Inherited disorders: polydactyly and cystic fibrosis

What you'll learn

This revision guide covers two important inherited disorders tested in AQA GCSE Biology: polydactyly and cystic fibrosis. You'll learn how these conditions are passed from parents to offspring through different inheritance patterns, understand the genetic basis of each disorder, and practise drawing and interpreting genetic diagrams. These topics link directly to inheritance, variation and evolution in your exam specification.

Key terms and definitions

Inherited disorder — a medical condition caused by faulty alleles passed from parents to offspring through gametes

Allele — a version of a gene; alleles can be dominant or recessive

Dominant allele — an allele that produces its characteristic effect even when only one copy is present (represented by a capital letter, e.g. D)

Recessive allele — an allele that only produces its characteristic effect when two copies are present (represented by a lowercase letter, e.g. f)

Homozygous — having two identical alleles for a particular gene (e.g. DD or dd)

Heterozygous — having two different alleles for a particular gene (e.g. Df)

Genotype — the combination of alleles an individual has (e.g. Ff)

Phenotype — the observable characteristics resulting from the genotype and environmental factors (e.g. having cystic fibrosis)

Carrier — an individual who is heterozygous for a recessive allele; they do not show the condition but can pass it to offspring

Core concepts

Polydactyly: a dominant inherited disorder

Polydactyly is a genetic condition where a baby is born with extra fingers or toes. The condition affects approximately 1 in 1,000 births and varies in severity — some individuals may have fully formed extra digits while others have small skin tags.

Key characteristics of polydactyly inheritance:

• Caused by a dominant allele (we can represent this as D)
• The normal allele is recessive (represented as d)
• Only one copy of the faulty allele is needed to have the condition
• If one parent has polydactyly, there is a significant chance their children will inherit it
• A person with genotype DD or Dd will have polydactyly
• Only individuals with genotype dd will have the normal number of digits

Polydactyly genotypes and phenotypes:

The condition is usually not life-threatening and can often be treated with surgery shortly after birth if required. Because the allele is dominant, affected individuals cannot be carriers — if they have the allele, they display the condition.

Cystic fibrosis: a recessive inherited disorder

Cystic fibrosis (CF) is a serious inherited disorder affecting internal organs, particularly the lungs and digestive system. It is caused by a recessive allele that affects cell membranes.

What happens in cystic fibrosis:

• Thick, sticky mucus is produced in the lungs and digestive system
• The mucus accumulates in the lungs, making breathing difficult
• It increases the risk of lung infections
• The mucus blocks pancreatic ducts, preventing digestive enzymes reaching the small intestine
• This leads to difficulty digesting food and absorbing nutrients
• Patients require daily physiotherapy to clear mucus from lungs
• Life expectancy has improved with treatment but remains reduced compared to the general population

Inheritance pattern of cystic fibrosis:

• Caused by a recessive allele (we can represent this as f)
• The normal allele is dominant (represented as F)
• Two copies of the faulty allele (ff) are needed to have the disorder
• Parents who are carriers (Ff) do not have symptoms but can pass the allele to offspring
• If both parents are carriers, there is a 25% chance with each pregnancy that the child will have CF

Cystic fibrosis genotypes and phenotypes:

Approximately 1 in 25 people in the UK are carriers of the CF allele. The condition is more common in people of European ancestry and affects around 1 in 2,500 newborns in the UK.

Drawing genetic diagrams for inherited disorders

Genetic diagrams (also called Punnett squares) show how alleles are inherited and help predict the probability of offspring inheriting particular conditions.

Steps for drawing a genetic diagram:

1. Choose letter symbols — use the first letter of the dominant characteristic; capital for dominant, lowercase for recessive
2. Write parents' genotypes — state what alleles each parent has
3. Show gametes — write the possible alleles in each parent's gametes (eggs and sperm)
4. Draw the Punnett square — place one parent's gametes along the top, the other parent's down the side
5. Fill in offspring genotypes — combine alleles from each gamete
6. Work out ratios — count how many of each genotype appear

Important points for genetic diagrams:

• Sex cells (gametes) contain only one allele for each gene due to meiosis
• Fertilisation combines one allele from each parent
• Use a 2×2 grid for crosses involving two heterozygous parents
• Always show your working clearly in exams

Probability and inheritance patterns

Understanding probability is essential when predicting inheritance of disorders.

Key probability concepts:

• Each pregnancy is an independent event — previous children do not affect the probability for the next child
• A 1 in 4 chance (25%) means that on average, one in four offspring will be affected
• This does not guarantee exactly one affected child in a family of four
• Probabilities can be expressed as ratios (3:1), fractions (¼), percentages (25%), or decimals (0.25)

Family pedigree analysis:

Family trees (pedigrees) show how disorders pass through generations. From these you should be able to:

• Identify whether a disorder is dominant or recessive
• Determine whether individuals are homozygous or heterozygous
• Work out the probability of future offspring being affected

Clues in pedigrees:

• If two unaffected parents have an affected child → the disorder is recessive and both parents are carriers
• If an affected parent has unaffected children → the disorder is likely dominant
• If the disorder appears in every generation → likely dominant
• If the disorder skips generations → likely recessive

Comparing polydactyly and cystic fibrosis

Understanding the differences between these disorders helps you answer comparison questions effectively.

Testing and genetic counselling

Carrier testing:

• People with a family history of cystic fibrosis can be tested to see if they carry the recessive allele
• This involves DNA analysis from a blood or saliva sample
• Prospective parents can use this information when planning families
• If both partners are carriers, they can receive genetic counselling

Antenatal screening:

• Pregnant women can have tests to check if their fetus has inherited certain conditions
• Amniocentesis or chorionic villus sampling can detect CF alleles before birth
• These procedures carry small risks and raise ethical considerations
• Results allow parents to make informed decisions and prepare for specialist care

Worked examples

Example 1: Polydactyly inheritance cross (4 marks)

Question: A man heterozygous for polydactyly (Dd) has children with a woman who does not have polydactyly (dd). Draw a genetic diagram to show the possible genotypes of their children. Calculate the probability that a child will have polydactyly.

Answer:

Parents' genotypes: Dd × dd

Parents' phenotypes: man has polydactyly × woman has normal digits

Gametes: D or d (from father); d (from mother)

Punnett square:

        |  D  |  d  |
    ----|-----|-----|
    d   | Dd  | dd  |
    ----|-----|-----|
    d   | Dd  | dd  |

Offspring genotypes: 2 Dd : 2 dd (or 1:1 ratio)

Offspring phenotypes: 2 with polydactyly : 2 with normal digits

Probability = 2 out of 4 = ½ = 50% chance of having polydactyly

Mark scheme guidance: 1 mark for correct parent genotypes, 1 mark for gametes, 1 mark for correct Punnett square, 1 mark for probability calculation.

Example 2: Cystic fibrosis carrier cross (5 marks)

Question: Two parents are both carriers of the cystic fibrosis allele but do not have the condition themselves. Use a genetic diagram to show the probability of their children having cystic fibrosis.

Answer:

Key: F = normal allele (dominant), f = cystic fibrosis allele (recessive)

Parents' genotypes: Ff × Ff

Parents' phenotypes: both are healthy carriers

Gametes: F or f (from each parent)

Punnett square:

        |  F  |  f  |
    ----|-----|-----|
    F   | FF  | Ff  |
    ----|-----|-----|
    f   | Ff  | ff  |

Offspring genotypes: 1 FF : 2 Ff : 1 ff

Offspring phenotypes:

• 3 without cystic fibrosis (1 FF + 2 Ff)
• 1 with cystic fibrosis (ff)

Probability of having CF = 1 in 4 = 25%

Additional notes: The 2 Ff children would be carriers like their parents.

Mark scheme guidance: 1 mark for correct parent genotypes with key, 1 mark for gametes shown, 1 mark for fully correct Punnett square, 1 mark for offspring genotypes/phenotypes, 1 mark for correct probability.

Example 3: Explain question (3 marks)

Question: Explain why two parents without cystic fibrosis can have a child with the condition.

Answer:

• Cystic fibrosis is caused by a recessive allele (f)
• Both parents must be carriers with genotype Ff
• They do not have CF because they have one dominant normal allele (F) which masks the recessive allele
• Each parent can pass the recessive f allele to their child
• If the child inherits f from both parents (ff genotype), they will have cystic fibrosis

Mark scheme guidance: 1 mark for stating CF is recessive, 1 mark for explaining both parents are carriers, 1 mark for explaining child inherits two recessive alleles.

Common mistakes and how to avoid them

• Confusing dominant and recessive — Remember: dominant alleles show their effect with just one copy; recessive alleles need two copies. Polydactyly = dominant (D), cystic fibrosis = recessive (f).

• Using the wrong case letters — Always use capital letters for dominant alleles and lowercase for recessive. Never use different letters for the same gene (e.g., don't use P and f together — it should be F and f for cystic fibrosis).

• Forgetting that each pregnancy is independent — A 25% probability means each child has a 1 in 4 chance, not that exactly 1 in 4 children will definitely be affected.

• Writing genotypes in gametes incorrectly — Gametes contain only ONE allele per gene because of meiosis. A parent with Ff produces gametes with F or f, not Ff.

• Confusing genotype and phenotype — Genotype is the alleles (Ff), phenotype is the observable characteristic (carrier without symptoms). Don't use these terms interchangeably.

• Mixing up carriers and affected individuals — Only recessive conditions have carriers. Someone with Dd has polydactyly (affected, not a carrier). Someone with Ff doesn't have CF but is a carrier.

Exam technique for inherited disorders questions

• Command word "State" — Give short, factual answers without explanation. "State the type of allele that causes polydactyly" requires just "dominant" (1 mark).

• Command word "Explain" — You must give reasons or mechanisms. Link cause and effect. For 3 marks, make at least three separate points explaining why/how something happens.

• Genetic diagram questions — Always show: (1) parents' genotypes, (2) gametes, (3) Punnett square with all offspring genotypes, (4) a clear ratio or probability statement. Marks are awarded for each step, so show full working even if you're confident.

• Use correct ratios and probabilities — Express answers in the form requested. If asked for a ratio, give 1:1 or 3:1. If asked for probability, give a fraction (¼), percentage (25%), or statement (1 in 4 chance). These are usually worth 1 mark.

Quick revision summary

Polydactyly (extra digits) is caused by a dominant allele — individuals need only one copy to be affected, and no carriers exist. Cystic fibrosis is caused by a recessive allele affecting mucus production in lungs and digestive system — individuals need two copies (ff) to be affected, while heterozygous individuals (Ff) are healthy carriers. Use genetic diagrams with Punnett squares to predict inheritance patterns. Always show parents' genotypes, gametes, and offspring genotypes clearly. Remember that each pregnancy is independent with its own probability.