There are many different types of genetic tests for identifying genetic diseases and defects. The scientific basis for all these tests is similar – identifying defects in the DNA. However, there are certain differences between the tests, such as in the resolution of defect identification, price range and accessibility.
Certain tests are condition-specific, and are designed to identify a specific disease, whereas others are more “general” tests – these are generally more expensive and more difficult to interpret.
These are the main genetic tests carried out today:
Karyotype tests are the oldest and simplest genetic tests, used to analyze the chromosomes.
The test includes creating a stained model of all the chromosomes in the cell. The test is usually carried out on dividing white blood cells.
Our body contains 23 pairs of chromosomes. Each pair comprises one chromosome that comes from the father and one that comes from the mother.
The karyotype test evaluates the number of chromosomes and can detect cases of chromosomal abnormality – having fewer or too many chromosomes.
For example, Down syndrome is a condition in which a person has an extra pair of chromosome 21. This can be detected using a karyotype test.
Turner syndrome is caused by a missing X chromosome (one chromosome instead of two).
Since karyotype testing includes staining of the chromosomes, it can detect chromosome inversion and other defects.
This test cannot detect individual mutations or small abnormalities, and it requires the interpretation of a specialist technician. The test can be performed on a sample of amniotic fluid or chorionic villus sampling, to detect chromosomal abnormalities. It can also sometimes be used to determine the child’s sex, in rare instances when the sex is unclear and a identification of the sex chromosomes is required.
A FISH test identifies specific genetic disorders and is more sensitive than a regular karyotype test.
The name of the test – an abbreviation for Fluorescent in situ hybridization – relates to its method of action. It uses the ability of DNA particles to chemically connect with other DNA strands. This process is also known as hybridization. In this test, pieces of DNA strands are marked with special fluorescent dye and are mixed with chromosomes taken as a sample from the body. When these dyed strands attach to the chromosomes, glowing dots show on the sample. This can allow identifying smaller chromosomal changes than those that can be identified in regular karyotype tests.
The FISH test is very useful in oncology, where it is used to identify genetic changes that appear in malignant cells. There are certain genetic disorders that cannot be identified with a karyotype test, in these cases a FISH test is necessary.
PCR tests (which stands for Polymerase Chain Reaction) are used to fully sequence segments of DNA, such as a gene or segments or different sizes.
In this test, enzymes are used to amplify (copy) the DNA numerous times. This large amount of amplified DNA can be quantified and examined. The copied DNA undergoes several divisions, which allow diagnosing the specific sequence they are made of. To multiply a certain segment, two single-stranded DNA pieces – known as “primers” – indicate the beginning and end of the segment that required multiplying, this creating the “edges” of the region to be copied.
This test is considered revolutionary in the medicine, and even earned its inventor a Nobel Prize.
PCR tests allow an accurate, fast, and relatively inexpensive sequencing of any selected segment of the genome.
It allows the identification of specific genetic defects, even single changes in individual nucleic acids in the DNA.
PCR tests are especially useful in diagnising the specific genetic disorder a patient is suffering, in cases where suspision arrises. The disadvantage of this test is that it enables testing a specific suspicious gene, and does not allow general testing for finding numerous genetic defaults.
Genome sequencing is the most through, and most expensive, genetic test. It enables to identify undiagnosed genetic diseases and to analyze the whole genome of the patient. Whole genetic sequencing is very expensive and is carried out only on rare occasions.
This test uses genetic chips that map DNA sequences using hybridization. It can identify specific changes throughout the entire genome.
Whole genome sequencing is targeted mainly when a genetic disease is suspected, but it is uncertain which gene to test and what disease to test for. Therefore, a PCR or FISH test will not be useful. For instance, when a child is diagnosed with an intellectual disability for no apparent reason, a whole genome sequencing can allow to identify the disease and point out which further genetic tests will be required for family members.
The main disadvantages of this test are its high cost, and the fact that it does not allow to distinguish between relevant and irrelevant changes in the DNA. There are many SNA changes that are not pathologically relevant, and an accurate analysis requires long and complex comparisons to existing databases.
The future of genetic testing will most likely see price reductions of full sequencing tests. Improvements in the field of bioinformatics will allow more efficient test analysis. This will lead to a new era of genetic testing.