What does it mean to be ‘carrier’ of a recessive genetic disorder?
We all have two copies of every geneThe basic unit of inheritance. A gene is comprised of DNA, and contains the code for a specific product, usually a protein such as an enzyme.. An individual is referred to as being a ‘carrier’ if they have one copy of a gene that contains a potentially harmful genetic change from one parent and one normal copy from the other parent.
The human genome contains around 25,000 different genes. It is estimated that everyone is a carrier of around half a dozen rare recessive genetic mutationsStructural changes in a gene – they can be alterations to a gene’s size, arrangement, or molecular sequence. that could lead to disease – this is actually normal and not something to be uncomfortable about.
What is the significance of being a carrier of a recessive disorder?
The significance of being a carrier of a recessive gene is dependent on whether your partner is also a carrier. In most couples it is unlikely that partners will carry the same mutations as most recessive disordersA recessive disorder requires both parents to carry a specific disorder-related mutation and pass it onto their child for the disorder to be inherited. are exceedingly rare. However, when partners come from the same ethnic group e.g. Jewish, there may be some genetic conditions that are more common in that group.
Being a carrier of a recessive genetic disorder does not affect your own health because the one normal copy of the gene that you have compensates for the mutated copy. You can, however, pass the mutation on to your children. Children who inherit one copy of the mutated gene from one of their parents will be carriers. There is only a risk of a child being affected with the genetic disorder if they inherit the mutated gene from both parents.
If you are a carrier of a recessive Jewish genetic disorder e.g. Tay Sachs disease, your partner (either now or in the future) should be tested. If you both carry a mutation in the same gene then there will be an increased risk of having an affected child.
If your partner is not a carrier for the same recessive disorder, the risk of having an affected child will be very remote. There will, however, be a 1 in 2 chance of your child being a carrier, like yourself, but this will not affect their health.
What if both partners are carriers of the same recessive disorder?
Where both partners are carriers of the same disorder-causing mutation for a recessive condition, there will be a 1 in 4 chance of having an affected child.
Given the serious nature of many Jewish genetic disorders, it is important to understand the implications of being a carrier couple. It is advisable to be referred to your regional genetics service where you can meet with a genetic counsellor to discuss the implications and options available that may reduce the risk of passing on a disorder should you be planning children.
What is the significance of being a carrier of a dominant disorder?
Although the term ‘carrier’ is commonly used in reference to recessive disorders, it can also be used in the context of dominant conditions. Dominant genetic disorders are those in which a mutation in just one copy of the gene pair is required for the disorder to develop.
An individual who carries a mutation for a dominant disorder usually manifests the disorder and therefore tends to be known as being affected by, rather than a carrier of, that disorder. For some dominant disordersA dominant disorder requires only one copy of a specific disorder-related mutation to be passed onto their child for the disorder to be inherited, only a percentage of individuals who carry the mutated gene will develop the disorders e.g. DYT1 generalised dystonia and breast/ovarian cancers (BRCA 1 and 2).
Whether affected or not, an individual who carries the mutation for a dominant disorder has a 1 in 2 chance of passing the mutation onto their children – greatly increasing the risk of the children developing the disorder.
Any individual who carries a mutation for a dominant disorder is advised to meet with a genetic counsellor to discuss the implications regarding their own health and that of their children.
More information about recessive and dominant inheritance can be found in the introduction to genetics section.
What options are available to people who are at risk of passing on a genetic disorder?
A summary of the various options available for couples who are at risk of having a child affected with a recessive or dominant genetic disorder are outlined briefly below and can be explained further by a genetic counsellor:
Prenatal Diagnosis (PND):
Prenatal diagnosis, also known as prenatal testing, is performed during pregnancy to assess the health status of the foetus. The two main types of prenatal diagnostic tests are:
- Chorionic villus sampling (CVS) involves removal and testing of cells from the early placenta (chorion) surrounding a developing foetus. It is performed at around the 12th week of pregnancy and results tend to take 1 to 2 weeks to process.
- Amniocentesis involves removing and testing of cells from the amniotic fluid that surrounds the developing foetus. It is performed from around the 15th week of pregnancy and results usually take up to 2 weeks to process, although for certain tests results can take longer.
The two tests differ in timing, process and risk of miscarriage, however, the genetic testing procedure on the extracted sample cells tends to be the same for both.
If the results of the prenatal diagnosis find that the genetic disorder has been passed onto the foetus, further information and support are available to you from your genetic counsellor and also from an organisation called ARC (Antenatal results and choices).
Pre-implantation Genetic Diagnosis (PGD):
This is a technology which utilises in vitro fertilisation techniques (IVF or test-tube baby) to create an embryo in the laboratory using eggs and sperm collected from the couple. The embryos are tested for the gene in question, in order to identify the ones that will be affected with the genetic condition. Unaffected embryos can then be transferred into the womb, in the hope that a pregnancy will result which will not develop the genetic condition tested for.
In the UK there are a small number of centres that offer PGD treatment, see the HFEA website for details. Before being referred to a PGD clinic you should be referred to your regional genetics service to discuss all the options fully.
If both members of the couple are carriers then another way to avoid having an affected child is to use a sperm or egg from a donor individual who is not a carrier.
Some couples who are at risk of having a child with a genetic condition decide to adopt children thereby removing the chance that they will pass the disorder onto their children.
No (more) children:
In some situations couples feel that none of the above options are appropriate for them, and so they decide not to have children, or not to extend their family if they already have children.