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CHROMOSOMAL ANALYSIS: THE KARYOTYPE

A karyotype analysis involves the use of fresh viable tissue cells grown in culture and arrested in the stage of cell division known as metaphase. In metaphase, the chromosomes condense and become distinguishable from the other chromosomes, as they align in the center of the dividing cell. The process of karyotyping involves the pairing and ordering of all the chromosomes, providing a genome-wide view of the individual’s chromosomes. The karyotype can reveal missing chromosomes, extra chromosomes, or deletions, duplications, and translocations of parts of chromosomes. A common abnormality which can be identified by karyotyping is Down syndrome in which there is trisomy of chromosome 21. Other commonly identified disorders using karyotyping include Turner syndrome, Klinefelter syndrome, and fragile X syndrome.

FLUORESCENCE IN SITU HYBRIDIZATION (FISH)

Fluorescence in situ Hybridization (FISH) is a well-established technique for the detection of numerous structural genome abnormalities. These include deletions, insertions, and translocations associated with genomic material which is missing, redundantly copied, or moved to other locations within the genome. As such, this is a common technique used in cancer diagnosis because many cancers have well-known genome alterations. The figure depicts translocation of genes in a sample of cancer cells.

FISH proceeds by first creating a collection of short DNA sequences representing a target of interest, such as a gene which is commonly translocated in a specific cancer. These short sequences are called “probes,” and they are modified through the addition of a fluorescent label to each fragment. The principle of this test is that the fluorescently labeled probes will bind specifically to a region in the sample DNA to which they are complementary, and the presence of the associated fluorescent tag will indicate the presence, abundance, and chromosomal location of the target sequence.

MOLECULAR GENETIC TESTING: CLINICAL TESTING USING MOLECULAR GENETIC ANALYSIS

Molecular genetic analysis is used to investigate a variety of clinical questions. It can be used with tissue containing cancer cells to determine the mutations in a tumor. A blood sample, known as a liquid biopsy in an evaluation for the presence of cancer, may also be used to identify circulating tumor cells or the DNA associated with the tumor cells. Molecular genetic analysis can be used with blood cells to determine if there is an inherited genetic abnormality. Molecular genetic analysis can also identify genes relevant to abnormal drug–gene interactions. This diagnostic testing is known as pharmacogenomics. Some individuals are poorly responsive or hyper-responsive to medications based upon their own genetic composition. Finally, a sample which may contain infectious organisms can be studied with molecular genetics to assess for the presence and definitive identification of organisms.

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