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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/9780195375336.003.0017

Series: Oxford Monographs on Medical Genetics

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THE MICROSCOPE has long been the classical instrument of the cytogeneticist, and large-scale variation in the human genome has been known since the early days of the discipline. As identified by the examination of banded chromosomes, variation can be normal or abnormal. Normal variation includes rearrangements or visible differences that are seen in the general population, and which are without clinical significance. These include the common pericentric inversion of chromosome 9, and the morphological and staining differences between homologs of the acrocentric short arms, and of the pericentromeric heterochromatin of certain chromosomes, and these have been discussed in the preceding chapter. Abnormal variation includes balanced rearrangements such as inversions, reciprocal translocations, and Robertsonian translocations, and unbalanced rearrangements such as derivative chromosomes and supernumerary marker chromosomes. Thus, the traditional study of human chromosomes with banding techniques has identified large-scale genomic changes, typically referred to as chromosome abnormalities, for over 40 years.

The new instrument of the twenty-first century cytogeneticist is the microarray (as described in detail in Chapter 2). This new approach has revealed a new layer of variation: genomic variation ranging from a few to several million base pairs of DNA, in clinically normal individuals. The interpretation of such variation can be that these are normal, abnormal, or of unclear clinical significance. This chapter will explore normal genomic variation and discuss when the lines blur between normal and clinically abnormal, as well as what tools are useful for distinguishing this variation.

Chapter.  3761 words. 

Subjects: Clinical Genetics

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