Genetic testing
Does your grandmother or grandfather have heart trouble? Does a particular type of cancer run in your family?
Until recently, family histories were the only tools available for identifying an inherited disease. These histories can now be combined with genetic testing.
Genetic tests look at a person's genetic material: genes or DNA. Such tests can compare the base sequences in sections of DNA, look at the results of a change or mutation that is present in the DNA or examine the shape and structure of chromosomes.
The DNA is usually taken from a blood sample, but other body fluids or tissues may be used. The tests may look for predisposition to disease, or confirm a genetic mutation in an individual or family. As well as studying changes to chromosomes or genes, genetic testing also includes biochemical tests for certain proteins that indicate disease-causing gene variations.
Carrier testing can determine if a couple is ‘carrying’ a particular gene mutation for an inherited disorder (such as cystic fibrosis) that they may pass on to their children.
Predictive genetic testing focuses on tests that identify if someone will develop a disease before any symptoms appear, such as Huntington disease. These tests can be useful for early detection, diagnosis, prognosis and treatment (if available).
Genetic testing is also used to identify people with an increased risk or predisposition of developing a particular condition, such as certain cancers. This information may be useful in helping to prevent, treat or manage the disease, but it also raises many issues for our community.
Looking at DNA
Small changes in genes cannot be seen using a microscope. Other techniques are used to detect tiny changes in the DNA code. These usually involve extracting the DNA from a tissue sample, such as blood or saliva, from the person being tested and making many copies of the gene being tested for.
One method to detect changes is to cut the DNA into small fragments using restriction enzymes. The cut DNA samples are then compared with other samples from people with and without the particular mutation.
For some types of DNA tests, gene probes are used. These are short sequences of DNA that have base sequences exactly matching the gene change that is being tested for. If the altered sequence is present in a sample of a person's DNA, the probe will bind onto that piece of DNA, which indicates the presence of the disease-causing gene.
Currently, gene tests are available for a number of diseases, including: cystic fibrosis, some forms of haemophilia, muscular dystrophy, Huntington disease, thalassaemia and Tay-Sachs disease.
For information on gene probes, go to the What is Biotechnology? chapter.
For fact sheets on these genetic disorders, go to: http://www.genetics.com.au/factsheet/default.htm
Genetics Education, Murdoch Children's Medical Research Institute
Looking at chromosomes
Some genetic disorders occur due to changes in chromosome number and structure. These can be detected by examining a person's cells using high-powered microscopes. When cells divide, chromosomes reproduce themselves and then coil up into compact shapes to make cell division easier.
Genetics Education, Murdoch Children's Medical Research Institute
In this state (called metaphase) the chromosomes can be stained, photographed and arranged for easy identification and comparison. These photographs are called karyotypes.
Human chromosomal abnormalities - work sheet [PDF 386kb | 6 pages]
Genetics Education, Murdoch Children's Medical Research Institute
Sometimes, when eggs or sperm are being produced, the chromosome pairs do not separate properly resulting in an egg or sperm cell that has more or less than the usual 23 chromosomes. If an egg or sperm carrying 24 chromosomes combines with an egg or sperm carrying the usual 23 chromosomes, the result will be an individual with cells in which there are 47 chromosomes instead of 46.
An extra copy of chromosome 21 is responsible for Down syndrome and is called Trisomy 21. An extra copy of chromosome 18 results in Edward syndrome.
For more information on genetic conditions associated with chromosomal abnormalities, go to: http://www.genetics.com.au/factsheet/default.htm
When a gene code is altered - work sheet [PDF 46kb | 2 pages]
Zoom in to find where DNA is in the body - interactive
Find the gene responsible - interactive