Connect mutation meiosis random mating and random fertilisation as population variation sources
Four sources explain origins and combinations without guaranteeing a phenotype or sample ratio.
Section 18.1 · Variation
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
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
Which list includes all four required genetic-variation sources?
What you will learn
- Explain all four genetic-variation sources
- Retain sequence, chromosome, probability and assessment limits
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Mutation | Random DNA sequence change; can form new alleles |
| Meiosis | Genetically different haploid combinations |
| Random mating | Different parental combinations |
| Random fertilisation | Different gamete combinations |
Explain all four genetic-variation sources
A gene mutation is a random change in the DNA base sequence. It can produce a changed allele version in the syllabus account, but is not directed toward a needed advantage and need not visibly affect the measured phenotype. Compare a stated locus and alignment before identifying a sequence change. A short fragment does not supply whole-gene function, a clinical interpretation or transmission through an unspecified reproductive lineage.
Meiosis is a source of genetic variation through genetically different haploid combinations. In the original two-type source model, AA, AB, BA and BB each retain one chromosome of each type, so n is two rather than four. The source labels are not dominant/recessive symbols or full DNA sequences. The set reduction and the different combinations are distinct contributions; a finite sample or one event need not show every possible source combination.
Retain sequence, chromosome, probability and assessment limits
Random mating brings different compatible parental contributions together, while random fertilisation combines gamete contributions. In the stated Aa × Aa model, four gamete-pair positions group into AA:Aa:aa = 1:2:1. The model uses existing A and a versions, so those combination events do not themselves demonstrate a changed allele. Keep parental-class possibilities, gamete combinations, genotype categories and observed records separately labelled.
The complete Variation pathway maps all eight Core and two Supplement outcomes to original explanations, worked examples and self-checked guided, independent and application practice. It includes continuous and discontinuous investigation reasoning, not a certified practical result. Adaptive features has its own pathway; selection and later Biology sections still require their own full sequences; completion here is not whole-qualification mastery or completion of the wider Cambridge curriculum.
Pause and explain
What does finishing these written lessons establish?
Worked example
Connect one same-locus base change, four meiosis source combinations and Aa × Aa gamete combinations.
Show the worked solution
- The base change supplies a changed sequence version; the source combinations retain a complete n = 2 haploid set in the stated model.
- Aa × Aa combines existing gametes into genotype probabilities 1/4, 1/2 and 1/4. Separate these steps from random parental pairing.
Answer Mutation, meiosis, random mating and random fertilisation are four required variation sources, with different sequence-origin and combination roles and explicit evidence limits.
Try a topic test
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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.
Name all four genetic-variation sources and distinguish the role of mutation from the stated unchanged-allele combination models.
Give me a hint
Origin of a sequence version differs from its combination with other versions.
Compare my reasoning
- Name mutation, meiosis, random mating and random fertilisation.
- Mutation can form changed alleles; the supplied other models combine existing information.
The four sources are mutation, meiosis, random mating and random fertilisation. Mutation can form new allele versions; the bounded combination models reshuffle or pair existing information.
Look for these in your work
- All four named
- Origin/combination distinction
A model has four possible source combinations with n = 2 gametes; a separate Aa × Aa model predicts 80 offspring. Give the chromosome count per gamete and the expected genotype counts.
Give me a hint
Use separate models and denominators.
Compare my reasoning
- The source model has two chromosomes per complete haploid gamete, not four.
- AA:Aa:aa fractions 1/4, 1/2 and 1/4 give 20, 40 and 20 in an expected total of 80.
Two chromosomes per supplied gamete; expected AA 20, Aa 40 and aa 20 in the separate cross. Neither sample is guaranteed to contain every possible category in exact proportions.
Look for these in your work
- Models separated
- Chromosome and genotype counts correct
A learner solves every Variation problem and claims certified practical competence and completion of Biology and all A Levels. Evaluate the claims and identify the next content boundary.
Give me a hint
Mapped access and self-checking differ from comprehensive human assessment.
Compare my reasoning
- Selection and later Biology sections, plus wider qualifications, remain unfinished.
The claims are unsupported. The pathway supplies mapped written teaching and self-checking; the next full Biology sequence to build is Selection, with wider teaching and assessment work still remaining.
Look for these in your work
- Assessment limits
- Remaining curriculum boundary
Common mistakes
- Omitting either random mating or random fertilisation
- Treating combination counts as chromosome counts
- Treating teaching availability as whole-qualification approval
What can you explain now?
What does finishing these written lessons establish?
Compare with the explanation
Access to mapped teaching and self-checked practice
Teacher approval, practical competence and wider mastery remain separate.
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Make it stick
Without your notes, explain all four genetic-variation sources and retain sequence, chromosome, probability and assessment limits. 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.