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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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