Inherited disorders — AQA GCSE Combined Science
Test yourself on Inherited disorders with AQA GCSE practice questions.
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Inherited disorders explained
Inherited disorders are conditions passed from parents to offspring because they are coded for by particular alleles carried in gametes.
Read the full explanation
A disorder allele is a version of a gene that alters the protein or trait it controls, so the condition appears when that allele is present in the offspring's cells. Some disorder alleles are dominant, so one copy is enough; others are recessive, so two copies are needed. For example, a recessive disorder such as cystic fibrosis requires both parents to pass on the recessive allele, while a dominant disorder appears if either parent passes on the dominant allele. Genetic diagrams using letters show the possible allele combinations and the probability of offspring being affected, unaffected or carriers.
Polydactyly (having extra fingers or toes) is caused by a dominant allele.
Polydactyly is a condition in which a person has extra fingers or toes. It is caused by a dominant allele, so only one copy of that allele is needed for the condition to appear. A person with polydactyly may be heterozygous, with one dominant allele and one recessive allele, or homozygous for the dominant allele. If a parent with polydactyly is heterozygous and the other parent is homozygous recessive, each child has a 50% chance of inheriting the dominant allele and therefore the condition. Genetic diagrams using letters such as P for the dominant polydactyly allele and p for the recessive allele show these possible outcomes and help to explain why the condition can appear in every generation.
Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
Cystic fibrosis is an inherited disorder affecting cell membranes, particularly in the lungs, pancreas and digestive system, where thick sticky mucus is produced. It is caused by a recessive allele, so a person must inherit two copies of the faulty allele, one from each parent, to develop the disorder. Someone with one faulty allele and one normal allele is a carrier: they do not have cystic fibrosis but can pass the faulty allele to their children. In a genetic cross, two carrier parents (Ff × Ff) have a 1 in 4 chance of a child with cystic fibrosis (ff), a 2 in 4 chance of a carrier child (Ff) and a 1 in 4 chance of a child with two normal alleles (FF). The disorder is not caused by infection or lifestyle, and it is not sex-linked.
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 such as cystic fibrosis before implantation. Students should use given information to weigh economic, social and ethical issues and reach an informed judgement. Economic issues include the cost of screening and treatment to the health service and families. Social issues include reducing suffering, the impact on families and questions about who should have access. Ethical issues include the status of the embryo, disability rights, designer babies and parental choice. A strong judgement considers both benefits and drawbacks, uses the information provided and reaches a clear conclusion. For example, screening can prevent a child being born with a serious disorder, but some people object because it involves discarding affected embryos.
Your focus
- State that inherited disorders are caused by the inheritance of certain alleles.
- Distinguish between dominant and recessive inheritance of disorder alleles.
- Use a genetic diagram to predict the chance of offspring inheriting a disorder allele.
Show all 12 objectives
- Describe polydactyly as having extra fingers or toes.
- Explain that polydactyly is caused by a dominant allele.
- Use a genetic diagram to predict the probability of offspring inheriting polydactyly.
- State that cystic fibrosis is caused by a recessive allele and affects cell membranes.
- Use genetic diagrams to predict the probability of offspring inheriting cystic fibrosis from carrier parents.
- Distinguish between an affected person (ff) and a carrier (Ff) and explain why carriers do not show symptoms.
- Identify economic, social and ethical issues concerning embryo screening from given information.
- Weigh benefits and drawbacks of embryo screening to reach an informed judgement.
- Justify a conclusion about embryo screening using evidence and considering different viewpoints.
Inherited disorders exam tips
Marking Points
- Inherited disorders are caused by alleles passed from parents to offspring in gametes, not by lifestyle or infection.
- A gene can have different alleles; a disorder allele is a version that produces a faulty or altered protein leading to the condition.
- Dominant disorder alleles cause the condition when at least one copy is inherited.
- Recessive disorder alleles cause the condition only when two copies are inherited, one from each parent.
- Carriers have one recessive disorder allele but do not show the condition; they can pass the allele to offspring.
- Genetic diagrams and Punnett squares use letters to predict the probability of offspring inheriting disorder alleles.
- Polydactyly means having extra fingers or toes.
- It is caused by a dominant allele, so one copy of the allele is sufficient for the condition to be expressed.
- A person with polydactyly can have the genotype PP or Pp.
- A person without polydactyly has the genotype pp.
- In a cross between a heterozygous affected parent and an unaffected parent, the expected probability of an affected child is 50%.
- Genetic diagrams using P and p alleles can predict the possible genotypes and phenotypes of offspring.
- Cystic fibrosis is caused by a recessive allele, so the disorder only appears when two recessive alleles are inherited, one from each parent.
- A person with one recessive allele and one dominant normal allele is a carrier; carriers do not show symptoms but can pass the recessive allele on.
- The disorder affects cell membranes, leading to thick sticky mucus that affects organs such as the lungs and pancreas.
- A genetic cross between two carriers (Ff × Ff) gives a 1 in 4 probability of an affected child (ff), 2 in 4 carrier children (Ff) and 1 in 4 unaffected non-carrier children (FF).
- Because the allele is recessive, an affected person must have the genotype ff; a dominant allele F would mask the recessive allele.
- Family pedigree diagrams can be used to identify carriers and to work out the probability of inherited disorders appearing in later generations.
- Economic issues include the cost of embryo screening and IVF, the cost of lifelong care for a child with a disorder, and who pays, such as the NHS or families.
- Social issues include reducing suffering, supporting families, the availability of screening and the effect on people already living with the disorder.
- Ethical issues include the moral status of the embryo, the right to life, disability rights, parental choice and concerns about designer babies.
- An informed judgement weighs benefits against drawbacks, uses the given information and reaches a clear, justified conclusion rather than listing points.
- Embryo screening can identify embryos with recessive alleles such as those causing cystic fibrosis, so parents can choose which embryos to implant.
- Different stakeholders, including parents, doctors, religious groups and taxpayers, may reach different judgements because they value the issues differently.
Examiner Tips
- 💡Define the key terms allele, dominant, recessive and carrier before applying them to a named disorder.
- 💡When using a genetic diagram, state the parent genotypes, gametes and possible offspring genotypes clearly.
- 💡Link each outcome to the alleles inherited, for example two recessive alleles are needed for a recessive disorder to be expressed.
- 💡State clearly that polydactyly is dominant and that one dominant allele is enough.
- 💡Show parent genotypes and gametes in a genetic diagram before working out offspring probabilities.
- 💡Use the terms genotype and phenotype correctly when describing the results of a cross.
- 💡Use a Punnett square or genetic diagram with clear parent genotypes, gametes and offspring genotypes to show how the recessive allele is inherited.
- 💡State probabilities as fractions, decimals or percentages and link them to the genotypes in the cross, for example 1 in 4 for ff.
- 💡When explaining why a person is a carrier, refer to one recessive allele being present but masked by the dominant normal allele.
- 💡Use the information provided in the question and refer to it explicitly when making your judgement.
- 💡Structure your answer to cover economic, social and ethical issues, then state and justify your overall conclusion.
- 💡Use connectives such as however, therefore and on balance to show that you are weighing up both sides.
Common Mistakes
- Thinking inherited disorders are caught from the environment: correct this by stating that the cause is the inheritance of certain alleles from parents.
- Assuming a dominant disorder must be more common than a recessive one: correct this by explaining that dominance affects whether one copy is enough, not how frequent the allele is in a population.
- Confusing a carrier with an affected person for a recessive disorder: correct this by stating that a carrier has one recessive allele and one dominant allele and does not show the condition.
- Writing that two dominant alleles are needed for polydactyly: correct this by stating that a dominant allele causes the condition when at least one copy is inherited.
- Using the same letter case for dominant and recessive alleles: correct this by using an uppercase letter for the dominant allele and the matching lowercase letter for the recessive allele.
- Confusing genotype with phenotype: correct this by stating that genotype is the allele combination, such as Pp, while phenotype is the observable feature, such as having extra fingers or toes.
- Thinking that a carrier of cystic fibrosis has the disorder in a mild form; correction: carriers are unaffected because the dominant normal allele masks the recessive allele.
- Assuming that two carrier parents will definitely have an affected child; correction: each child has a 1 in 4 probability, and chance has no memory between pregnancies.
- Confusing cystic fibrosis with a dominant disorder such as polydactyly; correction: cystic fibrosis is recessive, so two copies of the faulty allele are needed.
- Listing economic, social and ethical points without reaching a judgement; correction: finish with a clear conclusion that weighs the issues and refers to the information given.
- Treating embryo screening as the same as gene therapy or editing; correction: screening tests and selects embryos, whereas gene therapy aims to treat a person.
- Assuming everyone shares the same ethical view; correction: recognise that different groups and individuals may reach different judgements for valid reasons.