Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
A note about DNA, genes and chromosomes
Your body is made up of millions of tiny cells. Different types of cells form the different structures of the body – skin, muscles, nerves, organs, etc. In the centre (nucleus) of most cells in your body you have 46 chromosomes arranged in 23 pairs. One chromosome from each pair comes from your mother and one from your father. Chromosomes are made of DNA. DNA stands for deoxyribonucleic acid. DNA forms your genetic material.
A gene is the basic unit of your genetic material. It is made up of a sequence (or piece) of DNA and sits at a particular place on a chromosome. So, a gene is a small section of a chromosome. Each gene controls a particular feature or has a particular function in your body. For example, dictating your eye colour or hair colour, making all the various proteins in your body, etc. Each gene is part of a pair. One gene from each pair is inherited from your mother, the other from your father. Each chromosome carries hundreds of genes.
Humans have between 20,000 and 25,000 genes altogether. The total of all your genes is called your genome.
As our cells are multiplying all the time, our genetic information needs to stay the same. Normally, there are excellent mechanisms in place to make sure each cell gets the exact same copy of DNA, the material that makes up our genes. But sometimes the copying mechanism makes mistakes or other problems can occur with your genetic material. Problems and abnormalities in genes can lead to genetic diseases.
What is genetic testing?
What is gene testing?
Gene tests look for abnormalities in DNA taken from a person’s blood, body fluids or tissues. The tests can look for large mistakes such as a gene that has a section missing or added. Or tests can look for small changes within the DNA. Other mistakes that can be found include genes that are too active, genes that are turned off, or those that are lost entirely.
Genetic tests examine a person’s DNA in a variety of ways. Some tests use probes. These are short strings of DNA which form the other half of an altered gene. Probes look for DNA which matches up with this altered gene within a person’s genome. If an altered gene is found, the probe binds to it, identifying the alteration.
Another type of gene test directly compares the sequence of DNA in a person’s gene to a normal version of the gene.
What is chromosome testing?
Chromosome tests look at features of a person’s chromosomes, including their structure, number, and arrangement. These tests look for changes, such as pieces of chromosomes being switched or being in a different location.
Types of chromosomal tests include:
- Karyotype testing – this test gives a picture of all of a person’s chromosomes from the largest to the smallest.
- Fluorescent in situ hybridisation (FISH) analysis – this can find small pieces of chromosomes that are missing or have extra copies. These small changes can be missed by the karyotype test.
There are also tests which can detect low levels of proteins. This may indicate that the gene which makes the protein is faulty.
Why is genetic testing done?
Genetic testing can be done for many different reasons, including to:
- Give a diagnosis if someone has symptoms suggestive of a genetic disease.
- Show whether a person is a carrier for a genetic disease. Carriers have an altered gene, but don’t have the disease. However, they can pass the altered gene on to their children, who then have a chance of inheriting the disease.
- Help expectant parents to know whether an unborn child will have a genetic condition. This is called prenatal testing.
- Screen newborn infants for abnormal or missing proteins that can cause disease. This is called newborn screening.
- Show whether a person has an inherited disposition to a certain disease before symptoms start.
How is genetic testing done?
Genetic testing usually involves taking a sample of blood or tissue.
In adults and children this usually involves taking a blood sample from a vein.
In pregnancy, a sample may be taken from the baby by amniocentesis or chorionic villus testing. In amniocentesis a sample of the liquid (amniotic fluid) that surrounds a baby is taken. It is done by putting a needle though the abdomen (tummy) into the uterus. In chorionic villus testing a sample of part of the placenta is taken. This is either done by inserting a needle into the abdomen like in amniocentesis or by putting a thin tube into the cervix (neck of the womb). Both tests involve a small risk that you may suffer a miscarriage as a result of having the test. If you are offered these tests doctors will discuss the risks involved to help you to make a choice about whether to have the test or not.
In newborns, routine screening for conditions such as phenylketonuria happens as part of a baby’s newborn check-up.
After the sample has been taken it is sent to the laboratory for testing. It may take some time for the results of all the tests to come back. Your doctor should advise you how long the results will be.
What about genetic counselling?
The information obtained from genetic testing can have a profound impact on your life. Genetic counselling is available to anyone undergoing, or thinking of undergoing, any form of genetic testing. Genetic counselling is not a psychological therapy. It aims to provide you with all the information you need to make a decision about whether you should have a genetic test.
This may include information about:
- The risk of inheriting or passing on a genetic condition.
- The implications of testing positive for the condition, including the psychological impact and other consequences.
- Whether to inform relatives of your intention to test, or not to test.
- The usual pattern of progression of the disease you are being tested for and its potential treatments.
Post-test counselling is also available to help you deal with the results of the test.
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