What you need to know
- 2.16Effects of cystic fibrosisA recessive genetic disorder caused by mutations in the CFTR gene. Thick, sticky mucus affects the lungs, pancreas and reproductive system. on gaseous exchange, digestive and reproductive systems.
- 2.14(ii)A genetic disorder caused by mutationA change in the sequence of bases in DNA..
- 2.15(ii)Autosomal recessiveAn allele that is expressed in the phenotype only when no dominant allele is present (when homozygous). inheritance in context.
This is an explanation of disease mechanisms, not individual medical advice. The notes use simplified transport and inheritance models; effects vary among people.
By the end, you should be able to…
- Build a geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. → protein → transport → organ-function explanation.
- Explain effects in lungs, pancreas and reproductive systems.
- Apply autosomal recessive inheritance without stereotyping outcomes.
Build your understanding
From an allele to a transport problem
cystic fibrosisA recessive genetic disorder caused by mutations in the CFTR gene. Thick, sticky mucus affects the lungs, pancreas and reproductive system. is associated with disease-causing variants in the CFTRA membrane channel protein that transports chloride ions out of cells. Faulty CFTR causes cystic fibrosis. geneA sequence of DNA bases that codes for a polypeptide or a functional RNA.. CFTR is a membrane protein involved in chloride ion transport. Different variants can affect how much functional protein reaches the membrane or how well it works.
In the airway model used here, reduced chloride movement into the surface fluid, together with altered ion transport, reduces the water retained at the airway surface. Water movement follows water-potential differences by osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane.. Mucus becomes less hydrated and more difficult to clear. CFTR transports ions; it is not a pump carrying water or mucus.
Gaseous exchange
Thick, sticky mucus obstructs airways and is less effectively moved by cilia. Microorganisms are retained, increasing the likelihood of infection and inflammationA local response to infection or damage: histamine makes blood vessels dilate and become leakier, causing heat, redness and swelling and bringing in phagocytes.. Repeated damage can impair the surfaces and ventilation needed for effective gas exchange.
Connect airway obstruction with reduced delivery of fresh air to exchange surfaces and therefore a reduced oxygen gradient in poorly ventilated regions. Do not describe mucus as filling every alveolus in every person; severity and effects vary.
Digestion
Obstruction of pancreatic ducts can prevent sufficient digestive enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. reaching the small intestine. Less effective breakdown of large food molecules means fewer absorbable products, which can lead to poor nutrient absorption and difficulty maintaining growth or body mass.
The key chain is blocked ducts → fewer enzymes in the intestine → reduced digestion → reduced absorption of nutrients. Enzymes digest food; they do not directly absorb it through the gut wall.
Reproduction and inheritance
In males with cystic fibrosis, the vas deferens is often absent or obstructed, preventing sperm from entering semen even when sperm production occurs. In females, thick cervical mucus can make sperm passage more difficult and reduce fertility. Cystic fibrosis does not mean that every person is unable to have children.
For a simplified recessiveAn allele that is expressed in the phenotype only when no dominant allele is present (when homozygous). model, two carrier parents Ff × Ff have a 1/4 chance of an affected ff child at each conception. CFTR is not on a sex chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere., so this is autosomal inheritance, not sex-linkedDescribes a gene carried on a sex chromosome, usually the X chromosome, so the pattern of inheritance differs between males and females. inheritance. Actual disease severity cannot be predicted from this Punnett square alone.
NOT: CFTR does not carry mucus out of the lungs.
- Two carrier parents: 1/4 chance of an affected child in the simple model
- The gene is autosomal, so it is not X-linked
- Airways, pancreas and reproductive ducts can be affected
- Severity is not predicted by the Punnett square alone
Two traps this diagram blocks
- Do not say CFTR moves mucus. The mucus change is an indirect result of ion and water movement.
- Do not describe the inheritance as X-linked.
From the diagram alone
Question
Explain how reduced CFTR function can make airway mucus harder to clear. [4]
Show a model answer
Less functional CFTR means less chloride-ion transport into the airway surface fluid. The water potential of that fluid changes, so less water moves into it by osmosis. The mucus is less hydrated and therefore thicker and harder to clear. CFTR itself does not transport the mucus.
Make your reasoning visible
Link each level of explanation
mutationA change in the sequence of bases in DNA. → altered functional CFTRA membrane channel protein that transports chloride ions out of cells. Faulty CFTR causes cystic fibrosis. → altered ion movement → water movement by osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. → less hydrated mucus → a named organ effect.
Separate digestion from absorption
Reduced enzyme delivery impairs digestion first, reducing the small soluble products available for absorption.
Avoid absolute claims
Use “can”, “often” or the conditions supplied. Do not state that all people with CF have identical symptoms or are infertile.
Key explanations
- The changed ion balance changes water potential. Less water is retained at the surface by osmosis.
Avoid this misconception
The claim: CFTR moves mucus out of the lungs.
The correction: CFTR is involved in ion transport across membranes; the effect on mucus hydration and clearance is indirect.
From the CFTR gene to the symptoms
Cystic fibrosis is caused by recessive alleles of the CFTR gene. Order the chain from the gene to the mucus, then the consequences in two body systems.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Lungs or Pancreas.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the chain in order, from the alleles to the effect on gas exchange.
2 of 6 in the right place
The reproductive system is also affected (blocked sperm ducts, thick cervical mucus); that is a third, parallel effect, not a later step. Variants differ in how severe they are.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain how a membrane-protein defect can affect mucus hydration.
Show suggested answer
Reduced functional CFTR alters ion transport at the airway surface. The resulting water-potential relationships reduce surface hydration, so mucus becomes more concentrated and harder to clear.
Practice question
Retrieval and explanation · suggested answer
Why can blocked pancreatic ducts affect body mass even when a person eats food?
Show suggested answer
Fewer pancreatic enzymes reach the intestine, reducing digestion and the supply of absorbable nutrients. Eating food does not guarantee that its nutrients are absorbed.
Practice question
Retrieval and explanation · suggested answer
Two heterozygous carrier parents have one affected child. What is the modelled probability that their next child is affected?
Show suggested answer
1/4 at the next conception; the previous outcome does not change the gamete probabilities.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A fictional intervention restores chloride transport in airway cells but has no effect on pancreatic cells. Predict one possible benefit and one reason it would not correct every consequence of CF.
Show a hint
Follow the tissue in which transport has changed.
Explore the reasoning
Improved airway-surface hydration could improve mucus clearance. Pancreatic duct problems may remain, so digestion may still be impaired. A tissue-specific effect cannot be assumed to correct the whole body.
Recall this lesson
Lesson recall questions
Question 1 of 5: Some mutations cause ________ or genetic disorders, but many mutations have ________ observable effect.
Mutations and their effects
Practice question
Retrieval and explanation · suggested answer
Some mutations cause ________ or genetic disorders, but many mutations have ________ observable effect.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
One protein, several specialist teams
2C · Cystic Fibrosis: A Genetic DiseaseThe Cystic Fibrosis Foundation describes effects across several organs. Understanding the connections calls on respiratory, nutritional and reproductive expertise rather than treating CF as only a lung condition.…Specification 2.16Read & use the scienceClose article
Why this matters
The Cystic Fibrosis Foundation describes effects across several organs. Understanding the connections calls on respiratory, nutritional and reproductive expertise rather than treating CF as only a lung condition. A molecular defect can have consequences that depend on the tissue in which transport is disturbed. In the airways, altered secretions affect clearance; in the digestive system, obstruction can interfere with delivery of pancreatic enzymes and hence digestion. A dietitian’s nutritional questions and a physiologist’s functional measurements can therefore concern different consequences of the same underlying condition. This is why multidisciplinary thinking matters: explaining the gene or channel alone does not yet explain the full pattern of effects across organs.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.16 understand how the expression of a gene mutation in people with cystic fibrosis impairs the functioning of the gaseous exchange, digestive and reproductive systems
Use the science
Why might a physiologist and a dietitian ask different questions about the same CFTR defect?
Explore a possible response
The physiologist may investigate transport and ventilation; the dietitian may investigate digestion and nutrient absorption. Both trace consequences of the same underlying disorder.
Explore the sources
- Cystic Fibrosis Foundation: about CFCystic Fibrosis Foundation
- Your CF Care TeamCystic Fibrosis Foundation
Why one CF medicine does not fit every mutation
2C · Cystic Fibrosis: A Genetic DiseaseCFTR modulators target defects in the protein rather than only managing the consequences of thick mucus. Correctors can help some CFTR proteins fold and reach the cell surface;…Specification 2.16 · 2.14(ii) · 2.6(iii)Read & use the scienceClose article
Why this matters
CFTR modulators target defects in the protein rather than only managing the consequences of thick mucus. Correctors can help some CFTR proteins fold and reach the cell surface; potentiators improve the activity of channels that are present. The distinction follows directly from sequence, structure and function. A treatment that helps an existing channel open cannot solve the same problem in a cell making no usable channel at all. This is an example of personalised medicine based on the molecular effect of a variant. Improving protein function can help symptoms, but it does not change the inherited DNA sequence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.16 understand how the expression of a gene mutation in people with cystic fibrosis impairs the functioning of the gaseous exchange, digestive and reproductive systems
- 2.14(ii) know that some mutations will give rise to cancer or genetic disorders, but that many mutations will have no observable effect
- 2.6(iii) understand the significance of a protein’s primary structure in determining its secondary structure, three-dimensional structure and properties (globular and fibrous proteins and the types of bonds involved in its three-dimensional structure)
Use the science
Compare why a potentiator might help a gating defect but not a mutation that prevents any CFTR protein being produced.
Explore a possible response
A gating defect leaves a channel that the drug can act on. With no protein there is no channel target; a different approach is required.
Explore the source
- CFTR Modulator TypesCystic Fibrosis Foundation