What you need to know
- 2.17(i)Carrier identification, PGD and prenatal testing including amniocentesis and CVS.
- 2.17(ii)Implications of prenatal genetic screeningTesting people, embryos or fetuses for particular alleles, such as the allele that causes cystic fibrosis..
- 2.18Ethical and social viewpoints, including religious and moral considerations.
By the end, you should be able to…
- Distinguish carrier testing, embryo testing and prenatal testing.
- Explain the difference between screening probability and diagnosis.
- Construct a balanced, evidence-based ethical discussion.
Build your understanding
Ask what the test is intended to establish
Carrier testing can identify an alleleA different version of the same gene, found at the same locus. associated with a recessiveAn allele that is expressed in the phenotype only when no dominant allele is present (when homozygous). condition in someone who does not show that condition. Results can inform reproductive choices and testing of relatives, but the meaning depends on the condition and the variants the test detects.
Screening estimates the chance of a condition or identifies who may benefit from further testing. A diagnostic test aims to establish whether the specific condition or variant being tested for is present. A positive screen is not automatically a diagnosis, and a negative result is not a guarantee against every genetic condition.
Testing before implantation
Pre-implantation genetic diagnosis (PGD), the term used in the specification, involves testing cells from embryos produced through IVF before selecting embryos for transfer. Testing for a known single-gene condition is now commonly called PGT-M. It tests for the targeted condition; it does not alter the embryo’s DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code..
This can reduce the chance of transferring an embryo with the tested condition, but IVF and testing have physical, emotional and financial burdens. There may be no suitable embryo to transfer, and a transferred embryo may not result in a birth. Detailed eligibility rules vary and are not required recall here.
Testing during pregnancy
Amniocentesis samples amniotic fluid containing fetal cells. Chorionic villus sampling (CVS) samples placental tissue; it can usually be performed earlier in pregnancy than amniocentesis. The sample is analysed for the genetic or chromosomal condition of interest.
Both are invasive procedures with a risk of pregnancy loss and other complications. The precise risk depends on circumstances; use any figures supplied in an exam question rather than inventing a universal percentage. A result can help with preparation and care planning or inform reproductive decisions. It does not force one course of action.
Evaluate evidence and uncertainty
Ask about false positives, false negatives, conditions covered, predictive certainty and sample limitations. Finding a variant may not predict the severity of symptoms. Families may need counselling to understand the result and what it means for relatives.
Compare information gained with possible harms: anxiety, procedure risks, uncertainty and pressure to make decisions. Informed consent includes understanding the choices and the possibility of declining a test or choosing not to know.
Discuss viewpoints respectfully
An autonomy-based view emphasises informed choice. A welfare-based view weighs potential reductions in suffering against the burdens and risks of testing. A justice-based view asks whether access is fair and whether testing could contribute to stigma or discrimination against disabled people.
Some moral or religious viewpoints give strong weight to the status of embryos or oppose pregnancy termination; others place different weight on parental choice or preventing suffering. Beliefs vary within as well as between communities. State a viewpoint and its reasoning rather than assigning one opinion to an entire religion.
A balanced answer acknowledges competing values, evidence limits and confidentiality. It should respect people living with genetic conditions and the right to make different informed choices. All discussion cases here are fictional; no family or student genetic information is requested.
NOT: a positive screen is not a diagnosis, and none of these tests repairs the allele.
- PGD does not change the embryo’s DNA
- Use any miscarriage risk given in the question; do not invent one
- A result may not predict how severe symptoms will be
- People can decline a test. Views about embryos differ
Two traps this diagram blocks
- Do not swap the samples: fluid for amniocentesis, placenta for CVS.
- Do not say PGD or prenatal diagnosis treats the condition. They provide information.
From the diagram alone
Question
Distinguish amniocentesis from chorionic villus sampling, and state one limitation they share. [3]
Show a model answer
Amniocentesis samples amniotic fluid containing fetal cells. CVS samples placental tissue and can usually be performed earlier. Both are invasive and carry a risk of pregnancy loss, and neither result by itself decides what a person should do.
| Approach | Material or stage | Main purpose |
|---|---|---|
| Carrier testing | Sample from an individual | Identify a relevant recessive allele |
| PGD / PGT-M | IVF embryo before transfer | Test for a specified inherited condition |
| Amniocentesis | Amniotic fluid during pregnancy | Diagnostic testing for specified conditions |
| CVS | Placental tissue during pregnancy | Diagnostic testing, usually earlier than amniocentesis |
Make your reasoning visible
Distinguish the sample
Amniocentesis: amniotic fluid. CVS: placental tissue. PGD: embryo cells before transfer following IVF.
Discuss rather than list
Explain why a benefit or concern matters, whose interests are involved and any evidence limitation. A conclusion can be conditional.
Do not confuse testing with treatment
A test provides information; PGD does not repair an alleleA different version of the same gene, found at the same locus., and prenatal diagnosis does not itself cure a condition.
Avoid this misconception
The claim: A positive genetic screeningTesting people, embryos or fetuses for particular alleles, such as the allele that causes cystic fibrosis. result proves that the baby has every symptom of the condition.
The correction: Screening and diagnosis differ, and even a diagnostic result may not predict severity or every symptom.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Compare carrier testing with PGD for a recessive condition.
Show suggested answer
Carrier testing assesses a person’s alleles to inform risk. PGD tests IVF embryos before transfer for the specified condition. Neither changes an allele.
Practice question
Retrieval and explanation · suggested answer
What material is sampled by amniocentesis and by CVS?
Show suggested answer
Amniocentesis samples amniotic fluid containing fetal cells; CVS samples placental tissue.
Practice question
Retrieval and explanation · suggested answer
A screening test gives a higher-chance result. Give one reason to seek further information before making a decision.
Show suggested answer
The result may be a false positive and does not establish diagnosis. The person needs information on confirmatory testing, its risks and what the original screen actually detects.
Practice question
Retrieval and explanation · suggested answer
Give an autonomy argument and a justice argument about genetic screening.
Show suggested answer
Autonomy: people should be able to make informed, voluntary choices, including declining testing. Justice: access and support should be fair, and results should not promote stigma or unequal treatment.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
In a fictional group of 1,000 pregnancies, 10 have a condition. A screen flags 9 of those and also flags 99 of the 990 without it. What fraction of positive screens are true positives, and why does this matter?
Show a hint
Count all positive screens before dividing. These figures are invented, not test-performance claims.
Explore the reasoning
There are 108 positive screens, of which 9 are true positives: 9/108 = 1/12 ≈ 8.3%. Even a screen that detects most affected pregnancies can have many false positives when the condition is uncommon. A positive result needs careful explanation and appropriate follow-up.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare amniocentesis with chorionic villus sampling (CVS).
Genetic screening uses
Practice question
Retrieval and explanation · suggested answer
Compare amniocentesis with chorionic villus sampling (CVS).
Lesson: 2C 5: Genetic Screening
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
What embryo testing can tell you
2C · Genetic ScreeningThe HFEA explains PGT-M as testing embryos for specific monogenic conditions. Its information distinguishes testing from a guarantee of pregnancy or a child without any health condition. An…Specification 2.17(i) · 2.17(ii) · 2.18Read & use the scienceClose article
Why this matters
The HFEA explains PGT-M as testing embryos for specific monogenic conditions. Its information distinguishes testing from a guarantee of pregnancy or a child without any health condition. An embryology laboratory needs a clearly defined testing target and a method suited to the family’s situation. The result addresses that target, not every possible inherited variant or future environmental influence. Technical limitations and uncertainty remain relevant to informed decisions, alongside personal and ethical considerations. Students can separate three questions: what was tested, how reliable is that result, and what does it imply? This prevents the popular phrase “designer baby” from replacing a careful account of what a specific test actually measures.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.17(i) understand the uses of genetic screening, including the identification of carriers, pre-implantation genetic diagnosis (PGD) and prenatal testing, including amniocentesis and chorionic villus sampling
- 2.17(ii) understand the implications of prenatal genetic screening
- 2.18 be able to identify and discuss the ethical and social issues relating to genetic screening from a range of ethical viewpoints, including religious, moral and social implications
Use the science
Why is “an embryo tested for one condition” more accurate than “a guaranteed healthy embryo”?
Explore a possible response
A targeted test addresses specified genetic findings. It cannot rule out every other condition or predict every later health outcome.
Explore the source
A negative DNA test is not an all-clear for life
2C · Genetic ScreeningA commercial genetic test may examine a selected set of variants rather than every possible disease-associated change. The FDA explains that different companies can test different sets, so…Specification 2.17(i) · 2.18Read & use the scienceClose article
Why this matters
A commercial genetic test may examine a selected set of variants rather than every possible disease-associated change. The FDA explains that different companies can test different sets, so a negative result has to be interpreted within the test’s scope. It does not remove environmental risks, unknown variants or changes that were never tested. This extends genetic screening beyond the technology to the meaning of a result and the claims used to sell it. A scientifically responsible report distinguishes the absence of a detected variant from a guarantee of health, and makes uncertainty visible rather than hiding it in advertising.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.17(i) understand the uses of genetic screening, including the identification of carriers, pre-implantation genetic diagnosis (PGD) and prenatal testing, including amniocentesis and chorionic villus sampling
- 2.18 be able to identify and discuss the ethical and social issues relating to genetic screening from a range of ethical viewpoints, including religious, moral and social implications
Use the science
Two companies test different variants for the same condition and return different results. Why does this not by itself prove that one laboratory made an analytical error?
Explore a possible response
Their panels may measure different targets. Different scope can produce different findings even with accurate assays; interpretation also depends on evidence for each variant.