, and the breaks unite (Fig. 9–1). The rearranged chromosome consists of a central inverted segment, and flanking distal, or noninverted segments. If the inverted segment includes the centromere, the inversion is pericentric; if it does not, it is paracentric. Figure 9–2 depicts two different pericentric inversions of chromosome 3. Note that the pericentric inversion has one break in the short arm and one in the long arm, whereas in the paracentric inversion both breaks occur in the same arm. Thus, when reading cytogenetic nomenclature, one can readily tell which is which: for example, 46,XX,inv(3)(p25q21) is pericentric and 46,XY,inv(11)(q21q23) is paracentric (inv = inversion). The clinical relevance of inversion chromosomes is that they can set the stage for the generation of recombinant (rec) gametes that may lead to abnormal pregnancy.

The heterozygote is, other things being equal, a phenotypically normal person. The reorientation of a sequence of genetic material apparently does not influence its function, and breakage and reunion at most sites do not perturb the smooth running of the genome. Some inversions of the X may be an exception to this rule: a breakpoint involving the X long arm within the “critical region” can cause gonadal insufficiency. Some pericentric breakpoints occur at preferential sites, including 2p13, 2q21, 5q13, 5q31, 6q21, 10q22, and 12q13 (Kleczkowska et al., 1987); and certain paracentric breakpoints are likewise overrepresented (Madan, 1995).

Chapter.  12240 words.  Illustrated.

Subjects: Clinical Genetics

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