Genetic Screening (Edexcel International A Level Biology): Revision Note

Uses of Genetic Screening

• Some circumstances, e.g. in a pregnancy where there is a family history of a genetic disorder, may require individuals to determine if they have a particular allele present in their genome

• This can be determined by genetic screening

• There are three main uses of genetic screening

  • Identifying individuals who carry an allele at a gene locus for a particular disorder

  • The screening of embryos prior to implantation during fertility treatment; this is Preimplantation Genetic Diagnosis (PGD)

  • Testing a foetus before birth; this is prenatal testing

Identification of carriers

• Carrier testing is offered to individuals with a history of genetic disorders in their family

• It can show whether people who have no symptoms carry the allele for particular disorders, such as cystic fibrosis

  • Cell samples can be extracted from, e.g. blood or saliva, before being tested for the presence of specific alleles

• Couples can be tested prior to having children to determine the probability of future children inheriting the disorder

• Some of the benefits of carrying out genetic screening at this stage include

  • Families can make informed decisions before having children

  • Women can decide whether to have prenatal testing during pregnancy

Preimplantation Genetic Diagnosis (PGD)

• IVF, or in vitro fertilisation is a type of fertility treatment during which fertilisation is carried out in the lab; embryos produced in this way can be implanted into the uterus where they develop into a foetus

• Sometimes a couple might choose to have a child by IVF if they know that they are at increased risk of certain genetic disorders, as it allows them to screen any embryos produced using PGD

• PGD involves analysis of the DNA of an embryo prior to implanting it into the uterus

  • The sample of DNA to be analysed can be obtained by taking cell samples from embryos produced during IVF

• Benefits of PGD include

  • Evidence suggests that the process does not harm the embryo in any way

  • Reduces the chances of having a baby with a genetic disorder

  • It avoids abortion as it is carried out before implantation of the embryo

    • It is worth noting that some people believe that an embryo is as worthy of human status as a foetus or a baby, so the discarding of affected embryos after PGD is not ethically straightforward for everyone

Prenatal testing

• Prenatal testing is offered to pregnant women with a family history of genetic disorders

• It involves testing the foetus for genetic diseases during the course of a pregnancy

• The DNA can be obtained by chorionic villus sampling or amniocentesis in the uterus

• Chorionic villus sampling

  • This involves removing and testing a small sample of cells from the placenta using a fine needle

  • The cells contain foetal DNA which can be analysed for genetic disorders, allowing parents to make informed decisions about the pregnancy and foetus

  • Advantages of this process include

    • It is carried out at around 11-14 weeks of pregnancy, so any problems can be picked up early on

    • A relatively large tissue sample can be collected, providing plenty of cells to analyse

    • Results are available rapidly

  • Potential implications of chorionic villus sampling that should be considered include 

    • The process has a  1-2 % risk of miscarriage

    • The process cannot pick up any disorders that are caused by alleles of genes located on the paternal X chromosome, as this chromosome is inactive in the placental cells sampled

• Amniocentesis

  • This involves removing and testing a small sample of cells from amniotic fluid using a fine needle

    • The amniotic fluid is the fluid that surrounds the foetus within the uterus

  • The fluid contains foetal cells which contain DNA to be analysed

  • Potential implications of amniocentesis that should be considered include

    • It is carried out at around 15-20 weeks of pregnancy; this is relatively late in the pregnancy, making decisions about abortion more difficult for some parents

    • The procedure has a 1 % risk of miscarriage

    • It takes 2-3 weeks for results to be available

• Benefits of prenatal testing include

  • The tests allow parents to make informed decisions about the progress of a pregnancy

  • Results can help parents prepare for the future care of the child, including medical treatment

Implications of Prenatal Genetic Screening

• Prenatal genetic screening involves analysis of the DNA of a foetus taken from the uterus during pregnancy

  • Examples of prenatal screening methods include chorionic villus sampling and amniocentesis

• Both forms of prenatal screening come with associated risks and potential ethical dilemmas, so it is important that parents considering this type of screening have thought about the implications

  • Positive implications

    • Prenatal screening can provide advance notice of the birth of a child with a genetic disorder, giving time for parents to be educated and to make decisions about medical treatments

    • The risk of harm to the foetus during prenatal screening are low; around 1-2 % for both methods

    • Some parents may feel able to make the decision to end a pregnancy on discovering that a foetus has a life-limiting or potentially fatal diagnosis

    • Some parents may choose to continue with a pregnancy after receiving a fatal diagnosis, but they have time to process the results and grieve before the birth of a child who may not survive for long outside the uterus

    • Genetic counsellors are available to help parents think through the results of prenatal testing

  • Negative implications

    • Both methods of prenatal screening bring a small risk of miscarriage

    • Amniocentesis provides results relatively late in pregnancy, making decisions about termination of a pregnancy very difficult for some parents

    • Prenatal screening is not 100 % accurate

      • False positive results may lead to the termination of a healthy pregnancy

      • False negative results may give false expectations for the health of a foetus

    • Parents may experience pressure from society or their medical team to abort a foetus with a genetic disorder

    • Some parents may believe that a foetus has human status even very early on during a pregnancy, meaning that they would not consider abortion to be an option even after early testing