- Use gene → protein → cell/tissue → phenotype to explain a genetic disease.
- CF is an autosomal recessive disease involving the CFTR protein.
- Thick mucus is a consequence of CFTR dysfunction, not its cause.
Year 12 Biology
Module 8 · IQ2
⏱ ~45 min
Practice bank · 3 Short Answer
Lesson 7 of 21
Genetic Disease: From Gene Change to Body Effect
A DNA change can alter a protein, change how cells work and cause a phenotype. Use cystic fibrosis to build that chain, then apply it to PKU, Huntington's disease and Type 1 diabetes.
Today's hook: Two people without cystic fibrosis can have a child with cystic fibrosis. How can that happen, and how does a change in the CFTR gene lead to thick mucus in the lungs?
0/5TASKS
Warm up first
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Worksheets
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
Lesson map
Trace the cause, not just the symptoms
Start with a DNA change, then explain its effect on a protein, cells or tissues, and the observable phenotype.
- 1Name the gene change.State the gene or allele involved.
- 2Explain the protein change.Say what the protein normally does and what changes.
- 3Link to the phenotype.Trace the effect through cells, tissues and the person.
Know what matters
- PKU involves an enzyme needed to process phenylalanine.
- Huntington's disease is inherited in an autosomal dominant pattern.
- Type 1 diabetes involves genetic susceptibility and autoimmune destruction of beta cells.
- Different variants in the same gene can affect a protein in different ways.
- A genetic contribution does not always mean a simple Mendelian pattern or a certain outcome.
- Early screening can prevent consequences even when the DNA change remains.
0
Predict first: healthy parents, affected child?
connectBoth parents carry one working CFTR allele and one non-working allele. Which statement best explains how they can have a child with cystic fibrosis?
Inheritance support: an allele is one version of a gene. A carrier has one altered recessive allele and usually does not show the condition.
1
Key vocabulary, translated
vocabMutationa change in a DNA sequence
Proteina molecule that does a cell job, such as transport or catalysis
Phenotypean observable feature or effect of genes and environment
Recessivetwo altered alleles are needed for the condition
Dominantone altered allele can be enough for the condition
True or false: a phenotype is the protein itself.
2
Worked example: cystic fibrosis
buildNormal job: the CFTR protein helps move chloride ions across epithelial cell membranes. Water follows, helping keep mucus hydrated.
Gene
Changes in the CFTR gene can alter the CFTR protein. F508del is a common CF-causing variant, but it is not the only one.
Protein and cells
Too little working CFTR at the membrane reduces chloride movement. Less water moves into the airway surface.
Phenotype
Mucus becomes dehydrated and thick, which can block airways and ducts and make infection more likely.
Build the CF explanation+7 XP
Put the five steps in a clear cause-to-effect order.
- Less water reaches the airway surface.
- A person inherits two CF-causing CFTR alleles.
- Thick mucus can block airways and increase infection risk.
- Cells make too little working CFTR protein at their membrane.
- Chloride movement across epithelial cells is reduced.
HSC exam move
For an “explain” question, do not begin with “CF causes thick mucus”. Begin with the altered gene/protein, then use arrows or linking words to reach the symptom.
3
Apply the same chain to other diseases
comparePKU
PAH gene change → reduced phenylalanine hydroxylase enzyme activity → phenylalanine can build up → untreated high levels can damage the developing brain. Early dietary management reduces this risk.
Huntington's disease
HTT gene change → altered huntingtin protein → progressive damage to particular brain cells → movement, thinking and mood changes. It follows an autosomal dominant pattern.
Type 1 diabetes
Genetic susceptibility plus other factors → autoimmune attack on pancreatic beta cells → little or no insulin → high blood glucose. This is more complex than a single-gene Mendelian condition.
Which example is the best comparison for a multifactorial condition rather than a simple single-gene condition?
4
Choose your route
differentiateUse the frame to explain CF.
Compare the inheritance pattern of CF and Huntington's disease.
Explain why Type 1 diabetes should not be described as caused by one “diabetes gene”.
5
Exit retrieval
retrieveMemorise
Gene → protein → cell/tissue → phenotype; recessive; dominant; carrier.
Understand
CFTR dysfunction changes ion and water movement; thick mucus follows.
Apply
Use the cause-to-effect chain for PKU or Huntington's disease.
Avoid
Do not call a symptom the cause, or assume every genetic condition is simple Mendelian inheritance.
01
Multiple Choice
+5 XP
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
02
Short Answer, 14 marks
+5 XP
ApplyBand 4(4 marks) 1. Use the gene → protein → cell/tissue → phenotype model to explain how altered CFTR can lead to thick airway mucus in cystic fibrosis.
AnalyseBand 4–5(5 marks) 2. Compare cystic fibrosis and Huntington's disease. Include their inheritance pattern and one link from altered protein to phenotype for each.
EvaluateBand 5–6(5 marks) 3. Evaluate the claim: “A genetic condition is always caused by one gene and is certain to occur.” Use Type 1 diabetes and one single-gene condition in your response.
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
SA1 (4 marks): Gene: a CFTR mutation on chromosome 7 (commonly F508del) [1]. Protein: the mutant CFTR misfolds and is degraded in the ER before reaching the membrane, so epithelial cells have no functional Cl⁻ channel [1]. Cellular consequence: Cl⁻ cannot be secreted into the airway lumen, so water does not follow by osmosis, the airway surface liquid is depleted and mucus becomes thick, viscous and dehydrated [1]. Lung consequences: dehydrated mucus cannot be cleared by cilia (mucociliary clearance fails) → accumulates → bacterial colonisation (Pseudomonas, Staphylococcus) → chronic infection and inflammation → progressive lung damage and respiratory failure [1].
SA2 (5 marks): (a) PKU: PAH mutation → non-functional phenylalanine hydroxylase that cannot convert phenylalanine to tyrosine, loss of function [1]. Huntington's: CAG expansion → mutant huntingtin with a long polyQ tract that misfolds into toxic aggregates, gain of function [1]. (b) PKU is recessive because one normal PAH allele produces enough enzyme (50% activity is adequate), loss of one allele does not cause disease. Huntington's is dominant because one normal HTT allele does not protect against the toxic mutant protein produced by the other allele [2]. (c) PKU is managed by diet because the damage depends on accumulation of dietary phenylalanine, restricting intake removes the substrate. Huntington's cannot be managed by diet because the toxic huntingtin is produced endogenously regardless of diet, there is no dietary substrate to restrict [1].
SA3 (5 marks): Supporting genetic classification: clear genetic risk factors, HLA-DR3/DR4 in ~90% of Type 1 diabetics; first-degree relatives have 5–10× increased risk; 50+ risk loci identified; autoimmunity has a genetic basis [1]. Complicating evidence: identical twin concordance is only ~50%, since twins share 100% of DNA, pure genetics would give ~100%; the 50% figure shows genetic predisposition alone is insufficient [2]. Environmental factors: enteroviral infections, early dietary exposures, gut microbiome, vitamin D status, proposed triggers in genetically susceptible individuals [1]. Conclusion: 'genetic disease' is appropriate in that genetic predisposition is necessary, but more accurately Type 1 diabetes is a disease of genetic predisposition requiring environmental triggering, a multifactorial disease; the bare label risks overstating genetic determinism and understating preventive potential [1].
Check what actually stuck
Take the full module quiz
quiz
A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.
Start the module quiz →Race Through Genetic Diseases!
Answer questions on cystic fibrosis, PKU, Huntington's disease and Type 1 diabetes. Pool: lessons 1–7.
How did your thinking change?
Return to your Think First responses and apply the Collins and Tsui 1989 CFTR discovery to your understanding. The CFTR Gly551Asp mutation produces a chloride channel that does not open, a loss-of-function mutation on chromosome 7q31. In Australia, CF affects 1 in 2,500 births (CFHA 2023), and median survival has risen from 5 years (1960) to 44 years (2022) as treatments have targeted the specific protein defect.
- Q1, unaffected parents, affected child: Both parents are CFTR carriers (Cc). CF is autosomal recessive, the child must inherit the mutated allele from both parents (cc). Can you now draw a Cc × Cc Punnett square showing the 25% probability of a cc child? This explains why CF appeared in this family despite no affected parents.
- Q2, Huntington's inheritance: Autosomal dominant because one mutant allele is sufficient, the gain-of-function mechanism (toxic polyglutamine protein) acts regardless of the other allele. This is fundamentally different from CFTR's loss-of-function recessive pattern.
- Write the gene → protein → phenotype chain for CF from memory, using the Collins & Tsui 1989 data (chromosome 7q31 → CFTR Gly551Asp mutation → non-functional Cl⁻ channel → thick mucus → lung damage).