Carrier Testing
WHAT IS A CF CARRIER?
People are born with cystic fibrosis; it is a genetic disorder. Inherited characteristics- such as eye and hair colour - are passed from parents to their children through their genes. Everyone possesses two copies of every gene: one copy comes from the mother, and one from the father. Scientists estimate that there are about 100,000 genes in all, and they are close to mapping, i.e. identifying all of the genes in the human body. Genes are organized into 23 pairs of chromosomes, and are present in each cell of the body.Cystic fibrosis is caused by a gene which is defective on chromosome 7. The disease is present when a child receives two copies of the defective gene, one from each parent.
Any person with one copy of the defective CF gene, and one copy of an unaffected gene, is a CF carrier. Carriers are not affected themselves by cystic fibrosis. However, if their partner is also a carrier, there is a one in four chance with each pregnancy that the baby will be born with cystic fibrosis.
Approximately 1 in every 25 Canadians carries the defective gene which causes CF.
WHO SHOULD BE TESTED?
At present, the Canadian College of Medical Geneticists recommends that carrier testing for cystic fibrosis be available for anybody with a family history of CF, or for those whose partners have a family history of CF, or for a medical condition (such as male infertility).In general, carrier testing is performed with family planning in mind. Since carriers are unaffected by the disease, the fact that they are carriers is relevant only if they - or their grown children - wish to have children. A genetic counselor can help you decide whether or not you would like to be tested.
HOW RELIABLE IS THE TEST?
Since the advent of new genetic technologies, and following the discovery of the CF gene in 1989, it has become possible, in most families, to detect the defective gene through laboratory tests using blood samples. Samples are sent to specialized molecular diagnostic laboratories for analysis. There are kits available on the market where people mail in mouth swabs but they are currently not widely used in Canada.
Genetic tests cannot yet detect all CF carriers. Currently, there are over 850 mutations, and medical diagnostic laboratories test for a range of the most common versions, between 15 and 30. In addition, the mutations being tested may vary somewhat between laboratories.
Detection rates may vary between countries, and even from one region to another, depending on the ethnic origins of the population. For example, in Canada at the present time, laboratory test can detect approximately 85% of CF carriers. In certain populations and ethnic groups, however, it is possible to detect over 95% of all carriers.
WHAT DOES A NEGATIVE TEST MEAN?
A negative test result means that your likelihood of being a CF carrier is very low. However, due to the limitations of the test, and because not all the defective genes can be identified, there is still a slight possibility that you may be a carrier.
WHAT DOES A POSITIVE TEST RESULT MEAN??
A positive test result means that you are a CF carrier. You will never get cystic fibrosis yourself, but you could have a child with CF, if and only if, your partner is also a CF carrier or your partner has cystic fibrosis.If your test result is positive and you are thinking of having a child, your partner should also be tested. If your partner’s test result is negative, and your own is positive, there is still a possibility of having a child with CF, because there is a small chance that your partner carriers a version of the defective gene which cannot be detected through testing.
WHAT IF MY PARTNER AND I BOTH TEST POSITIVE??
Each child produced by two carriers has a 1 in 4 risk of having cystic fibrosis. The possible outcomes of each pregnancy are as follows:
- 1 in 4 (25%) chance of a child who does not have cystic fibrosis, and is not a CF carrier.
- 1 in 2 (50%) chance of a child who does not have cystic fibrosis but does carry one defective gene, and is therefore a CF carrier but will not be affected by CF.
- 1 in 4 (25%) chance of a child who has two defective genes, and therefore has cystic fibrosis.
