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Structural chromosome abnormalities 

Structural chromosome abnormalities
Chapter:
Structural chromosome abnormalities
Author(s):

Dieter Meschede

DOI:
10.1093/med/9780199235292.003.9084
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date: 09 December 2019

The term ‘structural chromosome abnormalities’ encompasses pathological alterations of chromosome structure that are detectable through microscopic examination of banded metaphase preparations (Chapter 9.3.5). It excludes smaller lesions diagnosable only with molecular genetic methods. Medium-sized genomic alterations, e.g. microdeletions demonstrable through molecular-cytogenetic methods such as fluorescence in situ hybridization (FISH), may also be classified as structural chromosome abnormalities. Some structural rearrangements, such as Robertsonian translocations and marker chromosomes, imply a change in chromosome number. By convention, they are regarded as structural and not numerical chromosome abnormalities.

Reciprocal and Robertsonian translocations, inversions, marker chromosomes, X and Y isochromosomes, and Y-chromosomal deletions are of practical importance in male endocrinology (Fig. 9.4.6.1) (1–4). Other classes of structural chromosome abnormalities such as rings, insertions, duplications, three-way and other complex translocations, fragile sites, and chromosome breakage syndromes (5) play no appreciable role in clinical andrology and are not further considered here.

The distinction between balanced and unbalanced structural aberrations is pivotal. The former are characterized by a deviation from normal chromosome structure without accompanying net loss or gain of genetic material. In contrast, the genome of a carrier of an unbalanced aberration is not fully diploid, but nullisomic, monosomic, trisomic, or higher aneuploid for an entire chromosome or parts of it. If no important gene is disrupted at the breakpoints, balanced structural aberrations exert no negative effect on general health. They are of clinical importance through their potential to adversely affect fertility, and to give rise to unbalanced karyotypes in the carrier’s offspring (5).

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