Practise Inherited disorders. 14 exam-style questions plus unlimited generated ones 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Describe polydactyly and cystic fibrosisPolydactyly is having extra fingers or toes; cystic fibrosis is a disorder of cell membranes.
4
State which allele causes each disorderPolydactyly is caused by a dominant allele; cystic fibrosis by a recessive allele.
5
Explain what a carrier isA heterozygous person with one recessive allele who does not have the disorder but can pass it on.
6
Predict disorder risks using Punnett squaresFor example, two carriers of cystic fibrosis have a 0.25 probability of an affected child.
7
Discuss the issues of embryo screeningGive economic, social and ethical arguments for and against, using the information provided.
8
Evaluate embryo screening with a justified conclusionWeigh the benefits against the costs and concerns, and make a judgement supported by the information.
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
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.
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.
Cheatsheet
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
How to answer each type of question
Explain carriers and allele types
1 to 2 marks5
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 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).
Use a Punnett square for a disorder
3 to 4 marks6
Write the parents' genotypes (a CF carrier is Ff; a person with polydactyly is usually Dd).
Show the gametes and complete the square.
Answer exactly what is asked: affected, carrier, or unaffected.
Example. A man with polydactyly (Dd) and a woman without polydactyly (dd) plan to have a child. Use a Punnett square to find the percentage chance that their child will have polydactyly. [3 marks]
Show the model answer
D d
d Dd dd
d Dd dd
Gametes D and d from the man, d and d from the woman (1). Offspring Dd, Dd, dd, dd (1). 2 out of 4 have polydactyly, so 50% (1).
Evaluate embryo screening
4 to 6 marks8
Use the information given; examiners reward points taken from it.
Give arguments for (preventing suffering, cost of lifelong treatment).
Give arguments against (cost, destroying embryos, 'designer babies').
End with a conclusion that follows from your points.
Example. A couple are both carriers of cystic fibrosis. They are considering IVF with embryo screening, so that only embryos without cystic fibrosis are placed in the mother's uterus. Evaluate the use of embryo screening for this couple. [6 marks]
Show the model answer
Marked by levels; a top answer covers both sides and reaches a conclusion. For example: Each embryo has a 0.25 probability of having CF (1). Screening means their child will not have CF, so it prevents suffering from a lifelong disorder (1). It could also save the cost of long-term medical treatment (1). However, IVF and screening are expensive (1). Embryos with CF would be destroyed, which some people believe is wrong (1). Some people worry it could lead to choosing embryos for other characteristics (1). Conclusion: because the couple have a high risk of a child with a serious disorder, screening is justified if they accept the ethical issues, but it should be their own choice.
Shortcuts and 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.
Where marks are lost
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.
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.
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
Polydactyly is caused by a dominant allele, D. A man who is heterozygous for polydactyly has children with a woman who does not have polydactyly. Give the genotype of the man and the genotype of the woman.
Man Dd; woman dd
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 2Medium6 marks
Polydactyly is caused by a dominant allele, D. A man who is heterozygous for polydactyly has children with a woman who does not have polydactyly.
(a) Give the genotype of the man and the genotype of the woman.[2]
(b) Draw a Punnett square to show the possible genotypes of their children. Give the probability that a child will have polydactyly.[3]
(c) Explain why a person cannot be a carrier of polydactyly without having the condition.[1]
Show the answer and mark scheme
(a)Answer: Man Dd; woman dd
man: Dd
woman: dd
(b)Answer: Gametes D and d from the man, d and d from the woman; offspring Dd, Dd, dd, dd; probability 0.5
gametes: D and d (man), d and d (woman)
offspring: Dd, Dd, dd, dd
probability = 0.5 / 50% / ½
(c)Answer: The allele is dominant, so anyone with one copy has polydactyly
the allele is dominant so it is always expressed / one copy causes the condition
Question 3Hard6 marks
Huntington's disease is an inherited disorder of the nervous system. It is caused by a dominant allele, H. Symptoms usually start when a person is between 30 and 50 years old. Jo is 20 years old. Jo's father has Huntington's disease and is heterozygous (Hh). Jo's mother does not have the allele (hh).
(a) Give the probability that Jo has inherited the allele for Huntington's disease. Explain your answer.[2]
(b) Explain why Jo cannot tell from Jo's own health whether Jo has inherited the allele.[1]
(c) Jo has not been tested. Jo's partner does not have the allele. Calculate the probability that their first child will inherit the allele for Huntington's disease. Give your answer as a fraction or a decimal.[2]
(d) Suggest one reason why some people in Jo's position decide not to have a test for the allele.[1]
Show the answer and mark scheme
(a)Answer: ½: Jo's father (Hh) passes on H or h with equal chance, and Jo's mother (hh) can only pass on h
½ / 0.5 / 50%
the father (Hh) passes on H or h with equal chance, and the mother (hh) can only pass on h (so Hh : hh = 1 : 1)
(b)Answer: Symptoms do not usually start until the age of 30 to 50, and Jo is only 20
symptoms do not usually appear until 30 to 50 years old, and Jo is only 20
(c)Answer: ¼ (0.25)
probability that Jo has the allele (½) × probability that Jo passes it on if Jo has it (½)
= ¼ (0.25)
(d)Answer: There is no cure, so knowing could cause great worry
there is no cure, so knowing could cause worry / anxiety / some prefer not to know / concerns about insurance or employment