AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Inheritance, variation and evolution › Reproduction
Practise Inherited disorders. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Polydactyly (caused by a dominant allele) and cystic fibrosis (caused by a recessive allele), how they are inherited, and the economic, social and ethical issues of embryo screening. Expect Punnett squares, carrier questions and 4 to 6 mark 'evaluate' questions using information you are given.
Key facts
Polydactyly = extra fingers or toes; caused by a dominant allele
Cystic fibrosis = a disorder of cell membranes; caused by a recessive allele
Carrier = heterozygous for a recessive disorder: no disorder, but can pass the allele on
Embryo screening = testing embryo cells for the alleles that cause disorders
Economic = cost; social = effects on people and society; ethical = right or wrong
Notes
Two inherited disorders
Some disorders are inherited: they are caused by the inheritance of certain alleles.
Polydactyly (having extra fingers or toes) is caused by a dominant allele. One copy is enough to cause it, so there are no carriers.
Cystic fibrosis (CF) is a disorder of cell membranes, caused by a recessive allele. A person must inherit two copies to have it.
A person with one CF allele (heterozygous, e.g. Ff) is a carrier: they do not have the disorder but can pass the allele on.
Predicting the risk
diagram
Two carriers (Ff × Ff): probability of a child with CF = 0.25; of a carrier = 0.5; of a child who neither has CF nor carries it = 0.25.
Two carriers: a 1 in 4 chance of a child with cystic fibrosis, for every pregnancy.
A parent with polydactyly (Dd) and a parent without it (dd): probability of a child with polydactyly = 0.5.
The probability is the same for every pregnancy.
Embryo screening
In embryo screening, cells from an embryo are tested for the alleles that cause a genetic disorder.
Arguments for: it can prevent suffering from a serious disorder; it can save the long-term cost of treatment.
Arguments against: it is expensive; embryos with the disorder are usually destroyed, which some people believe is wrong; some fear it could lead to choosing embryos for other characteristics ('designer babies'); some feel it suggests people with disorders are less valued.
Economic issues are about money, social issues are about the effects on people and society, and ethical issues are about what is right or wrong.
Gene therapy, which aims to correct a faulty allele, is being researched as a possible future treatment.
How to answer each type of question
Explain carriers and allele types
1 to 2 marksGrade 5
Polydactyly: dominant allele, extra fingers or toes.
Cystic fibrosis: recessive allele, disorder of cell membranes.
Explain carriers using 'heterozygous' or 'one recessive allele'.
Example. Explain why a person can be a carrier of cystic fibrosis but cannot be a carrier of polydactyly. [2 marks]
Show the model answerHide the model answer
Cystic fibrosis is caused by a recessive allele, so a person with one copy (heterozygous) does not have the disorder but can pass it on (1). Polydactyly is caused by a dominant allele, so anyone with one copy has the condition (1).
Don’t lose marks
Saying cystic fibrosis is caused by a dominant allele, or polydactyly by a recessive one.
Saying a carrier has a mild form of the disorder. A carrier does not have the disorder.
Giving the probability of being a carrier when asked for the probability of having the disorder. Read the question carefully.
In 'evaluate' questions, giving only arguments against (or only for), or giving no conclusion.
More tips
Memory tricks
Poly = many, dactyl = digit: polydactyly means extra fingers or toes. D for Dominant, D for Digits.
C for Cystic fibrosis, C for Carrier, C for Cell membranes: recessive, so carriers exist.
Two carriers → 1 : 2 : 1 → a quarter affected, half carriers, a quarter clear.
Exam technique
For ethics questions, use the information provided and quote figures from it.
Label issues correctly if asked: economic = money, social = people and society, ethical = right or wrong.
Use the words 'carrier' and 'heterozygous' when describing a person with one cystic fibrosis allele.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe polydactyly and cystic fibrosisPolydactyly is having extra fingers or toes; cystic fibrosis is a disorder of cell membranes.
Grade 4
State which allele causes each disorderPolydactyly is caused by a dominant allele; cystic fibrosis by a recessive allele.
Grade 5
Explain what a carrier isA heterozygous person with one recessive allele who does not have the disorder but can pass it on.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Some disorders are inherited. They are caused by the inheritance of certain alleles. Describe what polydactyly is.
Having extra fingers or toes
Cystic fibrosis (CF) is caused by a recessive allele, f. The dominant allele, F, results in normal cell membranes. Two parents are both carriers of CF. Give the genotype of a carrier of CF.
Ff
A couple are both carriers of cystic fibrosis. They are thinking about using IVF with embryo screening. Give one argument for embryo screening.
It prevents a child being born with a serious disorder, avoiding suffering
Cystic fibrosis is caused by a recessive allele. Give the genotype of a person who has cystic fibrosis. Use F for the dominant allele and f for the recessive allele.
ff
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Some disorders are inherited. They are caused by the inheritance of certain alleles.
(a) What causes polydactyly? Tick (✓) one box.[1]
A bacterial infection
A dominant allele
A poor diet
A recessive allele
(b) Describe what polydactyly is.[1]
(c) Cystic fibrosis is a disorder of which part of the cell? Tick (✓) one box.[1]
Cell membrane
Cell wall
Mitochondria
Ribosomes
(d) Cystic fibrosis is caused by a recessive allele. What word describes a person who has one copy of this allele but does not have the disorder?[1]
(e) Human embryos can be tested for the alleles that cause genetic disorders. What is this process called?[1]
Show the answer and mark scheme
(a)Answer: A dominant allele
(b)Answer: Having extra fingers or toes
having extra fingers or toes
(c)Answer: Cell membrane
(d)Answer: Carrier
carrier
(e)Answer: Embryo screening
embryo screening
Question 2Medium5 marks
In a certain population, 1 in 25 people are carriers of a recessive allele that causes an inherited disorder.
(a) Calculate the percentage of the population who are carriers.[1]
(b) Out of 12 000 people in a town, estimate how many would be expected to be carriers.[2]
(c) Explain why being a carrier does not affect a person’s own health.[2]
Show the answer and mark scheme
(a)Answer: 4%
1 ÷ 25 × 100 = 4 (%)
(b)Answer: 480
12 000 ÷ 25
480
(c)Answer: A carrier has one recessive allele and one dominant allele; the dominant allele is expressed, so the recessive allele’s effect is masked
a carrier has one recessive allele and one dominant allele
the dominant allele is expressed, so the (disease-causing) recessive allele’s effect is masked / does not affect the phenotype