Genetic Tests for Birth Defects
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Genetic tests for birth defects?
If your doctor has recommended a Genetic test for birth defects for you or your child to diagnose any birth defects, there are various tests available to go for. Mostly all the birth defects are related to chromosomes which are due to structural or numerical changes, but there other tests which can help in further diagnosis. Here are some of the genetic tests which can help you and your physician pinpoint the exact diagnosis of what kind of birth defects.
- Karyotyping is a genetic test used to study an individual’s chromosomes for any structural or numerical abnormalities. The chromosomes stained and viewed under a special microscope to detect any abnormalities, this process is called "karyotype". A normal female karyotype is written 46, XX, and a normal male karyotype is written 46, XY. The standard analysis of the chromosomal material evaluates both the number and structure of the chromosomes, with very high accuracy. Chromosome analyses are usually done from a blood sample (white blood cells), prenatal specimen, skin biopsy, or other tissue samples. In a karyotype, the chromosomes can look bent or twisted. This is normal and is a result of how they were sitting on the slide when the photograph was taken. Chromosomes are flexible structures that condense and elongate during different stages of cell division. If you unravelled all of the DNA that makes up the 46 chromosomes, you would find over 7 feet of DNA from one single cell.
Fluorescence in situ hybridization (FISH)
- FISH is a laboratory technique used to determine how many copies of a specific segment of DNA are present in a cell. It is also used to identify structural abnormalities of chromosomes. In the lab, a segment of DNA is chemically modified and labelled so that it will look fluorescent (very brightly coloured) under a special microscope. This DNA is called a "probe." The Probes then bind with a specific location in chromosomes and produces bright colour to identify the defects. Probes can find matching segments of DNA when added to cells under certain conditions. For example, if a baby is suspected of having Down syndrome and amniocentesis is done on the pregnancy, a FISH study can be performed on the cells found in the amniotic fluid. A probe made for chromosome 21 can determine how many copies of chromosome 21 the baby has. Under a special microscope, the cells from a baby with trisomy 21 or Down syndrome would contain three "signals" or three brightly coloured areas, where the probe matched up with the three #21 chromosomes. A FISH study does not replace a chromosome study but is done in addition to a standard chromosome study, depending on the birth defect in question.FISH can be used to detect structural chromosome abnormalities (such as submicroscopic deletions) that are beyond the resolution of normal chromosomal studies.
Chromosomal microarray analysis (CMA)
- CMA is a new laboratory test used to detect chromosomal imbalance at a higher resolution than current standard chromosome or FISH techniques. A sample of DNA from the individual to be tested are arranged in a particular order (array) on a glass slide. Fluorescent dyes are attached to the DNA samples, these fluorescent DNA then bind with sample chromosomes. These slides are then placed in a special scanner that measures the brightness of each fluorescent area. This process looks for the identification of a change in DNA copy number. The changes in copy number may indicate a chromosomal abnormality, such as a chromosomal imbalance, loss, or gain. Types of chromosomal abnormalities may include small chromosomal rearrangements, small duplication of chromosomal material (trisomy), or small deletion of chromosomal material (monosomy).*Note: Trisomy: - where 3 duplicate chromosomes are present, Monosomy:- where only one chromosome is present instead of 2 duplicate chromosomes.