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Rosie enrolled for the Cambridge Immerse Biology Programme in 2015. She is now studying Natural Sciences at Emmanuel College, University of Cambridge.
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Would you want to know if you would succumb to a genetic disease in a few years? Would you rather not know? Genetic testing is a rapidly advancing field and is becoming increasingly common in both prenatal and neonatal screening. Testing allows the identification of genetically inherited conditions as well as susceptibilities to other diseases.
There are three different types of genetic testing that I will discuss in this article: diagnostic, carrier and predictive testing. Diagnostic testing involves testing to identify current disease states, e.g. prenatal and newborn screening. Carrier testing determines whether an individual carries a particular trait – this is often done before having children to see if there is a risk of both parents passing a recessive allele to their child. Predictive testing is used to determine if an individual has a mutation that makes them is particularly susceptible to a late-onset disorder.
Prenatal screening allows parents to test embryos/ fetuses to see if the future baby is likely to have specific birth defects. The tests are usually performed in the first and second trimesters, but some can be done in the third. The screening can only show a risk or probability of a disease. If there is a positive result, further diagnostic tests can be done to provide a definitive answer. These diagnostic tests (e.g. amniocentesis) are more invasive than genetic screening and can carry a small risk of miscarriage.
A positive diagnostic test for a condition such as Down syndrome or neural tube defects often leads the parents to consider an abortion. Abortion carries its own range of ethical issues that I shall not discuss in this article, but are something each parent has to consider for themselves. What the parents need to decide at this stage is if living with such as condition would be worse than not living at all. A genetic test can confirm that the fetus has the genetic disease, but it cannot determine the severity nor the impact on future lifestyle.
Screening of a newborn does not carry the same abortion-related problems of a prenatal screening. There is, however, a significant question of whether the parents have the right to know about their child’s “genetic destiny”. In some cases, it might be crucial in allowing preventative measures to be taken. For example, the recessive genetic condition phenylketonuria (PKU) results in an inability to break down the amino acid phenylalanine. This builds up in the blood and brain and can cause brain damage. However, it can be treated simply through a change in diet. Individuals with PKU who avoid foods with high levels of protein and aspartame are able to live normal, healthy lives.
On the other hand, there are many genetic conditions for which there is no treatment or cure. Should parents test for diseases where there is no treatment in their child if it makes no difference anyway?
This type of screening determines whether an individual is a carrier for a genetic disease. Each person has two copies of each chromosome – one from each parent. If a genetic disorder is dominant, the disease will be expressed, irrelevant of the allele on the other chromosome. These conditions are surprisingly rare as they are selected against by natural selection. This is because the presence of the disease-causing allele will significantly reduce that individual’s chance of reproducing and therefore it is less likely to be passed on to the next generation. Since the effect of the allele is never masked by another allele, its frequency in the population will decrease quickly as it does not get passed on through reproduction.
Recessive genetic conditions are more common. The disease state will only be expressed if the individual has inherited the disease-causing allele from both parents. When only one allele is inherited, that person is known as a carrier as they don’t have the disease themselves, but they could pass it on to their children.
Carrier screening is often done before a couple have a child to check to see if there are any recessive alleles that they might both pass to the child that would result in a genetic disease. Embryo screening is a possibility for parents where there is a chance that their child will have a genetic condition. This is where the embryos are screened for abnormalities and particular allelic combinations before implantation into the mother through IVF.
This is where individuals test to see if there is a risk of them inheriting a late-onset genetic disorder (e.g. Huntington’s disease) or are particularly susceptible to some diseases such as breast cancer.
Some combinations of alleles can make you particularly susceptible to getting some diseases. People with mutations in BRCA1 and BRCA2 genes have a much higher chance of getting breast cancer. It is, however, only an increased risk of getting it rather than a certainty. If you know that you have a susceptibility to breast cancer, there are preventative measures you could take such as a mastectomy. Removal of breast or ovarian tissue based on a test that is predictive, rather than diagnostic avoids potential future harm but could be unnecessary.
Huntington’s disease (HD) is an example of an adult-onset disease. HD is a dominant genetic condition which has no effect until the individual is well into adulthood. If you have a parent that had HD, there is a 50% chance that you will also have it, but if you have inherited it, you will certainly get the disease. Deciding whether or not to have a genetic test to see if you have inherited the HD allele is a very personal decision and the testing of children is strongly discouraged since it should be their own decision. Choosing to have a genetic test can help some individuals to plan their life more realistically in terms of reproduction and career planning. Many people would simply rather not know and are concerned about the psychological effect of knowing as well as possible discriminatory biases in their jobs and in getting health insurance.
The accuracy and capacity of genetic screening are improving all the time. It won’t be long now until genetic testing will be able to reveal far more than potential diseases. There are many ethical concerns over the ability to test for things like intelligence and sporting ability. Germ-line gene editing is illegal in almost all countries but there must also be careful legislation to stop some people from aborting embryos with characteristics that they don’t like. Policy-makers must be careful to balance personal freedoms with the protection of vulnerable individuals and be wary of the impact of genetic testing being taken as a full prediction of life outcome. If employers, insurers and other groups with access to your genetic data started making decisions based on your predicted future it could dramatically disadvantage individuals through no fault of their own.