2.4.3Unit 2: Cells, Development, Biodiversity and Conservation
Cystic Fibrosis — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 26 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Genetic Basis
Cystic fibrosis is a genetic disorder that affects cell membranes and the secretions of several organs.
CFTR gene
- CFTR gene This gene on chromosome 7 codes for a chloride ion channel protein found in the cell surface membrane.
Recessive allele
- Recessive allele Cystic fibrosis is caused by a recessive allele, so a person must be homozygous recessive to have the disorder.
Carriers
- Carriers Heterozygous people have one working allele and do not have the disorder, but they can pass the faulty allele on.
Faulty channel
- Faulty channel The mutation leads to non-functional chloride channels, which reduces the movement of chloride ions across the membrane.
Reduced osmosis
- Reduced osmosis With less chloride moving out, less water follows by osmosis into secretions, which makes the mucus thick and sticky.
Inheritance
- Inheritance Two carrier parents have a 1 in 4 chance of a child with the disorder with each pregnancy.
Part 2 of 5Effects on Body Systems
Effects on Body Systems
Respiratory system
- Respiratory system Thick mucus clogs airways and cilia cannot clear it, so microorganisms stay in the lungs and infections are frequent.
Gas exchange
- Gas exchange Mucus build-up reduces the surface area available for gas exchange and causes breathing difficulty.
Digestive system
- Digestive system Mucus can block the pancreatic duct, so digestive enzymes do not reach the small intestine and nutrients are poorly absorbed.
Reproductive system
- Reproductive system Thick mucus can block the tubes that carry sperm in males and can prevent sperm from passing the cervix in females.
Part 3 of 5Management and Probability
1
Physiotherapy helps to loosen the mucus in the airways and improve gas exchange.
2
Antibiotics are used to treat chest infections that occur because bacteria remain trapped in the thick mucus.
3
Enzyme supplements taken with meals replace the pancreatic enzymes that cannot reach the small intestine.
4
Carrier testing can show whether prospective parents carry the faulty allele, and so allow them to understand their risk.
5
If both parents are carriers, a child has a 25% chance of being affected, a 50% chance of being a carrier and a 25% chance of being unaffected.
Part 4 of 5Worked Example
Question
- Two carriers of cystic fibrosis have two children.
- Calculate the probability that both children have cystic fibrosis.
- CrossFf × Ff gives FF, Ff, Ff and ff, so probability of ff = 1/4 per child.
- The births are independent events, so multiply the probabilities.
- Probability both affected = 1/4 × 1/4 = 1/16 = 0.0625.
Answer 1/16, or 6.25%.
Part 5 of 5Exam Checklist & Tips
2.4.3 – Key Points for the Exam
- CFTR The gene that codes for a chloride ion channel protein.
- Recessive allele An allele that is only expressed when two copies are present.
- Carrier A heterozygous person who does not have symptoms but can pass on the allele.
- Mucus A sticky secretion that traps microorganisms and is normally cleared by cilia.
Common Mistakes
- Saying cystic fibrosis is caused by a dominant allele or is infectious.
- Writing that mucus is thick because it lacks water, without linking this to faulty chloride channels and osmosis.
- Stating that carriers have symptoms, when carriers are heterozygous and normally unaffected.
- Giving the 1 in 4 risk as if one in four children in a family must be affected.
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