What you need to know
- 2.14–2.18mutationA change in the sequence of bases in DNA., inheritance, cystic fibrosisA recessive genetic disorder caused by mutations in the CFTR gene. Thick, sticky mucus affects the lungs, pancreas and reproductive system. and screening ethics.
By the end, you should be able to…
- Retrieve key mechanisms before checking the answers.
- Apply ideas to unfamiliar numerical and experimental examples.
- Identify and correct missing steps in an explanation.
Build your understanding
Make your assumptions visible
State whether a model is autosomal or sex-linkedDescribes a gene carried on a sex chromosome, usually the X chromosome, so the pattern of inheritance differs between males and females. and whether dominance is complete. An uncertain genotypeThe alleles an organism has for a gene. should remain a set of possibilities.
Keep biological evidence separate from values
A test result informs a decision but does not dictate it. Evaluate accuracy, risks and scope before discussing autonomy, welfare and fairness.
Make your reasoning visible
Answer the command word
Describe the pattern, explain the mechanism or evaluate the evidence as requested. A correct fact may still miss the question.
Show workings and units
Write a calculation before its result, and distinguish fractions, percentages and percentage-point changes.
Use answers diagnostically
Attempt the question first, then identify the missing step rather than only checking whether your final number matches.
Avoid this misconception
The claim: A longer answer automatically earns more credit.
The correction: Relevance, accuracy and the requested reasoning matter.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Why can a one-base insertion in a coding sequence affect many amino acids?
Show suggested answer
It shifts the reading frame downstream, changing how subsequent bases are grouped into codons and potentially introducing a stop codon.
Practice question
Retrieval and explanation · suggested answer
For Ff × Ff under fully penetrant autosomal recessive inheritance, what fraction of unaffected offspring are carriers?
Show suggested answer
2/3: among the unaffected genotypes FF, Ff and Ff, two are heterozygotes.
Practice question
Retrieval and explanation · suggested answer
For XᴺXⁿ × XᴺY, calculate the probability of an affected son among all children, assuming equally likely XX/XY outcomes.
Show suggested answer
1/2 chance of a son × 1/2 chance he receives Xⁿ = 1/4. Half the sons are affected, but one quarter of all children are affected sons.
Practice question
Retrieval and explanation · suggested answer
Explain a digestive consequence of reduced functional CFTR.
Show suggested answer
Pancreatic duct obstruction can reduce delivery of digestive enzymes to the intestine, impairing digestion and reducing the absorbable nutrients available.
Practice question
Retrieval and explanation · suggested answer
A fictional couple are offered a prenatal test. Give one potential benefit, one limitation and one ethical consideration without recommending a decision.
Show suggested answer
Benefit: information for preparation or reproductive choices. Limitation: the test may not predict severity or all conditions, and an invasive procedure has risks. Ethical consideration: voluntary informed consent, privacy, fair access or differing beliefs about reproductive decisions.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A pedigree has an affected father and an affected son. Does this observation alone prove direct father-to-son transmission of an X-linked allele?
Show a hint
Identify which paternal chromosome a son receives, and consider the mother’s allele.
Explore the reasoning
No. In the XX/XY model the son receives Y from his father and X from his mother. He could share the phenotype through a maternal X-linked allele; the observation alone does not establish the inheritance route.
Recall this lesson
Lesson recall questions
Question 1 of 5: Explain, in causal order, why a single-base deletion often changes many amino acids.
Mutations and their effects
Practice question
Retrieval and explanation · suggested answer
Explain, in causal order, why a single-base deletion often changes many amino acids.
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Explaining a probability clearly
2C · Patterns of InheritanceGenetic counsellors help people understand genetic conditions, testing and family implications. Their work includes communicating uncertainty and supporting informed choices. A genetic counsellor translates a model into language…Specification 2.15(i) · 2.15(ii)Read & use the scienceClose article
Why this matters
Genetic counsellors help people understand genetic conditions, testing and family implications. Their work includes communicating uncertainty and supporting informed choices. A genetic counsellor translates a model into language a family can use, including what a test can and cannot establish. If two carriers of an autosomal recessive condition have children, the usual model predicts a 25% affected probability for each conception. Earlier outcomes do not use up that probability. Real counselling also considers whether the assumed genotypes and condition are established. The skill is explaining conditional probability accurately while supporting choices, rather than telling a family what decision to make.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.15(i) understand what is meant by the terms gene, allele, genotype, phenotype, recessive, dominant, codominance, homozygote and heterozygote
- 2.15(ii) understand patterns of inheritance, including the interpretation of genetic pedigree diagrams, in the context of monohybrid inheritance
Use the science
How would you explain a 1-in-4 recurrence probability without implying that one of every four children must be affected?
Explore a possible response
Describe it as a 25% chance for each conception under the model; family outcomes can differ from the expected proportions.
Explore the source
A negative DNA test is not an all-clear for life
2C · Genetic ScreeningA commercial genetic test may examine a selected set of variants rather than every possible disease-associated change. The FDA explains that different companies can test different sets, so…Specification 2.17(i) · 2.18Read & use the scienceClose article
Why this matters
A commercial genetic test may examine a selected set of variants rather than every possible disease-associated change. The FDA explains that different companies can test different sets, so a negative result has to be interpreted within the test’s scope. It does not remove environmental risks, unknown variants or changes that were never tested. This extends genetic screening beyond the technology to the meaning of a result and the claims used to sell it. A scientifically responsible report distinguishes the absence of a detected variant from a guarantee of health, and makes uncertainty visible rather than hiding it in advertising.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.17(i) understand the uses of genetic screening, including the identification of carriers, pre-implantation genetic diagnosis (PGD) and prenatal testing, including amniocentesis and chorionic villus sampling
- 2.18 be able to identify and discuss the ethical and social issues relating to genetic screening from a range of ethical viewpoints, including religious, moral and social implications
Use the science
Two companies test different variants for the same condition and return different results. Why does this not by itself prove that one laboratory made an analytical error?
Explore a possible response
Their panels may measure different targets. Different scope can produce different findings even with accurate assays; interpretation also depends on evidence for each variant.