Genetic screening is emerging as a and likely to raise many questions for parents. Below are frequently asked questions regarding genetic screening.
Genetic screening refers to a particular type of genetic testing whereby a person's DNA is analysed to check for specific genetic diseases, or susceptability to specific genetic diseases. It is called genetic 'screening' rather than testing because it is carried out on whole populations of people, not just the individuals who are considered to be at higher risk of having a particular genetic condition (e.g. because they have a family history of it).
Genetic screening tests to identify 'carriers' of Thalassemia are already offered to all pregnant women, and in certain high risk areas, Sickle Cell Anaemia and Haemoglobin disorders are also genetically screened for. A 'carrier' is someone who can pass the condition on to their offspring, but who does not show symptoms of the condition themselves. If a pregnant woman is found to be a carrier of Thalassemia Sickle Cell Anaemia or a Haemoglobin Disorder, the baby's father may also be tested. If he is found to be a carrier as well, the foetus may then be tested using a prenatal diagnostic test.
Newborn babies can also undergo genetic screening. There are three inheritable conditions that newborn babies can be genetically screened for: Phenylketonuria (PKU), Cystic Fibrosis and Sickle Cell Anaemia through a 'heel prick' or 'blood spot' test whereby a small amount of blood is taken from the heel to be genetically tested. These tests can identify those babies who are carriers of these conditions as well as identifying those babies who will go on to develop them.
Genetic screening is different to some of the other screening tests offered in pregnancy, e.g. Down's Syndrome screening, as genetic screening involves the analysis of genes, whereas Down's Syndrome may be identified through a blood test (which looks for particular hormones and proteins rather than genes) and an ultrasound scan which measures the nuchal translucency (fluid at the back of the foetus' neck). Whilst Down's Syndrome is caused by the presence of an additional chromosome, it is not generally considered to be an inheritable condition, meaning that it does not run in families in the same way as other genetic conditions and so there is no screening for genetic 'carriers' of Down's Syndrome.
Screening can be done at different times in a person's life. For example, newborn babies may be screened to see if they will develop a condition, or if they are 'carriers' of a condition. This is called newborn screening. However, adults may want to be screened before they have children to see if they have a chance of passing on a genetic condition to their children. This is called pre-conception genetic screening and is only available for particular conditions and in specific populations.
Another type of genetic screening is called antenatal genetic screening and this type of screening is done for women who are already pregnant to establish whether they are 'carriers' for a particular genetic condition. If the pregnant woman is found to be a carrier of a specific genetic condition, further genetic testing may be done to establish what the chances are of the foetus having the genetic condition.
The UK National Screening Committee (the UK agency that advises government ministers on the viability, effectiveness and appropriateness of screening programmes) announced in May 2013 that it is reviewing its policy on carrier screening for Spinal Muscular Atrophy, a neuromuscular condition causing varying degrees of muscle weakness. Spinal Muscular Atrophy is a autosomal recessively inherited condition, meaning that both parents need to be 'carriers' of SMA for a child to have it. It is possible to identify most carriers of SMA through a blood test and genetic analysis. It is estimated that anywhere between 1 in 40 and 1 in 60 people in the UK population are carriers of SMA, and when two carrier parents reproduce, they have a 25% chance with each pregnancy that the resulting child will have SMA.
SMA has a very variable presentation; in its most severe form, the condition can be fatal in the first two years of life. However, in its milder forms, SMA symptoms may not onset until adult life and lifespan may not affected. Genetic testing cannot currently tell parents how severely affected their child will be, only whether or not they have inherited it. There is no cure or effective treatments currently available for SMA, and when a prenatal diagnosis of SMA is made, termination of the pregnancy is usually offered.
The UK NSC is currently considering whether antenatal carrier screening (screening of all pregnant women) would increase parents' reproductive choices and enable them to make informed decisions about having a child with SMA. However, as with all screening programmes, there are many issues that need to be carefully considered before a decision can be reached about whether screening for SMA is recommended at this time.
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