Genetic testing can be used to diagnose genetic conditions. There are a number of different types of genetic testing that are outlined below. Testing is done by a clinical genetics department in a specialist genetics lab.
Diagnostic testing involves testing a fetus or a person for a suspected genetic condition. We can test a fetus for a genetic condition via amniocentesis. An example of this is antenatal testing for Down’s syndrome. Antenatal testing can have implications on the decision to continue the pregnancy. Where a specific condition is suspected, for example Turner syndrome, it is possible to test directly for that condition in a child or adult.
Predictive testing involves testing a person for a specific gene mutation that has implications for them in the future. Examples are the BRCA1 breast cancer gene or the gene for Huntington’s chorea.
Carrier testing involves testing parents or potential parents for the gene for a specific autosomal recessive condition in order to calculate the risk of passing it to their children. An example of this is testing for the cystic fibrosis gene.
Other Specific Scenarios
- Genealogical testing
- Forensic testing
- Paternity testing
There are significant ethical implications for genetic testing. It is essential to get consent and perform some level of genetic counselling before doing the test, and discuss the implications of the result. The greater the implications of the test, the more genetic counselling will be required.
For example, if a patient’s parent suffered with Huntington’s chorea, there is a 50% chance they have the gene and will inevitably develop the same highly disabling condition. Having a test that tells you whether or not you will definitely develop this condition is very different to getting a simply cholesterol check. It is important that the person is fully informed about the implications of the results, not only for them but also their family.
Karyotyping involves looking at the number of chromosomes, their size and basic structure. This is helpful in diagnosing conditions like Down’s syndrome (trisomy 21) and Turner syndrome (45 XO).
Microarray testing involves cutting up the genetic material from an individual using enzymes. Different genes will have different molecular weights. The chopped up genetic material is then applied to a plate that separates molecules of different weights into different locations. This can be used to see what genes the person expresses. For example, if you know that the gene for cystic fibrosis is a certain size, and when this gene is chopped out and applied to the plate it ends up in a specific location on the plate, you can test an individual to see whether they have a clump of molecules at that location. If they do, this suggests they are expressing that gene.
This has many applications, such as screening for chromosomal abnormalities and many common genetic conditions, looking for mutations in cancer cells and also for research aimed at matching genes with phenotypes.
Specific Gene Testing
Specific gene testing can be done by splitting the two strands of DNA and adding a “gene probe”. The gene probe is made of single stranded DNA that contains complementary genetic code for a specific gene you want to test for. When the strands of DNA are mixed with the gene probe and the gene probe matches the genetic material on the DNA, they will stick together. This suggests the specific gene that matches the gene probe is present. This is used to confirm whether a patient has a particular gene.
DNA sequencing is only used for research purposes, and has no role in routine clinical practice. This involves splitting the two strands of DNA and watching as individual nucleotides are added to a single strand of DNA, ultimately revealing the exact sequence of nucleotides in that section of DNA.
Last updated January 2020