What is a genetic test?

A genetic test can help identify if there is a faulty gene or chromosome that might cause a genetic condition. It is usually a blood or tissue test. There are a number of reasons why a person might take a genetic test.

Some of the reasons are listed below:

• You or your partner have a child with learning difficulties, developmental delay or health problems. The doctor thinks it might be a genetic condition.
• Your doctor thinks you may have a genetic condition and wants to confirm the diagnosis.
• There is a genetic condition that happens in your family. You want to know if you are at high risk of developing the condition during your lifetime.
• You or your partner have a genetic condition that might be passed on to your children.
• You have had another type of test that is done during pregnancy (such as an ultrasound, nuchal translucency scan or blood test). It has shown that there is an increased risk that your baby has a genetic condition.
• You or your partner have had a miscarriage or stillbirth.
• Particular types of cancer have occurred in several close relatives.
• There is an increased risk of having a child with a particular genetic condition because of your ethnic background. Examples of this include sickle cell disease in people of Afro-Caribbean descent, beta-thalassaemia in people of Mediterranean descent, cystic fibrosis in people of Western European descent and Tay Sachs disease in people of Ashkenazi Jewish descent. These conditions are more prevalent in these particular ethnic groups, but may occur in others.

It will not always be necessary to have a genetic test. They may be able to diagnose a genetic condition through a clinical examination, or tell you about your risk by looking at a detailed family history. Depending on the purpose of the test, genetic tests can be categorised as diagnostic tests, carrier tests or predictive tests.

Diagnostic testing

Diagnostic testing is used when someone is already experiencing symptoms to identify or confirm the genetic cause of these symptoms, if there is one. A doctor may already have a strong belief that the symptoms are caused by a specific condition, and use genetic testing to confirm this and identify the genetic fault causing the condition. In other cases the doctor may look at a large number of genes which can cause a particular set of symptoms, in the hope of finding out which gene or mutation is responsible. Knowing the molecular basis of a condition can sometimes help doctors target their treatment so that it is more appropriate to the patient.

Carrier testing

For some genetic conditions that need two copies of a faulty gene to cause a condition (recessive conditions), someone who only has one copy is called a carrier. Carriers of most conditions are themselves unaffected because the working copy of the gene overrides the faulty copy, but they can pass on a copy of the faulty gene to their children. Carrier testing allows you to know whether or not you and your partner carry a faulty copy of a gene, in order to make informed choices about having children in the future.

For chromosome translocations, a carrier is a person who has a balanced translocation whereby no chromosome material is lost or gained and is not usually affected by it.

Predictive testing

A predictive test can provide information about whether or not someone will develop or is likely to develop a specific condition, usually at a later stage in life. If a late onset genetic condition runs in your family, then you might decide that you would like to know whether or not you have inherited the faulty gene. We talk more about predictive testing here, as there are lots of factors to consider when deciding whether or not to have a predictive test.