Everyone has 23 pairs of chromosomes, which include 22 pairs of autosomes and one pair of sex chromosomes. The science that relates to the study of these chromosomes is referred to as cytogenetics. Trained cytogeneticists examine the number, shape, and staining pattern of these structures using special technologies. In this way, they can detect extra chromosomes, missing chromosomes, missing or extra pieces of chromosomes, or rearranged chromosomes.

Chromosome Analysis (Karyotyping)

Karyotyping begins with placing cells on glass slides and separating whole chromosomes from thenucleiof the cells. The slides are stained with special dyes and examined under a microscope. Then, pictures are taken of the chromosomes on the slides, and the picture is cut into pieces, so the chromosome pairs can be arranged and matched. Each chromosome pair is assigned a special number (from 1 to 22, then X and Y) that is based on its staining pattern and size.

Examining a persons whole chromosomes, called karyotyping, can diagnose a wide array of disorders.Down syndrome, in which an individual has an extra chromosome 21, can be determined by karyotyping studies. When there are three chromosomes in one group instead of a pair, it is referred to as a trisomy. Missing chromosomes can also be detected, as in the case ofTurner syndrome, in which a female has only a single X chromosome. When there is only one chromosome instead of a pair, it is referred to as a monosomy.

Sometimes, a piece of a chromosome will break off and attach to another chromosome. When this happens, it is referred to as a translocation or rearrangement. For example,chronic myelogenous leukemia (CML)is a disease caused by a translocation in which a part of chromosome 9 breaks off and attaches itself to chromosome 22 (BCRABL-1 fusion gene). Another example is Burkitt lymphoma, in which a piece of chromosome 8 attaches to chromosome 14. These chromosomal translocations cause disease because the broken piece usually attaches to the new chromosome near a special gene that then becomes activated and helps to produce tumor cells. Translocations can sometimes be seen under the microscope if a special stain is used via karyotyping.

Fluorescence in situ hybridization (FISH)

A special technique commonly called FISH for short can be used to view changes in chromosomes that result from genetic variations. A gene segment in a chromosome can be made to light up or fluoresce when it is bound by a special probe. By using more than one probe at once, cytogeneticists can compare to see if the probes are located in their normal positions or if they have moved to a new location on a different chromosome, or if there are more or fewer copies of a probe than in a normal cell.

Genetic changes in some cancers can be detected using this method. For instance, FISH is one of the methods used to determine increased copy number (amplification) of the gene ERBB2 (also known asHER2) in breast cancer. There are many other applications of FISH technology as well, such as detecting chromosomal deletions, in which a certain part of a chromosome is completely missing. In this case, the chromosome segment will not fluoresce compared to a normal set of chromosomes.

More here:
Genetic Testing Techniques | Lab Tests Online

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