2.4.4Unit 2: Cells, Development, Biodiversity and Conservation
Genetic Screening — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 24 of 26 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Uses of Genetic Screening
Genetic screening tests for the presence of particular alleles in a person, embryo or foetus.
Carrier testing
- Carrier testing Cells from blood or saliva are tested for a particular allele, so people can learn their chance of having an affected child.
PGD
- PGD Preimplantation genetic diagnosis analyses cells taken from embryos made by IVF before any embryo is placed in the uterus.
Prenatal testing
- Prenatal testing A foetus is tested during pregnancy, using DNA from cells obtained from the placenta or the amniotic fluid.
Who is offered it
- Who is offered it Screening is usually offered where there is a family history of a disorder such as cystic fibrosis.
DNA analysis
- DNA analysis The sample DNA is tested for the presence of specific alleles of the gene concerned.
Informed decisions
- Informed decisions The results help families to plan, to choose whether to continue a pregnancy and to prepare for the care of a child.
Section 2 of 4Prenatal Tests Compared
Prenatal Tests Compared
Chorionic villus sampling
- Chorionic villus sampling A small sample of placental cells is taken with a fine needle at around 11 to 14 weeks of pregnancy.
Amniocentesis
- Amniocentesis Amniotic fluid containing foetal cells is sampled with a fine needle at around 15 to 20 weeks of pregnancy.
Risk
- Risk Both tests carry a small risk of miscarriage, and the risk is a little higher for chorionic villus sampling.
Timing of results
- Timing of results Chorionic villus sampling gives earlier and quicker results, whereas amniocentesis results can take two to three weeks.
| Feature | Chorionic villus sampling | Amniocentesis |
|---|---|---|
| Time in pregnancy | About 11–14 weeks | About 15–20 weeks |
| Sample | Placental cells | Cells in amniotic fluid |
| Results | Rapid | 2–3 weeks |
| Miscarriage risk | About 1–2% | About 1% |
Section 3 of 4PGD Step by Step
1
Eggs are fertilised in the laboratory (IVF), and the resulting embryos are allowed to divide to a small number of cells.
2
One or two cells are removed from each embryo without, according to current evidence, harming its further development.
3
The DNA from these cells is analysed to see if the embryo carries the allele for the disorder in question.
4
Only embryos that do not have the disorder-causing genotype are chosen for transfer into the uterus.
5
This reduces the chance of a child with the disorder and avoids a later decision about ending a pregnancy.
Section 4 of 4Exam Checklist & Tips
2.4.4 – Key Points for the Exam
- Genetic screening Testing for the presence of specific alleles in an individual, embryo or foetus.
- PGD Analysis of embryo DNA before implantation in IVF treatment.
- Amniocentesis Prenatal test using foetal cells in the amniotic fluid.
- Chorionic villus sampling Prenatal test using cells from the placenta.
- IVF Fertilisation of eggs outside the body, followed by transfer of embryos to the uterus.
Common Mistakes
- Confusing which test is earlier;
- chorionic villus sampling is done at around 11–14 weeks and amniocentesis later.
- Writing that PGD is carried out on a foetus, when it is used on embryos before implantation.
- Stating that screening cures a disorder, when it only identifies alleles.
- Saying that carrier testing is only done during pregnancy, when it can be done before having children.
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