Link genes, proteins and inherited phenotypes
Explain the gene–protein–phenotype relationships for TYR, HBB, F8 and HTT examples.
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 connects a gene sequence to a phenotype in these examples?
What you will learn
- Relate gene alleles to protein structure or function and phenotype
- Use the specified TYR, HBB, F8 and HTT examples
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Starting information | Amino-acid sequence |
| Molecular relationship | Interactions influence folding |
| Possible result | Different protein shape |
| Evidence limit | Exact structure and function need evidence |
Build the genetic explanation
A gene’s nucleotide sequence affects the amino-acid sequence or expression of a protein. A changed allele can alter protein production, structure or function, which may affect phenotype. TYR codes for tyrosinase; variants associated with albinism reduce melanin synthesis. HBB codes for a haemoglobin polypeptide; the sickle-cell allele changes haemoglobin and red-cell properties. F8 codes for clotting factor VIII; variants can impair blood clotting and cause haemophilia A. HTT codes for huntingtin; disease-associated expansions alter the protein and are associated with Huntington’s disease.
Check the evidence and limits
A genotype can influence phenotype through protein function, but environment and other genes can also contribute. Avoid implying every allele difference causes disease or that all variants in these genes have the same effect.
Pause and explain
A student says every mutation in HTT causes Huntington’s disease. Correct the claim.
Worked example
Trace one causal chain from a gene to a phenotype using HBB.
Show the worked solution
- HBB is a gene coding for a haemoglobin polypeptide.
- A disease-associated allele changes haemoglobin and can alter red-cell properties.
Answer A variant in HBB changes the haemoglobin polypeptide; altered haemoglobin can cause red-cell sickling under particular conditions and contribute to sickle-cell disease.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Use TYR to trace a gene–protein–phenotype relationship.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- TYR codes for tyrosinase.
- Reduced enzyme function can reduce melanin synthesis.
TYR codes for tyrosinase. A disease-associated allele can reduce functional enzyme activity, lowering melanin synthesis and contributing to albinism.
Look for these in your work
- Genetic terms used correctly
- Steps linked
Explain how F8 variation can affect blood clotting.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- F8 codes for factor VIII.
- Reduced or altered factor function affects the clotting pathway.
F8 codes for clotting factor VIII. A variant that reduces functional factor VIII can impair the clotting process and cause haemophilia A.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A student says every mutation in HTT causes Huntington’s disease. Correct the claim.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- Disease association relates to particular variants or expansions.
- A gene can have many variants with different effects.
Not every HTT variant causes disease. Huntington’s disease is associated with particular expanded repeat alleles that alter huntingtin; other variants may have different or no detectable effects.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Claiming genes directly create visible traits without protein action
- Treating all variants in a gene as harmful
What can you explain now?
What connects a gene sequence to a phenotype in these examples?
Compare with the explanation
The protein product and its function
An allele can alter the protein and thereby affect cell or organism characteristics.
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Make it stick
How can an allele change a protein and thereby alter phenotype?
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.