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CHROMOSOME DISORDERS

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Alterations of the chromosomes (numerical and structural) occur in about 1% of the general population, in 8% of stillbirths, and in close to 50% of spontaneously aborted fetuses. The 3 × 109 base pairs that encode the human genome are packaged into 23 pairs of chromosomes, which consist of discrete portions of DNA, bound to several classes of regulatory proteins. Technical advances that led to the ability to analyze human chromosomes immediately translated into the revelation that human disorders can be caused by an abnormality of chromosome number. In 1959, the clinically recognizable disorder, Down syndrome, was demonstrated to result from having three copies of chromosome 21 (trisomy 21). Very soon thereafter, in 1960, a small, structurally abnormal chromosome was recognized in the cells of some patients with chronic myelogenous leukemia (CML), and this abnormal chromosome is now known as the Philadelphia chromosome.

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Since these early discoveries, the techniques for analysis of human chromosomes, and DNA in general, have gone through several revolutions, and with each technical advancement, our understanding of the role of chromosomal abnormalities in human disease has expanded. While early studies in the 1950s and 1960s easily identified abnormalities of chromosome number (aneuploidy) and large structural alterations such as deletions (chromosomes with missing regions), duplications (extra copies of chromosome regions), or translocations (where portions of the chromosomes are rearranged), many other types of structural alterations could only be identified as techniques improved. The first important technical advance was the introduction of chromosome banding in the late 1960s, a technique that allowed for the staining of the chromosomes, so that each chromosome could be recognized by its pattern of alternating dark and light (or fluorescent and nonfluorescent) bands. Other technical innovations ranged from the introduction of fluorescence in situ hybridization in the 1980s to use of array-based and sequencing technologies in the early 2000s. Currently, we can appreciate that many types of chromosome abnormalities contribute to human disease including aneuploidy; structural alterations such as deletions and duplications, translocations, or inversions; uniparental disomy, where two copies of one chromosome (or a portion of a chromosome) are inherited from one parent; complex alterations such as isochromosomes, markers, and rings; and mosaicism for all of the aforementioned abnormalities. The first chromosome disorders identified had very striking and generally severe phenotypes, because the abnormalities involved large regions of the genome, but as methods have become more sensitive, it is now possible to recognize many more subtle phenotypes, often involving smaller genomic regions.

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METHODS FOR CHROMOSOME ANALYSIS

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STANDARD CYTOGENETIC ANALYSIS

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Standard cytogenetic analysis refers to the examination of banded human chromosomes. Banded chromosome analysis allows for both the determination of the number and identity of chromosomes in the cell and recognition of abnormal banding patterns associated with a structural rearrangement. A stained band is defined as the part of a chromosome that is clearly distinguishable from its adjacent segments ...

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