Explain how mutation can form a new allele of an existing gene
A new gene version differs from reshuffling versions already present.
Section B17.1 · Variation
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall genes, alleles, genotype and phenotype from the inheritance pathway. Use the stated organism, units and sample; the records in these lessons are invented.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is an allele?
What you will learn
- Connect genetic change with new allele formation
- Separate new versions from new combinations
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Allele | A version at a gene |
| Sequence alteration | Can form a different allele version |
| Existing A + a | Aa is an allele combination |
| Observed frequency | Does not establish the origin event |
Connect genetic change with new allele formation
An allele is a version of a gene. Mutation is the way new alleles form in the syllabus account: a change in genetic information can produce a different version at the tracked gene. A new allele is not automatically a completely new gene, a new chromosome set or a new species. Name the relevant gene or locus and distinguish the version from the wider genetic complement.
In the original one-locus model, allele version one includes AGCTTAGC and a changed version includes AGCATAGC. The stipulated difference at position four makes these different sequence versions of the same tracked locus. The record does not say that the altered version codes a visibly different feature, is beneficial or will be inherited by every descendant. Those are separate questions needing functional and reproductive evidence.
Separate new versions from new combinations
Contrast creating a changed allele with combining existing alleles. A parent carrying existing A and a can supply either to its normal gametes. Combining A from one gamete with a from another gives Aa, a new combination for that offspring, but in this model neither allele A nor allele a has a changed internal sequence. Likewise, reordering tT as Tt does not produce a new allele or a different allele pair.
The origin of an allele and its subsequent frequency in a population are different matters. Reproduction can spread an existing allele and sampling can observe different proportions without a newly formed version in each event. The supplied models trace particular unchanged versions or a particular sequence change; they do not claim to describe every molecular recombination mechanism. No direction toward an organism’s needs, guaranteed advantage or whole-population replacement is established.
Sources for this lesson
Pause and explain
Existing A and a gametes combine as Aa with no stipulated sequence change. What is newly formed?
Worked example
Compare a changed same-locus DNA version with forming Aa from existing A and a gametes.
Show the worked solution
- AGCTTAGC changing to AGCATAGC changes the stipulated locus sequence.
- A and a combining as Aa preserves the supplied allele versions while forming an offspring genotype.
Answer The sequence change can form a new allele version; forming Aa from existing A and a is a new combination, not a supplied new-allele event.
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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.
Explain why a stipulated same-locus sequence change can represent a new allele rather than a new whole gene.
Give me a hint
Version and gene are different levels of description.
Compare my reasoning
- An allele is a version at the tracked gene/locus.
- Changing its sequence can make a different version while retaining that locus identity.
The change can create a new allele version of the tracked gene; it need not create a new gene or species.
Look for these in your work
- Gene/version distinction
- No unsupported new-species claim
A gamete with A joins one with a to form Aa. Both allele sequences are supplied as unchanged. Is this a new-allele event?
Give me a hint
Separate pairing from sequence alteration.
Compare my reasoning
- A and a existed before fertilisation.
- The event combines them without a stipulated internal sequence change.
No new allele is supplied. Aa is an offspring genotype combining existing A and a versions.
Look for these in your work
- Existing alleles retained
- Genotype combination identified
A population’s recorded a frequency increases between two samples. Does that observation require new a alleles to have arisen by mutation between samples?
Give me a hint
Frequency and origin ask different questions.
Compare my reasoning
- Existing a copies can be transmitted and sampled in different proportions.
- The records supply no sequence-origin evidence or control of sampling.
No. An existing allele’s observed frequency can change without a newly formed version being demonstrated. Origin, transmission and sampling need separate evidence.
Look for these in your work
- Origin versus frequency
- Appropriate evidence limit
Common mistakes
- Calling every new genotype a new allele
- Treating a new allele as a new species
- Assuming new means beneficial or guaranteed to spread
What can you explain now?
Existing A and a gametes combine as Aa with no stipulated sequence change. What is newly formed?
Compare with the explanation
An offspring allele combination
The model preserves A and a as existing versions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, connect genetic change with new allele formation and separate new versions from new combinations. Keep units, the stated sample and the limits of the evidence clear.
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.