Two breaks on different arms of a chromosome
Figure 3.8 Continued
Two q arms p arm
Centromere q arm p arm
Centromere q arm
Maldivision of the centromere
Two p arms
Abnormal U-type exchange between sister chromatids next to centromere
Figure 3.8 Intrachromosomal rearrangements. (A) Schematic examples are shown of some large scale chromosomal rearrangements involving either two breaks in the same arm of a chromosome or two breaks on different arms of a chromosome. Redrawn and adapted with permission from Strachan and Read (2004). (B) Generation of isochromosomes. Redrawn and adapted from Shaffer and Lupski (2000) with kind permission of Annual Reviews.
of about one in 13 000 people. Among those individuals with Turner syndrome, the percentage with an i sochromosome of Xq is high at 15%. The isochromosomes are characteristically dicentric with the breakpoint in the proximal short arm of the X chromosome, and may be of maternal or paternal origin (James et al. 1997). Any of the acrocentric chromosomes have also been found to form isochromosomes and these underlie a significant proportion of rearrangements seen in spe cific syndromes, for example Patau (chromosome 13), Prader-Willi or Angleman (chromosome 15), and Down syndrome (chromosome 21) (Shaffer and Lupski 2000).
Cytogenetics has been fundamental to our understanding of human genetic variation. At a microscopic level,
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