top of page

B12.7 Screening for Genetic Disorders

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: testing embryos and fetuses for inherited disorders, 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 embryos made by IVF can be screened before implantation
  • Describe how a fetus can be tested during pregnancy, and the risks
  • Sort arguments about embryo screening into economic, social and ethical issues
  • Make an informed, balanced judgement about screening, using data

Key words

  • Screening: Testing many embryos, fetuses or people to find those who have, or carry, a disorder.
  • IVF: In vitro fertilisation: eggs are fertilised by sperm in a dish in the lab.
  • Embryo screening: Testing one or two 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 piece 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 an embryo or fetus has inherited the faulty alleles. Every cell has the same DNA, so testing just one or two cells is enough.

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

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

Embryo screening in IVF

Couples at high risk can have IVF with embryo screening. Several embryos are made in the lab. When they are a few days old, one or two cells are removed from each and tested. Only embryos without the disorder are placed in the mother's uterus.

Worked example: how many embryos could be used?

Expected from Ff × Ff with 10 embryos: ff = 10 × 1/4 = 2.5; Ff = 10 × 1/2 = 5; FF = 10 × 1/4 = 2.5.
The actual results were 3 FF, 5 Ff and 2 ff, close to the prediction.
Embryos without CF = 3 + 5 = 8 (carriers are healthy). Percentage = 8 ÷ 10 × 100 = 80%.

Answer: 8 of the 10 embryos (80%)

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
  • Risk; CVS small extra risk of miscarriage (about 0.5%); Amniocentesis small extra risk of miscarriage (about 0.5%)

Worked example: risk in numbers (maths skill)

A health region carries out 1 800 CVS tests in a year. The extra miscarriage risk is 0.5%.
Expected miscarriages = 1 800 × 0.5 ÷ 100 = 9.

Answer: about 9 miscarriages caused by testing

Weighing up the issues

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

In the UK, embryo screening is tightly regulated. It is only allowed 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; 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. The timeline shows when two tests can be done on a fetus. Which bar, P or Q, shows chorionic villus sampling (CVS)? (Choose from: P, Q, neither.)

Answer: P

Working: CVS is done at 11–14 weeks (bar P). Amniocentesis is usually done later, at 15–20 weeks (bar Q).

Question 2. What is sampled in amniocentesis? (Choose from: Fluid around the fetus, which contains fetal cells, Blood from the mother's arm, A piece of the placenta, 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 shed by the fetus. Their DNA is tested.

Question 3. The extra miscarriage risk from amniocentesis is about 0.5%. How many miscarriages would you expect from 4 400 tests?

Answer: 22

Working: 4 400 × 0.5 ÷ 100 = 22.

Related lessons

Continue in the full course

This lesson is section B12.7 (AGCSC1207) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

bottom of page