Inherited disorders — AQA GCSE Biology
Test yourself on Inherited disorders with AQA GCSE practice questions.
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Inherited disorders explained
An inherited disorder is a condition passed from parents to offspring through alleles in gametes.
Read the full explanation
It is not caused by infection, diet or lifestyle, although environment can influence how a disorder affects a person. A person inherits one allele of each gene from each parent, so the combination of alleles they receive determines whether they develop the disorder. Some disorders, such as polydactyly, are caused by a dominant allele, so one copy is enough. Others, such as cystic fibrosis, are caused by a recessive allele, so two copies are needed. A carrier has one recessive allele but does not show the disorder and can pass the allele to their children.
Polydactyly (having extra fingers or toes) is caused by a dominant allele.
Polydactyly means a person is born with extra fingers or toes. It is caused by a dominant allele, so only one copy of that allele is needed for the feature to appear. We can write the dominant allele as P and the recessive allele as p. A person with genotype PP or Pp has polydactyly, while pp does not. Because the allele is dominant, an affected parent can pass it on even if the other parent has two recessive alleles. For example, a Pp parent crossed with a pp parent gives a 50% chance of Pp offspring with polydactyly and a 50% chance of pp offspring without it. This explains why the condition can appear in every generation of a family.
Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
Cystic fibrosis is a disorder that affects cell membranes, particularly in the lungs and digestive system, causing thick mucus. It is caused by a recessive allele, so two copies of the recessive allele are needed for a person to have the disorder. We can write the dominant allele as F and the recessive allele as f. A person with genotype ff has cystic fibrosis, while FF and Ff do not. People with Ff are carriers: they have one recessive allele but do not show the disorder, and they can pass the allele to their children. For example, if two carriers Ff × Ff have children, there is a 25% chance of a child with cystic fibrosis, a 50% chance of a carrier and a 25% chance of a child with no recessive allele.
Students should make informed judgements about the economic, social and ethical issues concerning embryo screening, given appropriate information.
Embryo screening involves testing embryos produced by IVF for alleles linked to inherited disorders before implantation. Students must use given information to weigh economic, social and ethical issues and reach a justified judgement. Economic points include the cost of screening and long-term treatment, and who pays. Social points include reducing suffering, parental choice, and possible pressure on families. Ethical points include the moral status of embryos, discarding affected embryos, designer babies, and disability rights. A strong answer considers benefits and drawbacks, uses the information provided, and reaches a clear conclusion. For example, screening could prevent a child being born with cystic fibrosis, but some people object to discarding embryos or to selecting characteristics.
Your focus
- Define an inherited disorder as a condition caused by alleles passed from parents.
- Explain how dominant and recessive alleles produce different patterns of inheritance.
- Describe the role of carriers in the inheritance of a recessive disorder.
Show all 12 objectives
- Define polydactyly as a condition caused by a dominant allele.
- Use genetic symbols to represent the dominant and recessive alleles for polydactyly.
- Predict the possible genotypes and phenotypes of offspring from a given parental cross.
- Define cystic fibrosis as a disorder caused by a recessive allele.
- Use genetic symbols to represent dominant and recessive alleles and identify carrier genotypes.
- Predict the probability of offspring inheriting cystic fibrosis from a given parental cross.
- Identify economic, social and ethical issues about embryo screening from given information.
- Weigh benefits against drawbacks to reach a justified judgement.
- Explain why different people may reach different conclusions about embryo screening.
Inherited disorders exam tips
Marking Points
- State that inherited disorders are caused by alleles passed from parents to offspring in gametes.
- Distinguish inherited causes from environmental or infectious causes of disease.
- Explain that the alleles a person inherits determine whether they develop the disorder.
- Recognise that dominant alleles cause a disorder when at least one copy is present, while recessive alleles cause a disorder only when two copies are present.
- Describe a carrier as a person with one recessive allele who does not show the disorder but can pass the allele on.
- Use family pedigree or genetic cross information to identify how a disorder may have been inherited.
- States that polydactyly is caused by a dominant allele, so only one copy of the allele is needed for the condition to be expressed.
- Uses correct genetic symbols, such as P for the dominant allele and p for the recessive allele, and identifies PP and Pp as genotypes showing polydactyly.
- Identifies pp as the genotype of a person who does not have polydactyly.
- Completes a monohybrid cross, such as Pp × pp, and states the expected ratio or percentage of offspring with and without polydactyly.
- Explains that an affected parent with genotype Pp can pass on either P or p, so some offspring may inherit the condition and some may not.
- Distinguishes dominant inheritance from recessive inheritance by noting that a dominant disorder can be passed on by a parent who has the condition.
- States that cystic fibrosis is caused by a recessive allele, so two copies of the allele are needed for the disorder to be expressed.
- Uses correct genetic symbols, such as F for the dominant allele and f for the recessive allele, and identifies ff as the genotype of a person with cystic fibrosis.
- Identifies FF and Ff as genotypes of people who do not have cystic fibrosis.
- Explains that a person with genotype Ff is a carrier who does not show the disorder but can pass on the recessive allele.
- Completes a monohybrid cross, such as Ff × Ff, and states the expected ratio or percentage of offspring with cystic fibrosis, carriers and unaffected non-carriers.
- Links the disorder to cell membranes, for example by stating that cystic fibrosis affects the movement of substances across cell membranes and causes thick mucus.
- Identify economic issues such as the cost of embryo screening, the cost of lifelong treatment for a disorder, and who bears those costs.
- Discuss social issues such as reducing suffering in families, giving parents informed choice, and the possible emotional or social pressure to use screening.
- Discuss ethical issues such as the moral status of embryos, the discarding of affected embryos, and concerns about selecting embryos for particular characteristics.
- Use the information provided in the question rather than relying only on general opinion, and refer to specific details from that information.
- Reach an informed judgement by weighing benefits against drawbacks and stating a clear, justified conclusion.
- Recognise that different people may reach different judgements because they hold different ethical or religious views, and that both views can be reasonable.
Examiner Tips
- 💡Use the terms allele, dominant, recessive and carrier accurately, because these words carry the marks in inheritance questions.
- 💡When explaining a family history, track alleles through each generation rather than describing only the people who show the disorder.
- 💡Link the cause of the disorder to the alleles inherited, not to the symptoms, so your answer addresses inheritance directly.
- 💡Always write a key before using letters in a genetic cross, so the examiner can follow your reasoning.
- 💡When asked to complete a Punnett square, fill in the parental gametes along the top and side, then combine them carefully to avoid copying errors.
- 💡Link the ratio or probability to the question wording, for example stating the chance that a child will have polydactyly rather than only giving the genotypes.
- 💡Use a key to define dominant and recessive alleles before drawing a genetic cross, and keep the same letters throughout your answer.
- 💡When calculating probability, count the number of squares in the Punnett square that match the required genotype and express this as a fraction, percentage or ratio.
- 💡Read the question carefully to check whether it asks about the chance of a child being affected, being a carrier or being completely unaffected.
- 💡Plan two or three economic, social and ethical points before writing, then give a final judgement that follows from those points.
- 💡Use connectives such as however, therefore and this suggests to show weighing rather than listing.
- 💡Refer explicitly to the information in the question, for example by naming a cost or a condition mentioned there.
Common Mistakes
- Saying that inherited disorders are caught from another person; correct this by stating that they are passed through alleles in gametes from parents.
- Assuming that a person with a recessive disorder must have a parent who also shows the disorder; correct this by explaining that both parents may be carriers who do not show the disorder.
- Confusing a carrier with someone who has the disorder; correct this by stating that a carrier has one recessive allele but does not show the characteristic.
- Thinking that two copies of the dominant allele are needed; the correction is that one copy of a dominant allele is enough for polydactyly to be expressed.
- Writing genotypes without a clear key; the correction is to define the symbols first, for example P for the dominant allele and p for the recessive allele.
- Confusing genotype with phenotype; the correction is to state that PP and Pp are genotypes that produce the phenotype of having extra fingers or toes, while pp produces the phenotype without polydactyly.
- Thinking that one copy of the recessive allele causes cystic fibrosis; the correction is that two copies are needed, so only ff shows the disorder.
- Assuming that a carrier must show symptoms; the correction is that a carrier has one recessive allele and one dominant allele, so they do not have the disorder but can pass it on.
- Forgetting that both parents must pass on a recessive allele for a child to be affected; the correction is to show in a genetic cross that each parent contributes one allele to the offspring.
- Listing only advantages or only disadvantages; correction: address both sides and then make a balanced judgement.
- Confusing embryo screening with gene therapy; correction: embryo screening tests and selects embryos, whereas gene therapy alters the genes of a living person.
- Giving an unsupported opinion without using the information supplied; correction: quote or paraphrase the given information and link each point to it.