Interpret autosomal linkage and epistasis
Explain how linked genes and gene interactions can alter expected dihybrid offspring patterns.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does epistasis describe?
What you will learn
- Explain autosomal linkage as loci on the same chromosome
- Explain epistasis as an interaction in which one gene affects expression of another
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Test cross | Recessive partner; informative offspring and finite-sample limits |
| Codominance and ABO | Both contributions; allele-specific genotype and phenotype mapping |
| Sex linkage | Gene location, X/Y contributions and group-specific probabilities |
| Scope | Self-checked practice; teacher, practical and cumulative assessment pending |
Build the genetic explanation
Autosomal linked genes lie on the same autosome and tend to be inherited together. Crossing over can separate alleles at linked loci, producing recombinant gametes; recombination frequency depends on the loci and is not necessarily one half. Epistasis occurs when an allele at one gene affects or masks expression of a different gene, changing the phenotype outcomes of a cross. The syllabus requires interpretation and construction of crosses but does not require memorising expected ratios for different epistasis types.
Check the evidence and limits
Do not treat linked genes as absolutely inseparable; crossing over may recombine them. Do not assume a standard independent-assortment ratio when loci are linked or gene interactions alter phenotype.
Pause and explain
A student memorises a special epistasis ratio and applies it to an unfamiliar cross without checking the genotypes. Explain the problem.
Worked example
Why might a linked dihybrid cross yield more parental-type offspring than recombinant types?
Show the worked solution
- The loci lie on the same chromosome.
- Crossing over occurs only in some meioses.
Answer Linked alleles tend to be inherited together; crossing over can produce recombinants, but parental combinations may remain more frequent.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how crossing over affects inheritance of two autosomal linked genes.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Linked loci are on the same chromosome.
- A crossover can create recombinant chromatids.
Linked alleles usually travel together on a chromosome, but crossing over between homologous non-sister chromatids can create recombinant allele combinations.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A cross produces a phenotype pattern inconsistent with independent assortment. Give two genetic explanations that could be tested.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Loci may be linked.
- An interaction such as epistasis may change phenotypes.
The loci could be linked, producing fewer recombinants than independent assortment predicts, or epistasis could alter how genotype combinations map to phenotypes. Further evidence is needed to distinguish these explanations.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A student memorises a special epistasis ratio and applies it to an unfamiliar cross without checking the genotypes. Explain the problem.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The ratio depends on the genetic interaction and cross.
- The stated syllabus does not require memorising epistasis ratios.
An expected ratio depends on the specific cross and interaction. Interpret the supplied genotypes and phenotype evidence; memorised ratios are not a substitute for constructing the cross, and this syllabus does not require recall of different epistasis ratios.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Assuming linkage means no recombination
- Assigning a memorised epistasis ratio when none is required
What can you explain now?
What does epistasis describe?
Compare with the explanation
One gene affecting expression of another
This interaction can alter the phenotype ratio in a cross.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do linkage and epistasis each change a dihybrid pattern?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.