Genetic Diseases — Cystic Fibrosis, PKU, Huntington's Disease, Type 1 Diabetes
Two healthy parents have a child born with cystic fibrosis. A 40-year-old develops uncontrollable movements that killed their parent at 50. A newborn's heel-prick test catches a disease before any symptom appears. Four genetic diseases, one framework: gene mutation → altered protein → physiological consequence — and four very different inheritance patterns.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Genetic disease: one of the four categories of non-infectious disease
Cystic fibrosis is caused by mutations in both copies of the CFTR gene. Two parents, neither of whom has cystic fibrosis, have a child diagnosed with cystic fibrosis at birth.
At first glance this seems contradictory — if it is genetic, why do two healthy parents produce an affected child? And why does only one of their three children have it, while the other two do not?
Before reading on, answer both questions:
Q1: Using what you know about genetics from Module 5, explain how two unaffected parents can have an affected child. What term describes the parents' genetic status?
Q2: Huntington's disease is also genetic, but behaves very differently — children of an affected parent have a 50% chance of inheriting it, and it almost always manifests if the gene is present. What inheritance pattern does this suggest, and how does it differ from cystic fibrosis?
Know
- The gene, protein, and mechanism of disease for CF, PKU, Huntington's, and Type 1 diabetes
- The inheritance pattern (autosomal recessive or dominant) for each disease
- The specific consequences of each protein dysfunction at the organ/system level
- The distinction between penetrance and carrier status
Understand
- Why a single amino acid change can abolish or dramatically alter protein function
- Why autosomal recessive diseases can appear in families with no history of the condition
- Why Huntington's disease progresses over decades despite being caused by a single mutation present from birth
- How Type 1 diabetes illustrates the interaction between genetic predisposition and environmental trigger
Can Do
- Trace the pathway from gene mutation → altered protein → physiological consequence for each disease
- Distinguish autosomal recessive from autosomal dominant inheritance using pedigree or descriptive evidence
- Explain why genetic diseases are non-infectious and why 'genetic' does not mean 'inevitable'
- Apply the mutation → protein → phenotype framework to an unfamiliar genetic disease scenario
Core Content
Every genetic disease follows the same three-step logic — gene mutation → altered protein → physiological consequence
Before memorising specific diseases, understanding the universal logic of genetic disease is essential. Every genetic disease can be explained by the same pathway: a mutation changes the DNA sequence of a gene → this changes the amino acid sequence of the protein → the altered protein fails to perform its normal function → specific physiological consequences follow.
Genetic diseases showing inheritance patterns and examples
Pedigree patterns showing autosomal recessive, dominant and X-linked
Proteins are the molecular machines of the cell — enzymes, structural components, transport channels, receptors, signalling molecules, and regulators of cell division. When a mutation produces an altered amino acid sequence, the protein may:
- Fold incorrectly and be degraded before reaching its target location (most CF mutations)
- Lack catalytic activity — an enzyme that cannot catalyse its reaction (PKU)
- Gain a toxic function — the mutant protein does something actively harmful (Huntington's)
- Trigger an immune response — the immune system misidentifies self-proteins as foreign (Type 1 diabetes)
The severity and nature of the disease depends entirely on what the normal protein does and how the mutation changes its behaviour. A mutation in a ubiquitous enzyme (needed in every cell) has widespread effects; a mutation in a tissue-specific protein has localised effects.
What to write in your book
- Universal chain: gene mutation → altered protein → physiological consequence.
- Mutation effects: misfolded/degraded (CF), no catalytic activity (PKU), toxic gain-of-function (Huntington's), immune trigger (T1D).
- Severity depends on what the normal protein does and how the mutation changes it.
- Exam answers must name the gene, the protein change, AND the consequence.
Every genetic disease follows the chain: gene mutation → altered _____ → physiological consequence.
Autosomal recessive — most common life-limiting genetic disease in Australians of European descent
Cystic fibrosis results from the absence of a functioning chloride ion channel in the membranes of epithelial cells — a single molecular defect that produces consequences in the lungs, pancreas, digestive system, sweat glands, and reproductive organs simultaneously.
Cystic Fibrosis — Disease Profile
Why carriers are unaffected
Carriers have one functional CFTR allele (C) and one mutated allele (c). The one functional copy produces enough CFTR protein to maintain adequate Cl⁻ secretion — a single functional allele is sufficient for normal mucus hydration. This is why autosomal recessive diseases can appear in families with no history: carriers show no symptoms and may be unaware they carry the allele until they have an affected child.
What to write in your book
- CF: CFTR gene (chr 7); F508del → misfolded protein degraded → no Cl⁻ channel.
- Cl⁻ not secreted → water doesn't follow → thick dehydrated mucus → lung infection, pancreatic malabsorption.
- Autosomal recessive — both alleles must be mutated; carriers (Cc) are unaffected.
- Cause = the gene/protein; thick mucus is the consequence.
Cystic fibrosis is inherited in which pattern?
Autosomal recessive — entirely manageable with early diagnosis and dietary intervention
PKU illustrates a different mechanism of genetic disease: not a dysfunctional channel, but a missing enzyme. When the enzyme that metabolises phenylalanine is absent, the amino acid accumulates to toxic levels — but the disease is entirely preventable if diagnosed at birth and managed with a low-phenylalanine diet.
Phenylketonuria (PKU) — Disease Profile
Why PKU is significant beyond the disease itself
PKU was one of the first genetic diseases for which newborn screening was implemented. Australia introduced universal newborn PKU screening in the 1960s. A child diagnosed at birth and maintained on a low-phenylalanine diet develops normally — the genetic mutation is present but its consequences are entirely prevented by removing the substrate (phenylalanine) that accumulates. This demonstrates that genetic diseases, even when they cannot be 'cured,' can be managed to prevent their most severe consequences.
What to write in your book
- PKU: PAH gene (chr 12) → no phenylalanine hydroxylase enzyme (loss of function).
- Phenylalanine can't convert to tyrosine → accumulates → toxic to developing neurons.
- Autosomal recessive; managed by a lifelong low-phenylalanine diet.
- Newborn Guthrie heel-prick screening (day 2–3) prevents brain damage.
PKU causes intellectual disability even when it is detected at birth and managed with a low-phenylalanine diet.
Cystic fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene.
Huntington's disease is an autosomal recessive disorder that typically appears in childhood.
Autosomal dominant — gain-of-function — late-onset — 100% penetrance (40+ repeats)
Huntington's disease is mechanistically distinct from CF and PKU — the problem is not a loss of function but a gain of toxic function. The mutation does not eliminate a useful protein; it produces a new, poisonous one. This is why Huntington's is autosomal dominant: one copy of the mutant allele is sufficient, because the mutant protein is toxic regardless of what the other allele produces.
Huntington's Disease — Disease Profile
Why Huntington's is different from CF and PKU
CF and PKU are loss-of-function diseases — the mutant protein fails to perform its normal role. Huntington's is a gain-of-function disease — the mutant protein does something new and toxic. This explains why Huntington's is dominant: having one normal HTT allele does not protect you, because the toxic mutant protein produced by the other allele continues to accumulate and damage neurons regardless. It also explains the late onset: the mutant huntingtin accumulates slowly over decades, and symptoms appear only after enough neuronal loss has occurred.
What to write in your book
- Huntington's: HTT gene (chr 4); CAG repeat expansion (36+) → toxic polyQ huntingtin.
- Gain-of-function: aggregates kill striatal/cortical neurons over decades → chorea, cognitive decline.
- Autosomal dominant (one allele enough) BECAUSE the protein is toxic — normal allele can't protect.
- Late onset (30–50 yrs); anticipation = earlier/severe onset in later generations.
Why is Huntington's disease autosomal dominant?
Polygenic predisposition — environmental trigger — autoimmune mechanism — onset typically childhood
Type 1 diabetes is the most complex of the four — not a single mutated gene producing an altered protein, but genetic predisposition interacting with environmental triggers to produce an immune system malfunction that destroys the cells that make insulin.
Type 1 Diabetes — Disease Profile
Type 1 vs Type 2 — revisited from L03
From L03: Type 1 = no insulin produced (beta cells destroyed); Type 2 = insulin produced but cells are resistant. From L06: Type 1 is primarily a genetic disease (autoimmune, polygenic predisposition); Type 2 is primarily nutritional/environmental with genetic predisposition. Both produce chronic hyperglycaemia through different mechanisms.
What to write in your book
- T1D: polygenic predisposition (HLA genes, chr 6), NOT a single-gene Mendelian disease.
- Autoimmune destruction of beta cells → no insulin → hyperglycaemia (ketoacidosis if untreated).
- Needs an environmental trigger (virus/diet/microbiome) — twin concordance ~50%.
- Managed by lifelong insulin replacement; gene+environment interaction.
Identical twin concordance for Type 1 diabetes is only ~50%. This shows that:
Gene → Protein → Phenotype: Match and Explain
For each statement, identify which genetic disease it describes and complete the gene-protein-phenotype chain.
- A patient in their 40s begins exhibiting uncontrolled jerking movements and progressive cognitive decline. Their parent died of the same condition in their 50s. Brain imaging shows significant loss of neurons in the striatum.
- A newborn's heel-prick blood test shows elevated phenylalanine. The parents are both healthy with no family history. The infant is immediately placed on a special formula and diet.
- A 9-year-old is admitted with extreme thirst, frequent urination, and weight loss despite eating normally. Blood glucose is 22 mmol/L. C-peptide is undetectable. Anti-islet cell antibodies are present.
- A 16-year-old with a chronic lung condition produces thick, sticky sputum and has had four respiratory infections this year. A sweat chloride test returns an abnormally high result.
- Compare CF and Huntington's disease in terms of inheritance pattern, mechanism of protein dysfunction (loss vs gain of function), and age of onset. Explain why the inheritance pattern follows from the mechanism.
Applying the Gene-Protein-Phenotype Framework to an Unfamiliar Disease
(a) State the gene-protein-phenotype pathway for FH. (b) Is FH autosomal recessive or dominant? Justify using evidence from the description. (c) Is this a loss-of-function or gain-of-function mutation? (d) Explain why FH is classified as a genetic non-infectious disease rather than an environmental disease, even though diet also elevates LDL in the general population.
Before newborn screening for PKU, children were typically diagnosed at 2–4 years of age when intellectual disability became apparent. By that point, the damage from years of phenylalanine accumulation was permanent — the brain damage could not be reversed by dietary change.
In 1963, microbiologist Robert Guthrie developed a simple blood test — the heel-prick Guthrie card — that could detect elevated phenylalanine in a newborn's blood within 48 hours of birth. Australia introduced universal newborn screening in the mid-1960s. Today, every Australian baby is screened for PKU (along with over 25 other metabolic conditions) within 48–72 hours of birth.
A child diagnosed with PKU at birth who is immediately placed on a low-phenylalanine diet typically develops with normal intelligence and a normal lifespan. The genetic mutation is still present — it cannot be 'fixed' — but by removing the substrate (dietary phenylalanine) that accumulates to toxic levels, the disease consequences are entirely prevented. This is one of the most powerful examples of how understanding the molecular mechanism of a genetic disease directly enables a treatment strategy.
Cystic Fibrosis
- Gene: CFTR (chr 7); mutation F508del → misfolded → no Cl⁻ channel
- Cl⁻ not secreted → water doesn't follow → thick mucus
- Organs: lungs (infection), pancreas (malabsorption)
- Autosomal recessive
PKU
- Gene: PAH (chr 12) → no phenylalanine hydroxylase
- Phenylalanine accumulates → toxic to neurons
- Managed: low-phenylalanine diet from birth
- Autosomal recessive
Huntington's Disease
- Gene: HTT (chr 4); CAG repeat expansion → polyQ huntingtin
- Gain-of-function toxicity → neuron death
- Onset 30–50 yrs; high penetrance (40+ repeats)
- Autosomal dominant
Type 1 Diabetes
- Genetic basis: polygenic (HLA genes)
- Mechanism: autoimmune destruction of beta cells
- Consequence: no insulin → hyperglycaemia
- Twin concordance ~50% (gene + environment)
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.
ApplyBand 4(4 marks) 1. Describe how a mutation in the CFTR gene leads to the development of cystic fibrosis. Trace the complete pathway from gene mutation to physiological consequences in the lungs.
AnalyseBand 4–5(5 marks) 2. Compare the mechanisms of Huntington's disease and PKU at the protein level. Explain (a) how each mutation alters protein function; (b) why this difference explains the difference in inheritance pattern (dominant vs recessive); (c) why PKU can be managed with diet but Huntington's currently cannot.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Type 1 diabetes is a genetic disease." Discuss the genetic evidence for and against this classification, explain the role of environmental factors, and conclude whether the classification is appropriate or whether a more nuanced description is more accurate.
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.
Activity 1 — Gene-Protein-Phenotype Identification
1. Huntington's disease. Gene: HTT (chr 4), expanded CAG repeats. Protein: mutant huntingtin with a long polyQ tract that misfolds and aggregates in striatal/cortical neurons. The aggregates disrupt proteasome function and mitochondrial activity → progressive neuronal death in the basal ganglia → chorea and cognitive decline. The affected parent is consistent with autosomal dominant inheritance (50% chance of inheriting the expanded allele).
2. PKU. Gene: PAH (chr 12). Both parents are healthy carriers (Pp) — one mutant + one normal allele; the child is pp (autosomal recessive). Carriers produce enough functional enzyme from the normal allele to metabolise phenylalanine. The child has no functional PAH → phenylalanine accumulates → toxic to neurons. Immediate dietary intervention is critical because brain damage begins in infancy and is irreversible.
3. Type 1 diabetes. Anti-islet cell antibodies confirm the autoimmune attack on beta cells. Undetectable C-peptide confirms insulin production has ceased (C-peptide is co-released with insulin). Blood glucose is elevated because without insulin, cells cannot take up glucose despite hyperglycaemia — the pancreas is present but its beta cells are destroyed. Chain: HLA variants → impaired self-tolerance → T cells attack beta cells → no insulin → hyperglycaemia/ketoacidosis.
4. Cystic fibrosis. Elevated sweat chloride: normally CFTR reabsorbs Cl⁻ from sweat before it reaches the surface; in CF, without functional CFTR, Cl⁻ cannot be reabsorbed → high sweat Cl⁻ (>60 mmol/L is diagnostic). Chain: CFTR F508del → misfolded protein degraded → no Cl⁻ channel → thick mucus / high sweat Cl⁻ → chronic infection, lung damage.
5. CF vs Huntington's: CF: autosomal recessive; loss of function (CFTR absent); present from birth. Huntington's: autosomal dominant; gain of function (toxic polyQ huntingtin); onset 30–50 yrs. Mechanism explains inheritance: Huntington's is dominant because the mutant protein is actively toxic — one normal allele cannot neutralise it. CF is recessive because one functional CFTR allele produces enough channel for normal Cl⁻ secretion, so carriers are unaffected.
Activity 2 — Unfamiliar Disease (FH)
(a) Gene = LDLR → non-functional/absent LDL receptor on liver cells → LDL cannot be taken up by the liver → LDL accumulates in blood → deposits in artery walls → atherosclerosis → premature cardiovascular disease.
(b) Autosomal dominant. Evidence: the heterozygous form (one mutant allele) is already affected (moderately elevated LDL, CVD risk) — one normal LDLR allele does not produce enough receptor to clear LDL adequately (a gene dosage effect). Compare with CF, where one normal allele is sufficient (carriers healthy = recessive).
(c) Loss-of-function — the LDL receptor is absent or non-functional and fails to perform its normal role; it does nothing new or toxic.
(d) FH is genetic because it is caused by a specific LDLR mutation present from birth that elevates LDL regardless of diet — a person with FH on a low-fat diet still has markedly elevated LDL. Diet affects LDL in the general population through LDL production, but FH specifically impairs the clearance mechanism (receptor-mediated uptake) — a molecular defect in a specific protein, not a response to dietary excess.
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].
Five timed questions on CF, PKU, Huntington's and Type 1 diabetes. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaAnswer questions on cystic fibrosis, PKU, Huntington's disease and Type 1 diabetes. Pool: lessons 1–7.
Return to your Think First responses at the start of the lesson.
- Q1 — unaffected parents, affected child: The parents are carriers (Cc). CF is autosomal recessive — both CFTR alleles must be mutated (cc). Can you now draw a Cc × Cc Punnett square showing the 25% probability of cc offspring?
- Q2 — Huntington's inheritance: Autosomal dominant because one mutant allele is sufficient — this follows from the gain-of-function mechanism (the toxic protein is produced regardless of the other allele).
- Write the gene → protein → phenotype chain for one disease from memory.