Cystic fibrosis (CF) is caused by a RECESSIVE allele — written as 'f' (faulty) while 'F' is the normal allele.
GENOTYPES:
FF — unaffected, not a carrier.
Ff — carrier — appears healthy but carries one copy of the faulty allele.
ff — affected — has cystic fibrosis.
For a child to have CF, they must inherit TWO recessive alleles — one from each parent.
Two carriers (Ff × Ff) have a 25% chance of an affected child per pregnancy.
EFFECTS OF CYSTIC FIBROSIS:
The faulty allele affects a protein that controls the movement of salt and water across cell membranes.
This causes a build-up of THICK, STICKY MUCUS in:
The LUNGS — makes breathing difficult, blocks airways, traps bacteria → repeated chest infections → lung damage.
The DIGESTIVE SYSTEM — blocks ducts from the pancreas → digestive enzymes cannot reach the gut → poor absorption of nutrients.
TREATMENT: physiotherapy to loosen mucus, antibiotics for infections, enzyme supplements with food. No cure (though gene therapy is in development).
CF is the most common serious inherited disorder in the UK — approximately 1 in 25 people carry the allele.
Polydactyly
Polydactyly is caused by a DOMINANT allele — written as 'D' while 'd' is the normal allele.
GENOTYPES:
DD — affected (rare — very few people have two copies of the dominant allele).
Dd — affected — only ONE copy needed to show the condition.
dd — unaffected.
A person with polydactyly has one or more EXTRA FINGERS OR TOES.
Because the allele is DOMINANT:
Only ONE copy is needed — so an affected parent (Dd) has a 50% chance of passing the condition to each child.
The condition appears in EVERY GENERATION that carries the allele.
An affected person has at least one affected parent (unless it arose from a new mutation).
Polydactyly is not life-threatening and can be surgically corrected.
KEY CONTRAST with cystic fibrosis:
CF = recessive — can skip generations (carriers appear normal).
Polydactyly = dominant — appears in every generation that carries it.
Genetic Testing and Ethical Issues
GENETIC TESTING can identify whether a person carries alleles for inherited conditions.
Types of testing:
PRE-CONCEPTION testing — couples who have a family history of a genetic condition can be tested to find out if they are carriers before having children.
PRE-NATAL testing — testing the embryo or foetus during pregnancy. Methods: amniocentesis (sampling amniotic fluid), chorionic villus sampling (CVS — sampling placental tissue).
NEWBORN SCREENING — blood spot test (heel prick) shortly after birth screens for several conditions including CF.
ETHICAL CONSIDERATIONS:
Privacy — who has access to genetic information? Could affect insurance or employment.
Decision-making — if a foetus tests positive, should the pregnancy continue? Raises difficult ethical questions.
Psychological impact — knowing you carry an allele for a serious condition causes anxiety.
Social stigma — discrimination against those known to have certain genetic profiles.
These are genuine ethical debates with no single correct answer — the key is to consider multiple perspectives.
⚠️ Common Mistake
Cystic fibrosis is RECESSIVE — both parents can be unaffected carriers. Polydactyly is DOMINANT — at least one parent will always be affected (unless it arose from a new mutation). Students often apply these rules the wrong way round. Remember: recessive conditions can SKIP GENERATIONS (via carriers). Dominant conditions CANNOT skip generations.
📌 Key Note
CF: recessive (ff), thick mucus in lungs and gut, 1 in 25 carriers in UK. Polydactyly: dominant (Dd), extra digits, appears every generation. Recessive can skip generations. Dominant cannot (unless new mutation).
🎯 Matching Activity — Cystic Fibrosis or Polydactyly?
Match each feature to the correct inherited disorder. — drag the symbols on the right to match the component names on the left.
Cystic fibrosis
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Polydactyly
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Cystic fibrosis
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Polydactyly
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Cystic fibrosis
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Polydactyly
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Caused by a dominant allele — only one copy needed to show the condition
Causes thick sticky mucus in lungs and digestive system
Causes extra fingers or toes — not life-threatening
Caused by a recessive allele — both copies needed for the condition to show
Can skip generations — carriers appear healthy
Appears in every generation — an affected person always has at least one affected parent
⚽ FIFA Worked Examples
CF Inheritance Cross
Two carriers for cystic fibrosis (Ff × Ff) are expecting a child. What is the probability the child will have CF?
F
Draw Punnett square: Ff × Ff. CF allele = f (recessive). Normal = F
I
Offspring: FF, Ff, Ff, ff → ratio 1:2:1
F
Only ff has cystic fibrosis = 1 out of 4 boxes
A
Probability of CF = 1/4 = 25%
⭐ Higher Tier Only
Embryo screening (PGD — preimplantation genetic diagnosis): embryos from IVF can be tested for genetic conditions before implantation. Allows selection of unaffected embryos. Ethical issues: is this a slippery slope to designer babies? Does it devalue people with that condition? Genetic counselling helps families understand inheritance patterns and risks without being directive about choices.
🎯 Test Yourself
Question 1 of 2
1. Both parents appear healthy but have a child with cystic fibrosis. What must be true of the parents?
2. Why does polydactyly appear in every generation, while cystic fibrosis can skip generations?
⭐ How Well Do You Understand This Topic?
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