“A type of test used for the detection of any changes in a DNA sequence, gene or chromosome which causes genetic abnormality called genetic test.”
DNA is a building block of every organism on earth. It is an inheritance unit, pass on genotypes to subsequent generations.
A functional stretch of DNA called a “gene”, only 3% portion of our genome in function, rest is non-function junk DNA.
The genes are located on chromosomes, 23 pairs of chromosomes are present in a typical somatic cell. Overall, all the DNA of us regulates our life.
Any change in a DNA sequence (called mutation), causes abnormal genotype and consequently results in abnormal phenotype.
A genetic test is used to detect mutations occurs in genes, DNA or chromosomes. Read more on mutation: Different Types Of Genetic Mutations.
DNA test and chromosome test are two common types of genetic test used not only for the diagnosis of disease but for other various purposes. For example, a DNA test is employed for DNA fingerprinting as well as HLA typing too.
In the present article, we will discuss the various type of DNA testing and its importance. the content of the article is,
- What is genetic testing?
- How is genetic testing done?
- Type of genetic testing
- Pros and cons of genetic testing
- Cost of genetic testing
Read our article on DNA: DNA story: The structure and function of DNA.
What is genetic testing?
Detecting mutations in either DNA or chromosome associated with a genetic disorder using a genetic technique is called genetic testing.
The genetic testing is also known as genome testing majorly used for diagnosis of inherited genetic disease, rare genetic conditions, chromosomal anomalies and cancer.
The genetic test is used for;
- Determining the reason for the occurrence of the genetic disease.
- For identifying mutations or changes associated with the genetic disease.
- Genetic status of an individual (whether he or she is normal, affected or carrier for particular disease).
- For identification of a gene associated with the disease.
- To determine the severity of the disease using gene expression assays.
Although many genetic conditions are now characterised, some are still unknown for us. For example, one extra chromosome (chromosome 21) is responsible for down syndrome, A point mutation is HBB gene causes sickle cell anaemia. But the exact reason for the occurrence of cancer is still unknown to us.
Fascinatingly, in recent days, genetic testing is practised in predicting genetic conditions.
Also, the chance of the occurrence of genetic disease in upcoming generations can be determined correctly.
The genetic testing is not a fully functional medical testing tool because not all the type of genetic conditions can be identified using genetic testing.
More than 2000 thousand genetic tests are now available for diagnosis of different genetic conditions.
We can say it is a part of genetic counselling where a genetic expert advice someone to go for a genetic test.
Moreover, genetic testing is a complex process, even a medical professional can not interpret results without taking help of a genetics expert.
Emotional, social issues and ethical issues are associated with genetic testing thus one must need a concern before doing a genetic test.
How is genetic testing done?
Depending upon what we want to study, two major types of the genetic test are employed; DNA test and chromosome test.
Both the test are different and are used for different purposes.
People consider the DNA test as only genetic testing but chromosome test is also one of the types of genetic test available. Now let’s understand how both the test works.
A DNA test is often known as molecular test or gene test used prominently for the diagnosis of the single-gene disorders.
The majority of DNA testing is based on the polymerase chain reaction.
It can even identify the genetic conditions whether the person is suffering from the diseases or he or she is only a carrier for the disease.
The very first step in DNA testing is DNA extraction.
Any of the bodily fluid of cells type are used for DNA testing. Blood, buccal swab, saliva, cell suspension, hair, amniotic fluid, chorionic villi, solid tissues or any other type of body tissue are used for DNA extraction.
Depending upon the type of cell, the methods for DNA extraction varies. Some of the common types of DNA extraction methods are enlisted in the article given below,
However, the main goal of DNA extraction is to provide high-quality DNA to perform a genetic test.
The extracted DNA must have purity nearby ~1.80 and quantity more than 100ng. Interestingly, It has one additional benefit over the chromosome test. DNA can be stored for a longer time.
Now, we have DNA, we can repeat the DNA test for a particular patient many times that can not possible in chromosome test.
Once the DNA is extracted, it is sent for the PCR amplification.
In a simple language, we can say that the polymerase chain reaction is a centre for molecular genetic testing. In almost, all molecular genetic techniques it required.
Single gene disorders like thalassemia, sickle cell anaemia and cystic fibrosis are now diagnosed only using the conventional gradient PCR.
The amplified DNA sample is now sent for DNA sequencing where the sequence of the DNA is determined.
Using DNA sequencing, known as well as unknown mutations associated with the particular genetic disorder are identified.
In addition to this, gene expression can determine using real-time PCR as well.
Gene expression: the amount of gene in particular tissue is estimated thus if the mutant gene is present, the amount of it can be estimated consequently, the severity of the disease can be measured.
The chromosome test is used for the detection or identification of numerical or structural abnormalities associated with chromosomes.
We have already discussed about the chromosome test in our previous article. Please read it here: Chromosome test.
Here I am only giving the outline of it,
The sample is proceeding for the cell culture aiming to get good amount of metaphase cells.
Once the cells are grown, banding is performed to identify the chromosomes.
The expert then arranges the chromosomes according to their numbers and trying to identify any changes in it.
The bunch of several genes deleted or inserted on a particular chromosome can be distinguished by chromosome banding.
Furthermore, numerical changes can also be encountered.
However, a lot of experience and expertise required to interpret the results of the chromosome test.
Type of genetic tests:
Disease diagnosis testing:
Chromosome test, as well as the DNA test, is used for the diagnosis of disease. Inherited genetic disease and rare abnormal conditions can be determined.
New mutations or changes related to the disease can also be identified.
Based on the phenotypic symptoms a particular test recommended by the geneticist. Diseases like sickle cell anaemia, thalassemia, Huntington’s disease, cystic fibrosis, cry-du-chat syndrome, down syndrome, patau syndrome, Edward syndrome etc are now being treated using the genetic testing based diagnosis.
[DNA test: sickle cell anaemia, thalassemia, Huntington’s disease and cystic fibrosis.
Chromosome test: cry-du-chat syndrome, down syndrome, Patau syndrome and Edward syndrome.]
Even several types of cancer are now diagnosed using the genetic testing method. For example, chronic myeloid leukaemia is now diagnosed using the chromosome test while breast cancer or cervical cancer are now diagnosed using molecular genetic methods.
A carrier is a genetic condition in which a person carries one copy of a gene but remains normal.
The sign and symptoms of the disease are not observed in a carrier, nonetheless, it can pass a disease gene to the next generation.
If the father and mother both are a carrier for one particular disease, there is a possibility that the fetus may carry the disease.
Genetic testing helps in identification of carriers as well as disease conditions.
Prenatal genetic testing gives the power to us for deciding the fate of the fetus. Before birth, using genetic testing, one can determine whether the fetus develops the disease, remains normal or carrier.
For the high-risk pregnancy or families having the previous history of any genetic abnormalities must go for genetic testing.
Structural and numerical chromosomal abnormality, as well as gene or DNA mutations, can be identified.
For prenatal diagnosis, the amniotic fluid or chorionic villi sample is collected for genetic testing, which proceeds either for cell culture, DNA extraction or both.
A drop of blood collected on the paper from the newly born baby and send for the DNA testing. The DNA is extracted from the sample and different genetic test are being performed on it.
In addition to this, several biochemical tests are also performed.
It is required in case of the genetic disease treated during the prenatal stage of the pregnancy.
Newborn screening for phenylketonuria, sickle cell anaemia, congenital hyperthyroidism are nowadays compulsorily done in almost every country.
If any genetic abnormality occurs treatment for that started earlier.
Predictive genetic testing:
By doing the pedigree analysis of a family, a genetic condition of the upcoming generation can be predicted and doing a genetic test based on that one can confirm, either the fetus remains normal or suffers from the disease.
This type of genetic testing is called presymptomatic or predictive genetic testing.
By identifying a particular mutation associated with the disease, one can actually predict the chance of occurrence of disease. It is often used in the prediction of cancer.
Preimplantation genetic testing:
The preimplantation genetic testing is used during the in vitro fertilization. A few cells are taken from the growing embryo and a DNA test is performed to detect genetic abnormalities if any.
Preimplantation genetic testing is one of the sensitive methods because we have only a couple of cells to perform DNA extraction.
Thus, instead of wasting it on different PCR-based diagnosis method, the DNA sample is directly sent for DNA sequencing or DNA microarray analysis.
Both techniques have the power to screen thousands of mutation in a single run. Notably, preimplantation genetic testing is one of the growing fields of genetics.
Pharmacogenetics is also a trending application of DNA testing in which a DNA test helps in determining which medicine and how much dose is effective for some disease.
Besides these DNA test is also used for several non-diagnostic application:
One of the important types of DNA testing is DNA fingerprinting used in the forensic analysis and identification of criminals.
DNA is someone’s unique identity, no two people in the world have a similar genetic profile.
A sample is collected from the crime scene (it might be hair, blood or any body fluid). The DNA of it is matched against the DNA of the suspected candidates.
Thus, it actually used for the identification of a person and used to establish a biological relationship between people.
Detection of contamination:
A DNA test has the power to distinguish two different cell population, if mixed or contaminated.
The DNA test is applied to the detection of maternal cell contamination.
One of the fascinating application of DNA test in recent days is to determine one’s ancestry or ethnicity.
Pros and cons of genetic testing:
The genetic testing is one of the best diagnostic method available now, yet it has several advantages as well as disadvantages.
Pros of genetic testing:
The genetic testing method is unmatched, no other diagnostic method is more effective than genetic testing.
Inherited as well as rare disorders are identified using the genetic testing method.
The genetic testing method is a more accurate and reliable method.
DNA testing method is rapid and gives results within a day, however, chromosome test takes 3 to 4 days for giving results.
A physical risk associated with genetic testing is very less.
The test results give a sense of relief for the families having a genetic test and help them to make a decision for their child.
Prevention is one of the important tasks, by doing the prenatal genetic testing, an inherited genetic disease can be prevented by aborting the affected child, however, the decision entirely depends on the parent.
A genetic test is a “lifesaver test”, preventive actions can be taken earlier if the results are positive.
Nonetheless, the negative results eliminate the need for unnecessary screening tests. Hence saves money.
Using the DNA test one’s ethical background can establish.
In a nutshell, a genetic test makes families happy.
Cons of genetic testing:
The chromosome test is time-consuming and the chance of results failure is also high in it.
The genetic testing is a costly diagnostic method.
Ethical, social, emotional and financial issues are involved in genetic testing. If someone’s baby is positive for a genetic test, he or she may felt guilty and undergo depression.
The use of invasive sample collection method makes it restricted to use.
Collecting amniotic fluid or chorionic villi carry the risk of pregnancy failure.
One of the major limitations of genetic testing is uncertainty.
A positive result does not always mean that the person may suffer by the disease, similarly, a negative result does not mean that the person is safe, or he or she may not suffer by the disease.
Related article: Common FAQs- Chromosome Test.
Cost of genetic testing:
The cost of genetic testing varies from test to test, some tests is too costlier, for example, whole-genome sequencing or whole chromosome microarray while some are cheap.
The test cost for a genetic test 100$ to 2000$ or more.
On an average, the cost of chromosome test is between 42$ to 50$ (3000 to 5000 INR), while the cost of the DNA test is between 50$ to 500$ (3500 to 35,000 INR).
I advise all to go for a genetic test once before the marriage, who knows you might be a carrier for some genetic disease. Respect the decision of your genetic counsellor, in case of positive results, take action with discretion.