Inversion

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I nve r s i o n

need to be incredibly long, sometimes greater than the size of the genome, as a consequence of which the method is not used.

Reference

Lake JA (1987) A rate-independent technique for the analysis of nucleic acid sequences: evolutionary parsimony. Molecular Biology and Evolution 4: 167±191.

See also: Phylogeny; Transition; Transversion Mutation

Inversion N Grindley Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0711

An inversion is a DNA rearrangement in which a segment of a chromosome is flipped (or reversed), so that the sequence reads in the opposite direction to the original. Genes contained within an inversion will map in the reverse order to normal and will be expressed in the opposite orientation. See also: Hin/Gin-Mediated Site-Specific DNA Inversion; Site-Specific Recombination

Inverted Repeats Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1881

Inverted repeats are two copies of the same DNA sequence repeated in opposite orientation in the same molecule. See also: Repetitive (DNA) Sequence

Inverted Terminal Repeats Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1882

Inverted terminal repeats are short related or identical sequences repeated in opposite orientation at the ends of some transposons. See also: Transposable Elements

Isochromosome M A Ferguson-Smith Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0717

An isochromosome is an abnormal metacentric chromosome formed by the duplication of one arm of a normal chromosome with deletion of the other arm. Both arms of the metacentric chromosome are thus genetically identical. It may arise from transverse instead of longitudinal division of the centromere during cell division or, more often, by an isochromatid break and fusion of the daughter chromatids above the centromere. In the latter case the isochromosome is dicentric. One of the two centromeres of a dicentric isochromosome usually becomes nonfunctional, so that the chromosome segregates normally during cell division. The commonest human isochromosome observed in livebirths is an isochromosome for the long arm of the X chromosome. This results in Turner syndrome (see Turner Syndrome), and it is found that the isochromosome is preferentially inactivated, forming larger than normal sex chromatin (Barr body; see Sex Chromatin). Isochromosomes of the Y chromosome are also found in livebirths, and can involve either the short or long arms. Short-arm Y isochromosomes cause male infertility as the testis-determining region is not lost despite the loss of spermatogenesis factors on the long arm. Long-arm isochromosomes of the Y are associated with female sex determination unless the isochromatid break lies distal to the sexdetermining region of the Y. Isochromosomes involving the human autosomes usually result in early spontaneous abortion; rare exceptions are isochromosomes for the short arms of chromosomes 9 and 12, and these are associated with severe mental and physical disability. See also: Sex Chromatin; Turner Syndrome; XChromosome Inactivation

Isolation by Distance N E Morton Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1426

Sewall Wright pioneered the study of how genetic similarity declines with geographic distance. His