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13.10 Screening for Genetic Disorders

GCSE Biology · AQA · Chapter 13: Reproduction

A free GCSE Biology lesson for AQA (8461) from OBA School: tests in pregnancy, embryo screening in IVF, and the economic, social and ethical issues. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how amniocentesis and chorionic villus sampling test a fetus for genetic disorders
  • Describe how embryos made by IVF can be screened before implantation
  • Identify economic, social and ethical issues about embryo screening
  • Make an informed, balanced judgement about screening using data

Key words

  • Screening: Testing many people (or embryos) to find those who have, or carry, a disorder.
  • IVF: In vitro fertilisation: eggs are fertilised by sperm in a dish in the lab.
  • PGD: Pre-implantation genetic diagnosis: testing cells from IVF embryos before one is placed in the uterus.
  • Amnio- centesis: Testing fetal cells from a sample of the fluid around the fetus, usually at 15–20 weeks.
  • CVS: Chorionic villus sampling: testing a tiny sample of the developing placenta, at 11–14 weeks.
  • Ethical issue: A question about what is morally right or wrong.

Why screen?

Some families know they carry the allele for a serious inherited disorder such as cystic fibrosis. Genetic tests can show whether a fetus or embryo has inherited the faulty alleles. Every cell has the same DNA, so testing just a few cells is enough.

Results help families to make decisions: to prepare for caring for a child with the disorder, or, in some cases, to end the pregnancy (a termination) or to choose an unaffected embryo.

Key fact: Screening finds a disorder; it does not cure it. The family still has to make a choice based on the result.

Tests during pregnancy

  • When; CVS 11–14 weeks; Amniocentesis usually 15–20 weeks
  • Sample; CVS a tiny piece of the developing placenta; Amniocentesis amniotic fluid containing cells from the fetus
  • How; CVS needle through the abdomen (or tube through the cervix); Amniocentesis fine needle through the abdomen, guided by ultrasound
  • Risk; CVS small extra risk of miscarriage (about 0.5%, up to 1 in 200); Amniocentesis small extra risk of miscarriage (about 0.5%, up to 1 in 200)

Worked example: risk in numbers (maths skill)

A region carries out 3 000 amniocentesis tests in a year. The extra miscarriage risk is 0.5%.
Expected miscarriages = 3 000 × 0.5 ÷ 100 = 15.

Answer: about 15 miscarriages caused by testing

Embryo screening in IVF

Couples at high risk can choose IVF with pre-implantation genetic diagnosis (PGD). Several embryos are made in the lab. When they are a few days old, one or a few cells are removed from each and tested. Only embryos without the disorder are placed in the mother's uterus.

Worked example: how many embryos can be used?

Two CF carriers (Ff × Ff) have 8 embryos made by IVF. Expected numbers: ff = 8 × 1/4 = 2; Ff = 8 × 1/2 = 4; FF = 8 × 1/4 = 2.
Embryos without CF = 4 + 2 = 6 (carriers are healthy).

Answer: about 6 of the 8 embryos

Weighing up the issues

  • Economic (money); Arguments for screening treating and caring for someone with a serious disorder can cost far more than testing; Arguments against screening tests and IVF are expensive; resources could be used elsewhere
  • Social (people and society); Arguments for screening reduces suffering; families can plan and prepare; Arguments against screening may increase prejudice against people living with disorders; false results can cause stress
  • Ethical (right and wrong); Arguments for screening parents have the right to choose a healthy child; Arguments against screening embryos are destroyed; some fear "designer babies" chosen for non-medical features; tests in pregnancy risk miscarriage

In the UK, PGD is tightly regulated: it is only allowed to test for serious medical conditions on an approved list, not to choose features such as eye colour.

Exam tip: For "evaluate" questions: give points for and against, label them economic, social or ethical if asked, use any data given, and finish with a justified conclusion.

Common mistakes

  • Saying screening cures a disorder. It only detects it.
  • Mixing up the tests: CVS samples the placenta earlier (11–14 weeks); amniocentesis samples the fluid later.
  • Calling every argument "ethical". Money is economic; effects on families and society are social.
  • Giving a one-sided answer to an "evaluate" question, or no conclusion.

Revision questions

Try these, then check your answers below each one.

Question 1. When in pregnancy is chorionic villus sampling (CVS) usually carried out? (Choose from: 11–14 weeks, 15–20 weeks, 28–32 weeks, After birth.)

Answer: 11–14 weeks

Working: CVS samples the developing placenta at 11–14 weeks; amniocentesis is usually at 15–20 weeks.

Question 2. What is sampled in amniocentesis? (Choose from: Fluid around the fetus, which contains fetal cells, A piece of the placenta, The mother's blood only, Cells from the father.)

Answer: Fluid around the fetus, which contains fetal cells

Working: A fine needle takes a sample of amniotic fluid, which contains cells from the fetus that can be tested.

Question 3. The extra miscarriage risk from amniocentesis is about 0.5%. How many miscarriages would you expect from 2 600 tests?

Answer: 13

Working: 2 600 × 0.5 ÷ 100 = 13.

Related lessons

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