Define an allele as an alternative form of a gene
An allele is an alternative form of the same gene. Gene identity, allele form, whole chromosome and protein product are different categories; labels alone do not establish dominance or fitness.
Section 17.1 · Chromosomes, genes and proteins
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall the nucleus, DNA base pairing, gametes, fertilisation, enzymes, membrane carriers and chemical synapses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the correct allele definition?
What you will learn
- Connect different allele forms with the same gene
- Distinguish an allele from a different gene or product
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Gene | One DNA information region |
| Alleles | Alternative forms of that gene |
| Position model | Corresponding marked region |
| Limit | Letters do not supply fitness or dominance evidence |
Connect different allele forms with the same gene
An allele is an alternative form of a gene. Different alleles concern the same gene, rather than any two unrelated genes in a chromosome. The definition connects information variants: the gene is a DNA region, and an allele is a particular form of that gene. A protein molecule or a whole chromosome should not be substituted for the word allele.
In a simplified paired-chromosome model, the same gene occupies corresponding positions, and its forms can be the same or different. The lesson uses forms labelled A and a to make the alternatives easy to track. Letter case is a naming convention here; it does not, on its own, supply evidence of dominance, fitness or a visible trait. Those inheritance rules need stated information.
Distinguish an allele from a different gene or product
A different gene at a different marked region is not automatically another allele of the first gene. Ask whether the two labels identify versions of the same gene or separate information regions. An original diagram can show the positions and labels, but colour differences alone do not prove a biological allele difference unless the key states what the colours mean.
The original allele key compares one gene with two named forms. It does not present a human test result or imply that one form is better. Practice focuses on the definition and category distinction; detailed monohybrid dominance and phenotype prediction belong to the separate monohybrid inheritance sequence.
Pause and explain
What does a supplied A and a label key establish?
Worked example
Why are two unrelated genes not necessarily two alleles?
Show the worked solution
- Identify alleles as forms of the same gene.
- Distinguish different gene identities.
Answer Alleles are alternative forms of one gene; two unrelated genes are not forms of that same gene.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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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.
Give a concise allele definition.
Give me a hint
Use alternative form and gene.
Compare my reasoning
- Name an alternative form.
- State that it is a form of a gene.
An allele is an alternative form of a gene.
Look for these in your work
- Use alternative form
- Link to the same gene
Explain why a gene for one protein and an unrelated gene for another protein are not automatically alleles.
Give me a hint
Gene identity differs from a form of one gene.
Compare my reasoning
- Identify two distinct gene regions.
- Require alternative forms of the same gene for alleles.
They are different genes, whereas alleles are alternative forms of the same gene; different product labels do not establish an allele relationship.
Look for these in your work
- Distinguish genes and forms
- Retain the same-gene condition
A fictional allele diagram labels two forms A and a but supplies no dominance or fitness information. What cannot be concluded from capitalisation?
Give me a hint
A label convention is not a functional test.
Compare my reasoning
- Reject automatic dominance inference from the key alone.
- Reject a universal better-form or fitness judgement.
The letters alone do not establish a dominance relationship or that A is universally fitter; those conclusions require stated evidence.
Look for these in your work
- Retain label limits
- Reject both unprovided claims
Common mistakes
- An allele is not a whole chromosome.
- Letter case alone does not establish dominance.
What can you explain now?
What does a supplied A and a label key establish?
Compare with the explanation
Two named forms of the same gene if the key says so
The relationship is supplied by the model.
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Make it stick
Explain connect different allele forms with the same gene and distinguish an allele from a different gene or product. Use the stated cell, chromosome sets and information pathway before opening the solutions.
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.