Explain Le and le alleles in gibberellin synthesis
Relate the functional Le allele to enzyme activity, gibberellin production and stem elongation.
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 the Le allele code for?
What you will learn
- Explain that Le codes for a functional enzyme in gibberellin synthesis
- Explain that le codes for a non-functional enzyme and predict the recessive phenotype
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT | Dominant allele present; pigment present |
| Tt | Dominant phenotype; t still carried |
| tt | No dominant allele; recessive phenotype |
| Population frequency | A separate question from dominance |
Build the genetic explanation
The dominant Le allele codes for a functional enzyme in the gibberellin synthesis pathway. Gibberellin promotes stem elongation. The recessive le allele codes for a non-functional enzyme; an le/le plant has reduced ability to synthesise the relevant gibberellin and shows reduced stem elongation compared with a plant carrying at least one functional Le allele. The genotype-to-phenotype chain links allele, enzyme, hormone and growth.
Check the evidence and limits
The allele does not directly encode gibberellin; it codes for an enzyme in its synthesis pathway. A dominant phenotype means one functional Le allele is sufficient in the stated model, not that it is more common.
Pause and explain
A student says le plants are short because le codes for a hormone that inhibits growth. Correct the causal chain.
Worked example
Predict the stem-growth phenotype of Le/le in the stated model.
Show the worked solution
- Le codes for a functional enzyme.
- One functional allele supports the pathway.
Answer Le/le has at least one functional allele and is expected to show the Le-associated stem-elongation phenotype in this model.
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.
Trace the effect of le/le on stem elongation.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Both copies code for non-functional enzyme.
- Gibberellin synthesis and elongation are reduced.
An le/le plant lacks the functional enzyme specified by Le in this model. Gibberellin synthesis is reduced, so stem elongation is reduced.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A plant is Le/le. State the expected phenotype and explain the allele relationship.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Le is dominant and codes for a functional pathway enzyme.
- One copy is sufficient in the stated model.
The plant is expected to show the Le-associated elongation phenotype because its one Le allele codes for a functional enzyme in gibberellin synthesis.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A student says le plants are short because le codes for a hormone that inhibits growth. Correct the causal chain.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- le codes for a non-functional enzyme.
- Reduced gibberellin synthesis lowers elongation.
The recessive le allele codes for a non-functional enzyme in the gibberellin synthesis pathway. Reduced hormone production, rather than a growth-inhibiting hormone encoded by le, reduces stem elongation.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Saying Le codes directly for gibberellin
- Saying the non-functional le enzyme actively blocks the functional enzyme
What can you explain now?
What does the Le allele code for?
Compare with the explanation
A functional enzyme in gibberellin synthesis
The enzyme supports production of gibberellin used in stem elongation.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does the Le/le allele pair affect gibberellin production and stem growth?
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.