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Genetic screening and ethical issuesEdexcel A-Level Biology A: Revision notes

Section 1

What is genetic screening?

Genetic screening is testing people, or embryos and fetuses, for the presence of a particular allele or chromosome abnormality. DNA is usually extracted from a blood sample, cheek swab, or cells from an embryo or fetus, and tested for the allele. Screening is used to identify carriers, to test embryos before implantation, and to test a fetus during pregnancy.

Key termsgenetic screening

Section 2

Identifying carriers

A carrier is a heterozygote who has one copy of a recessive allele and has no symptoms. Carrier screening, for example for the CF allele, shows whether a person could pass the allele on.

If both partners are carriers, each child has a 1 in 4 chance of having the disorder, a 1 in 2 chance of being a carrier and a 1 in 4 chance of being unaffected and not a carrier. Genetic counselling explains these risks and the options: PGD, prenatal testing, adoption, or deciding not to have children.

Key termscarriergenetic counselling

Section 3

Pre-implantation genetic diagnosis (PGD)

PGD is used for couples with a known risk of passing on a serious genetic disorder. Embryos are made by IVF. At the eight-cell stage, one or two cells are removed from each embryo. These cells are totipotent and unspecialised, so removing them does not stop the embryo developing. The DNA is tested for the faulty allele, and only unaffected embryos are placed in the uterus.

Advantages: no pregnancy has to be terminated. Disadvantages: IVF is expensive, involves hormone treatment and often fails, and affected embryos are discarded.

Key termsPGDIVFtotipotent

Section 4

Prenatal testing: amniocentesis and CVS

Amniocentesis is done from about 15 weeks. A needle passed through the abdomen takes a sample of amniotic fluid containing fetal cells, whose DNA or chromosomes are then analysed.

Chorionic villus sampling (CVS) is done from about 11 weeks. A sample of tissue from the chorion (placenta) is taken, using a needle or a tube through the cervix. The cells have the fetus's genotype, and results are available earlier.

Both carry a risk of miscarriage of roughly 1%. Prenatal tests do not treat the disorder: they give information.

Key termsamniocentesischorionic villus samplingamniotic fluid

Section 5

Implications of prenatal screening

A positive result means the parents must decide whether to continue the pregnancy, which can cause great distress. A negative result gives reassurance, but no test is 100% accurate. Waiting for results causes anxiety, and the tests themselves risk miscarriage. Results may reveal unexpected information, such as non-paternity. Parents might also feel pressure to terminate an affected pregnancy, or prepare early to care for a child with the condition.

Key termstermination

Section 6

Social and ethical issues

Examiners expect you to name a viewpoint and apply it.

  • Sanctity of life (often religious): human life begins at fertilisation, so discarding embryos or terminating is wrong.
  • Utilitarian: choose the action that reduces suffering and gives the greatest good for most people.
  • Autonomy: parents have the right to make informed decisions about their own reproduction.
  • Disability rights: screening may suggest that lives with a condition are less valuable.
  • Slippery slope: PGD might be extended to non-medical traits (designer babies).
  • Social issues: cost, unequal access, confidentiality and discrimination by insurers or employers.
Key termssanctity of lifeutilitarianautonomy
Exam tip

In discussion questions, give at least two different viewpoints and show how they disagree, then reach a justified conclusion if asked.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Genetic screening and ethical issues

  1. A couple who are both carriers of the cystic fibrosis (CF) allele are having fertility treatment. Eight embryos have been created outside the body. When each embryo reaches the eight-cell stage, one cell is removed and its DNA is tested for the CF allele. Only embryos that do not have CF will be placed in the woman's uterus.
    Explain why removing one cell at the eight-cell stage is unlikely to harm the development of the embryo.2 marks
  2. A woman who already has a child with a serious recessive disorder is 12 weeks pregnant. She and her partner are both carriers. She has been offered either chorionic villus sampling (CVS) now, or amniocentesis at 16 weeks. Each procedure carries a risk of miscarriage of about 1%.
    Suggest two reasons why the woman might decide not to have either test.2 marks
  3. A region has a high frequency of a serious recessive blood disorder. The local health authority offers free carrier screening to all adults who plan to have children. A cheek swab is taken and the DNA is tested for the faulty allele. People found to be carriers are offered genetic counselling.
    Explain how carrier screening and genetic counselling help couples to make informed decisions about having children.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).