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, apply it to PKU and Huntington's disease, then contrast it with Type 1 diabetes, which does not fit the chain.

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?

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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.

Name the gene change.State the gene or allele involved.
Explain the protein change.Say what the protein normally does and what changes.
Link to the phenotype.Trace the effect through cells, tissues and the person.

Know what matters

Must Know

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.

Should Know

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.

Going Deeper

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.

Predict first: healthy parents, affected child?

connect

Both 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.

Key vocabulary, translated

vocab

MutationA change in the DNA base sequence. It matters because DNA is the instruction for building a protein, so a changed instruction can build a faulty protein or none at all.Like this: cystic fibrosis usually comes from three bases missing in the CFTR gene, so the chloride channel it codes for never folds properly.

ProteinThe working molecule that cells are built from and run by. Enzymes, channels, receptors and structural fibres are all proteins, and a gene's whole job is to specify one.Like this: haemoglobin carries oxygen and CFTR moves chloride. Change the gene and the job simply stops being done.

PhenotypeWhat you can actually observe or measure: symptoms, appearance, test results. It comes from genes and environment together, not from genes alone.Like this: two children with the same PKU genotype have very different phenotypes if one follows a strict low-phenylalanine diet and the other does not.

RecessiveThe condition only appears if BOTH copies of the gene are faulty. One working copy is enough to stay healthy, so carriers show no symptoms at all.Like this: cystic fibrosis is recessive, so two healthy carrier parents have a 25% chance of an affected child in each pregnancy.

DominantOne faulty copy is enough to cause the condition, so an affected parent passes it to about half their children and it appears in every generation.Like this: Huntington's disease is dominant, so a single altered allele from one parent gives each child a 50% chance of inheriting it.

True or false: a phenotype is the protein itself.

The gene to protein to phenotype framework

explain

When Francis Collins and Lap-Chee Tsui published the CFTR gene sequence in 1989, they could trace a single amino acid change through the whole chain of consequences: altered protein, non-functional chloride channel, thick mucus, chronic lung infection, progressive lung damage. Cystic fibrosis, PKU and Huntington's disease all follow that same three-step logic, because each is a single-gene condition. Type 1 diabetes, the fourth disease in this lesson, does not, and card 8 sets out why.

Three ways a mutation changes a protein

A mutated protein may fold incorrectly and be degraded before reaching its target, as in most CF mutations. It may lack catalytic activity, like the enzyme in PKU. Or it may gain a toxic function, like the huntingtin protein in Huntington's disease.

How severe the disease becomes depends on what the normal protein does and how the mutation changes it. A mutation in an enzyme needed in every cell has widespread effects, while a mutation in a tissue-specific protein has localised effects. This is why one framework can explain diseases that look completely different in a patient.

The contrasting case: multifactorial disease

Not every disease with a genetic contribution fits that chain. Type 1 diabetes is multifactorial: risk is spread across many genes rather than caused by one mutated protein, an environmental trigger is also needed, and the damage is done by the person's own immune system destroying pancreatic beta cells. There is no single altered protein to put in the middle of the chain. Card 8 works through this case in full.

HSC exam move

For any "explain a single-gene disease" question, state three things: which gene is mutated, what protein is altered and how, and what physiological consequence follows. Naming only the gene, or only the symptoms, earns partial marks.

Book notes

Single-gene disease: gene mutation, altered protein, physiological consequence.
Loss of function (CF, PKU) or gain of toxic function (Huntington's).
Type 1 diabetes is the contrast: multifactorial, polygenic risk plus a trigger plus autoimmune beta-cell destruction, not one altered protein.
Full marks need the gene, the protein change, and the consequence.

Fill the gap: a single-gene disease follows the chain: gene mutation, altered [___], physiological consequence.

Recessive and dominant: predicting with Punnett squares

explain

In autosomal recessive inheritance, two altered alleles are needed for the condition to appear. A person with one altered allele is a carrier: usually unaffected, but able to pass the allele to a child. In autosomal dominant inheritance, one altered allele is enough, so the condition often appears in every generation of a family.

Two CF carriers (Cc x Cc) produce offspring in the ratio 1 CC : 2 Cc : 1 cc. Only the cc child is affected, a 25% probability in each pregnancy, while half the children are carriers like their parents. This is why recessive diseases can appear with no family history: carriers show no symptoms and often do not know they carry the allele.

Dominant inheritance predicts differently. A person with Huntington's disease is usually heterozygous, so each child has a 50% chance of inheriting the expanded allele. Penetrance adds one more layer: it is the proportion of people with a genotype who actually show the phenotype, and it is not always 100%.

Book notes

Recessive: two altered alleles needed; a carrier has one and is usually unaffected.
Cc x Cc gives 1 CC : 2 Cc : 1 cc, so 25% affected and 50% carriers.
Dominant: one altered allele is enough, so 50% risk per child of an affected heterozygote.
Penetrance: the proportion of a genotype that actually shows the phenotype.

Two healthy carriers of the CF mutation have a child. What is the probability that the child has cystic fibrosis?

Interactive · Punnett Square Predictor

Worked example: cystic fibrosis

build

Normal 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.

Common error Why does “cystic fibrosis is caused by thick mucus” lose marks? +

Thick mucus is the phenotype at the end of the chain, not the cause. It starts with the altered CFTR gene and protein.

Trace the whole chain: altered CFTR gene → misfolded or missing channel → reduced chloride and water movement → dehydrated, thick mucus → blocked airways. The mucus is a consequence of CFTR dysfunction.

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.

CF in depth: one missing channel, many organs

explain

The most common CF-causing variant, F508del, deletes a single amino acid (phenylalanine at position 508) from the CFTR protein on chromosome 7. The altered protein misfolds and is degraded in the endoplasmic reticulum before it reaches the cell membrane, so the epithelial cell ends up with too few working chloride channels, not merely faulty ones.

Normally, CFTR sits in the apical membrane of epithelial cells and lets chloride ions move out into the airway surface liquid. Water follows by osmosis, keeping mucus thin and mobile. Without working CFTR, chloride is not secreted, water does not follow, and mucus across the lungs, pancreas, intestine and reproductive tract becomes dehydrated and thick.

The organ consequences follow logically. In the lungs, thick mucus cannot be cleared by cilia, so bacteria such as Pseudomonas and Staphylococcus colonise and cause chronic infection and progressive damage. In the pancreas, blocked ducts prevent enzyme secretion, causing malabsorption. In males, absence of the vas deferens causes infertility.

In Australia, CF affects about 1 in 2,500 births and around 3,500 people are living with it; about 1 in 25 people of European descent is a carrier. Median survival has risen from about 5 years in 1960 to 44 years in 2022, because newer treatments target the specific protein defect rather than only the symptoms.

Book notes

F508del deletes phenylalanine 508; the protein misfolds and is degraded before reaching the membrane.
No chloride secretion, no water follows by osmosis, mucus dehydrates and thickens.
Organs: lungs (chronic infection), pancreas (malabsorption), male infertility.
Australia: about 1 in 2,500 births; median survival 5 years (1960) to 44 years (2022).

In cystic fibrosis, why does the airway surface liquid become dehydrated?

Apply the chain, and meet its limit

PKU: a missing enzyme, not a broken channel

explain

Phenylketonuria (PKU) shows the same framework with a different mechanism: not a broken channel, but a missing enzyme. Mutations in the PAH gene on chromosome 12 remove functional phenylalanine hydroxylase, the liver enzyme that normally converts the essential amino acid phenylalanine into tyrosine.

Without the enzyme, phenylalanine from food accumulates in blood and tissues. High levels are toxic to developing neurons, partly by competing for amino acid transport across the blood-brain barrier and disrupting myelin synthesis, so untreated PKU causes intellectual disability and seizures. Low tyrosine also means less melanin, which is why untreated children often have pale skin and hair.

PKU is autosomal recessive, and about 1 in 50 people carries a mutated PAH allele. The decisive point is management: the damage depends on dietary phenylalanine, so a lifelong low-phenylalanine diet removes the substrate and prevents the consequences even though the mutation itself remains.

Australia introduced universal newborn PKU screening in the 1960s using the Guthrie heel-prick test on day 2 to 3 of life. A child diagnosed at birth and kept on the diet typically develops normally. It is one of the most successful public-health uses of genetic knowledge, and a preview of the screening programs you study in L17.

Book notes

PAH gene (chromosome 12), no functional phenylalanine hydroxylase.
Phenylalanine accumulates and is toxic to developing neurons; low tyrosine means less melanin.
Autosomal recessive; about 1 in 50 people is a carrier.
Guthrie heel-prick screening at day 2 to 3; low-phenylalanine diet prevents the damage.

True or false: PKU causes intellectual disability even when it is detected at birth and managed with a low-phenylalanine diet.

Huntington's disease: gain-of-function toxicity

analyse

Huntington's breaks the pattern set by CF and PKU. The problem is not loss of function but gain of toxic function: the mutation does not remove a useful protein, it produces a poisonous new one. The HTT gene on chromosome 4 contains a run of CAG repeats; fewer than 36 repeats is normal, while 36 or more causes disease.

The expanded repeat encodes an abnormally long polyglutamine tract in the huntingtin protein. The mutant protein misfolds and forms aggregates that accumulate in striatal and cortical neurons, disrupting protein clearance and mitochondrial function until the neurons die. Symptoms (involuntary jerking movements called chorea, cognitive decline, mood disturbance) typically appear between 30 and 50 years of age.

This mechanism explains the inheritance pattern. One normal HTT allele cannot protect against a toxic protein made by the other allele, so one mutated copy is sufficient: autosomal dominant, with each child of an affected parent at 50% risk. Penetrance is high, because anyone with 40 or more repeats will develop the disease if they live long enough.

The repeat can also expand further during transmission, especially from fathers, so offspring may carry more repeats than their parent. This is anticipation: earlier onset and greater severity in successive generations, and the reason juvenile forms exist at very long repeat lengths.

Book notes

HTT gene (chromosome 4), CAG repeat expansion: under 36 normal, 36 or more disease-causing.
Long polyglutamine tract, misfolded huntingtin, toxic aggregates in striatal and cortical neurons.
Gain of function explains dominance: one mutated allele is enough, 50% risk per child.
Onset usually 30 to 50 years; anticipation means earlier onset in later generations.

Why is Huntington's disease autosomal dominant?

Type 1 diabetes: predisposition plus trigger

classify

Type 1 diabetes is the most complex of the four. There is no single "diabetes gene". Risk is polygenic: many genes contribute, especially HLA immune-recognition genes on chromosome 6. The high-risk variants DR3 and DR4 are found in about 90% of people with Type 1 diabetes, and first-degree relatives carry roughly a 5 to 10% lifetime risk against a 0.4% baseline.

The mechanism is autoimmune. In a genetically susceptible person, T cells stop recognising pancreatic beta cells as self and destroy them, possibly after a trigger such as an enterovirus infection, early dietary exposure or gut microbiome disruption. With the beta cells gone, no insulin is produced, blood glucose homeostasis fails, and untreated patients can progress to life-threatening ketoacidosis.

The strongest evidence that genes are not the whole story is twin concordance. Identical twins share 100% of their DNA, so a purely genetic disease should show near 100% concordance. The observed figure is only about 50%, which shows genetic predisposition is necessary but not sufficient: an environmental trigger is also required.

Contrast this with Type 2 diabetes from L03 and L06. Type 1 is autoimmune destruction, typically with childhood onset, and it requires insulin replacement. Type 2 is insulin resistance, largely nutritional and environmental, usually with adult onset. Both end in chronic hyperglycaemia, but by different mechanisms, and conflating them costs marks.

Book notes

Polygenic risk, especially HLA variants DR3 and DR4 (about 90% of patients).
Autoimmune T-cell destruction of beta cells, so no insulin and chronic hyperglycaemia.
Twin concordance about 50%: genes are necessary but not sufficient, a trigger is required.
Type 1 (autoimmune, no insulin) is a different disease from Type 2 (insulin resistance).

Two of these statements are true and one is a lie. Tap the lie.

Apply the chain, and meet its limit

compare

PKU

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.

Four genetic disorders compared: cystic fibrosis (autosomal recessive, mutated CFTR chloride channel blocking Cl- and water so mucus thickens), Huntington's disease (autosomal dominant, CAG trinucleotide repeat expansion over 36 repeats, with a dominant-inheritance pedigree and 50% risk per child), Down syndrome (trisomy 21, karyotype 47,XX/XY,+21) and the genetic-testing pathway from carrier screening through prenatal testing to diagnostic tests.

Cystic fibrosis (recessive) and Huntington's disease (dominant) as single-gene disorders, Down syndrome as a chromosomal disorder, and the genetic-testing and counselling pathway.

Which example is the best comparison for a multifactorial condition rather than a simple single-gene condition?

Interactive · Genetic Disease Matcher

Choose your route

Real-world anchor: the PKU screening story

example

Before newborn screening, PKU was usually diagnosed at 2 to 4 years of age, when intellectual disability became obvious. By then, years of phenylalanine accumulation had permanently damaged the brain, and changing the diet could not reverse it.

In 1963 the microbiologist Robert Guthrie developed a simple blood test, the heel-prick Guthrie card, that detects elevated phenylalanine within 48 hours of birth. Australia introduced universal newborn screening in the mid-1960s, and today every Australian baby is tested for PKU and more than 25 other metabolic conditions within 48 to 72 hours of birth.

The payoff is the whole point of this lesson's framework. Understanding the mechanism, a missing enzyme and an accumulating substrate, pointed directly at a management strategy: remove the substrate. A child diagnosed at birth and kept on a low-phenylalanine diet develops with normal intelligence, even though the mutation itself is still present.

Book notes

Pre-screening: PKU diagnosed at 2 to 4 years, after permanent brain damage.
Guthrie heel-prick test (1963); Australia began universal screening in the mid-1960s.
Mechanism knowledge gave the strategy: remove dietary phenylalanine, prevent the phenotype.

Three traps to avoid

analyse

"Genetic means inevitable." Only high-penetrance dominant conditions like Huntington's come close. Recessive diseases need two altered alleles, carriers are unaffected, and polygenic conditions like Type 1 diabetes also need an environmental trigger. Penetrance varies, so a genotype is a risk, not a sentence.

"CF is caused by thick mucus." This reverses cause and effect. The chain starts with the altered CFTR gene and protein; thick mucus is the phenotype at the end of the chain. Exam answers that open with the symptom and never mention the gene or protein lose most of the available marks.

"The mutation is present at birth, so the disease is present at birth." Huntington's shows why this fails: the expanded allele is present from conception, but the mutant protein accumulates slowly and neurons die over decades, so symptoms typically emerge only between 30 and 50 years of age.

Book notes

Genetic does not mean inevitable: penetrance, carriers and triggers all intervene.
A symptom is not a cause: always trace back to the gene and protein.
A mutation present from birth can still produce a late-onset disease.

Exam rehearsal: an unfamiliar disease

apply

Exam questions often hand you a disease you have never seen. Familial hypercholesterolaemia (FH) is a good rehearsal: mutations in the LDLR gene remove functional LDL receptors from liver cell membranes, so LDL cholesterol is not taken up from the blood and deposits in artery walls, causing premature cardiovascular disease.

Apply the framework. Gene: LDLR. Protein: a non-functional or absent LDL receptor, a loss of function. Phenotype: elevated blood LDL and early heart disease. Heterozygotes are moderately affected from middle age, while homozygotes are severely affected in childhood, so one altered allele already has an effect and the pattern is not simple recessive inheritance.

FH affects about 1 in 250 Australians and is significantly underdiagnosed. Diet raises LDL in everyone, which tempts students to call FH environmental. It is not: the mutation sets a baseline LDL level that diet alone cannot explain, which is exactly the distinction an "evaluate" question is hunting for.

Book notes

FH: LDLR mutation, loss of functional LDL receptor, LDL accumulates in artery walls.
Heterozygotes are affected from middle age; homozygotes in childhood.
Australia: about 1 in 250 people; genetic baseline, not just diet.

Choose your route

differentiate

Pick one route, whichever matches how confident you feel right now. Supported gives you the most structure, Stretch asks for the most independent judgement. You only need to complete one.

Supported

Use the frame to explain CF.

Cover A change in the CFTR gene changes the … protein. This reduces … so …

Core

Compare the inheritance pattern of CF and Huntington's disease.

Cover CF is … because … Huntington's is … because …

Stretch

Explain why Type 1 diabetes should not be described as caused by one “diabetes gene”.

Cover Include genetic susceptibility, autoimmune beta-cell destruction and non-genetic influences.

Exit retrieval

retrieve

Memorise

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.

Independent practice

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.

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].

Retrieve and reflect

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.

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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 most common CFTR mutation, F508del, deletes a single amino acid (phenylalanine at position 508) on chromosome 7q31, so the CFTR protein misfolds and is broken down before it reaches the membrane. This loss-of-function change leaves the cell with too few working chloride channels. 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 F508del mutation → misfolded CFTR degraded before it reaches the membrane → too few working Cl⁻ channels → thick mucus → lung damage).

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