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Chromosomal Abnormalities
Introduction
There are 22 pairs of autosomes and two sex chromosomes in humans, a total of 46 chromosomes. A male is described as having a chromosome set of 46 XY , a woman has a set of 46XX . This is how conventionally the nomenclature is handled.
Chromosomal abnormalities occur in about 0.4% of all births. They often cause mental retardation and are responsible for a number of congenital anomalies. Chromosomal abnormalities can affect the number or the structure of the chromosomes; they can also affect the autosomal chromosomes or the sex chromosomes separately. There can be missing chromosomes (deletions) and there can be micro deletions (also known as contiguous gene syndromes). In the following I will only mention the common chromosomal abnormalities. However, there are literally hundreds of syndromes that occur and that usually have the name of the person(s) that first described them followed by “syndrome”.
Unfortunately at this point in time there is no treatment for these complex genetic abnormalities. Sometimes the outer appearance can be partially corrected with plastic surgery, but the underlying genetic abnormalities cannot be changed .
Diagnosis
Lymphocytes that are isolated from the blood are used to diagnose a chromosomal abnormality. This test would not be done routinely, but only when the doctor suspects a genetic abnormality and this needs to be confirmed or ruled out. The lymphocytes are cultured together with phytohemagglutinin, which will make the cells divide. Colchicine is added to arrest the dividing cells at the stage when each of the chromosomes has divided into two chromatids, which are attached to the centromere. The cells are then stained with either the traditional Giemsa stain or a fluorescent banding stain technique. Further refining is possible with DNA probes containing fluorescent tags, which will help to locate genes or DNA sequences on a specific region of a chromosome. A useful technique is "fluorescence in situ hybridization" (FISH), where a double stranded DNA is broken down into single-stranded DNA. A probe of known amino acid sequence coding for a specific gene defect can then be introduced and this will attach to the affected DNA sequence by hybridization. Using fluorescent microscopy this band is made visible on the chromosome, which makes it easier for the geneticist to study the gene defect.
A geneticist who works with these tests all the time will be able to tell whether a set of chromosomes is normal or abnormal. Furthermore, with tests such as FISH it is possible to pinpoint the genetic defect of the following chromosomal abnormalities to a certain region within a chromosome, as will be explained in more detail for each of the conditions. In the following table use the links to find more details regarding the respective chromosomal abnormalities.
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