Biology • Year 12 • Module 8 • Lesson 17
Genetic Prevention: Screening and Engineering
Lock in the distinction between detection, selection and modification, the mechanism of a worked screening example, and the somatic versus germline contrast, the foundations for evaluating genetic prevention.
1. Complete the detection, selection and modification map
The diagram sorts genetic technologies by what they actually do. Write the missing labels into boxes A–H using the lesson Key Terms and Learn cards. 8 marks
| Box | Your label |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F | |
| G | |
| H |
2. Term–definition match
The definitions below are shuffled. Write the matching term from this list in the right-hand column: screening, PGT, gene therapy, genetic engineering, somatic editing, germline editing, genetic counselling, off-target effect, mosaicism, variant of uncertain significance. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | Editing DNA in body cells such as liver, retina or blood stem cells, so the change cannot be inherited by the patient's children. | |
| 2.2 | A change in the DNA sequence that has not been observed often enough to know whether it causes disease, so it gives a family no actionable decision. | |
| 2.3 | Testing to identify risk or an affected individual before symptoms appear. | |
| 2.4 | A deliberate change to genetic material or to gene expression. | |
| 2.5 | Editing DNA in gametes or a very early embryo, so the change appears in every cell including the resulting person's own gametes. | |
| 2.6 | Testing IVF embryos before implantation so that embryos without a known variant can be transferred. | |
| 2.7 | Non-directive support that explains the inheritance pattern, the numerical risk and every available option, then supports whichever decision the family reaches. | |
| 2.8 | Adding, replacing or altering genes in order to treat a disease. | |
| 2.9 | An individual who is a patchwork of edited and unedited cells because editing succeeded in some cells and not others. | |
| 2.10 | A cut made at a similar sequence elsewhere in the genome when the guide RNA binds the wrong site. |
3. True or false, with correction
Circle T or F. If the statement is false, write the corrected version on the line below. 8 marks (1 T/F + 1 correction where needed)
3.1 A low-phenylalanine diet started in the first weeks of life corrects the faulty PAH allele. T / F
3.2 Preimplantation genetic testing edits the disease-causing allele out of the embryo before transfer. T / F
3.3 Somatic gene therapy corrections are passed on to the patient's children. T / F
3.4 Most people who carry a recessive disease allele are healthy and have no family history of the condition. T / F
4. Function recall
Answer each question in 1–2 sentences using precise lesson terminology. 10 marks (2 each)
4.1 What is the function of the enzyme phenylalanine hydroxylase, and what happens when it is absent?
4.2 What is the function of the guide RNA in CRISPR-Cas9?
4.3 What is the function of the viral vector used in gene therapies such as Luxturna?
4.4 What is the function of reproductive carrier screening, and why is family history an unreliable substitute?
4.5 What is the function of non-directive genetic counselling after a screening result?
5. Build a concept map
Draw labelled arrows between the six terms below. Each arrow must carry a linking phrase (e.g. "detects", "selects between", "changes", "cannot address"). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: genetic screening · PGT · somatic editing · germline editing · genotype · phenotype.
6. Cloze, fill in the blanks
Complete the paragraph using words from the word bank below. Each word is used once. 10 marks
Newborn screening for phenylketonuria is _______________, not genetic engineering. Early dietary management prevents the _______________, while the _______________ remains unchanged and is still passed on. Preimplantation genetic testing is _______________ rather than modification: it identifies embryos without a known variant, but it requires a full cycle of _______________ and never alters any embryo's DNA. Only editing changes DNA. _______________ editing changes body cells, so it can treat the patient without being inherited, whereas _______________ editing changes gametes or a very early embryo, which makes the change _______________ and is prohibited in Australia. Even so, genetic prevention will never cover most disease, because conditions such as Type 2 diabetes and most cancers are _______________ and have no single gene to target, and because cost and _______________ limit any method to the families who can reach it.
Q1, Labelled detection, selection and modification map
A: newborn bloodspot screening (the NSW heel-prick test). B: NIPT, non-invasive prenatal testing (a screening test). C: amniocentesis (a diagnostic test). D: PGT, preimplantation genetic testing. E: PGT-M (testing for a known monogenic variant). F: somatic editing, or somatic gene therapy. G: CRISPR-Cas9. H: germline editing.
Q2, Term–definition matches
2.1 somatic editing · 2.2 variant of uncertain significance · 2.3 screening · 2.4 genetic engineering · 2.5 germline editing · 2.6 PGT · 2.7 genetic counselling · 2.8 gene therapy · 2.9 mosaicism · 2.10 off-target effect.
Q3, True / false with correction
3.1 False. Correction: the diet does not touch the allele. It removes the substrate the broken pathway cannot handle, keeping blood phenylalanine below the toxic range so the developing nervous system is not damaged. The genotype is unchanged, so the faulty allele is still present and still heritable; what has been prevented is the phenotype.
3.2 False. Correction: PGT is selection, not modification. Embryos created through IVF are biopsied at the blastocyst stage and tested, and embryos without the variant are identified and transferred. No embryo's DNA is altered at any point.
3.3 False. Correction: somatic editing changes DNA in body cells such as liver, retina or blood stem cells. Those are not the cells that become sperm or eggs, so the change cannot be inherited. The patient may be cured while their children face exactly the same risk they always did, which is why somatic editing is treatment rather than prevention of inheritance.
3.4 True.
Q4, Function recall answers
4.1 Phenylalanine hydroxylase: It converts phenylalanine from dietary protein into tyrosine. When a mutation in the PAH gene leaves the enzyme missing or non-functional, phenylalanine cannot be converted and accumulates in the blood and brain, where high concentrations damage developing neurons and cause severe, permanent intellectual disability unless intake is restricted early.
4.2 Guide RNA: The guide RNA gives CRISPR its specificity. It is designed to be complementary to a chosen DNA sequence, so it base pairs with that target and carries the Cas9 nuclease to that exact site, where Cas9 cuts both strands. If the guide binds a similar sequence elsewhere, Cas9 cuts the wrong gene, which is an off-target effect.
4.3 Viral vector: A modified virus, usually an adeno-associated virus stripped of its ability to replicate, carries a working copy of the gene into the target tissue. The cells then transcribe and translate it to make the missing protein. Because the vector must reach the right cells and the gene must fit inside it, each therapy targets one gene in one accessible tissue, and the immune system can attack the vector, which limits repeat dosing.
4.4 Reproductive carrier screening: It tests prospective parents before or early in pregnancy to identify whether both carry a variant for the same recessive condition, since two carriers face a one in four chance of an affected child in every pregnancy. Family history is unreliable because carriers of a recessive allele are perfectly healthy, one working copy being enough, so most carriers have no family history at all. Around one in twenty people screened in Mackenzie's Mission carried a variant for at least one of cystic fibrosis, spinal muscular atrophy or fragile X syndrome.
4.5 Non-directive counselling: The counsellor explains the inheritance pattern, the actual numerical risk and every available option, then supports whichever decision the family reaches rather than recommending one. Screening only produces a benefit if what follows is genuinely useful to the family, and telling people what to choose would be both unethical and, in practice, ineffective.
Q5, Sample concept map
Correct maps should include arrows such as:
- genetic screening detects but does not change the → genotype
- genetic screening enables early management that can prevent the → phenotype
- PGT selects between embryos with different → genotype (without altering any of them)
- somatic editing changes the genotype of body cells only, so it alters the → phenotype without being inherited
- germline editing changes the genotype of every cell, so the change is heritable → genotype
- germline editing is prohibited in Australia, unlike → somatic editing
Award 1 mark per correct labelled arrow (maximum 6). Do not award a mark for any arrow implying that screening or PGT alters DNA.
Q6, Cloze answers (in order)
detection · phenotype · genotype · selection · IVF · somatic · germline · heritable · multifactorial · access