Distinguish random mating from random fertilisation as variation sources
Parental pairings and gamete pairings occur at different steps.
Exam scope and assessment notes
Extended only (Supplement) · 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 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 does random fertilisation combine?
What you will learn
- Separate mate choice and gamete combination
- Use bounded probability models without implying new allele formation
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Random mating | Different compatible parental pairings |
| Random fertilisation | Different gamete combinations |
| Aa × Aa positions | AA / Aa / aA / aa |
| Grouped genotypes | AA:Aa:aa = 1:2:1 under the supplied model |
Separate mate choice and gamete combination
Random mating and random fertilisation are two different required sources of genetic variation in populations. Mating combines genetic contributions from different compatible parents in a population. Fertilisation then combines one gamete’s genetic information with another’s. These processes can produce different offspring combinations using allele versions already present; a particular new allele is not automatically created at every pairing.
In an original stipulated equal-choice mating model, parent class P1 or P2 can pair with compatible class Q1 or Q2. The possible parent combinations are P1–Q1, P1–Q2, P2–Q1 and P2–Q2. Under that specifically supplied independent equal-choice assumption each has probability one quarter. The labels identify parental classes, not gene alleles, and the assumption is a teaching model rather than a rule that real mating is always random or equal.
Use bounded probability models without implying new allele formation
For the separate fertilisation example, two known Aa parents each supply gamete allele A or a with equal probability. Combining gametes gives AA, Aa, aA and aa in four equal-probability positions. Aa and aA name the same genotype, so the predicted genotype ratio AA:Aa:aa is 1:2:1. A and a remain the existing supplied versions; the offspring allele pair is the combination.
Keep the denominators and evidence distinct. Four parental-class pairings are not automatically four different genotypes, and four gamete-pair positions can group into three genotype categories. Nor does a finite family have to match exact predicted counts. These bounded records illustrate variation through combinations; they do not establish every reproductive system, actual mate preferences or universal transmission probabilities without the relevant stated assumptions.
Pause and explain
How many genotype categories arise from AA, Aa, aA and aa?
Worked example
Compare the P1/P2 and Q1/Q2 mating model with Aa × Aa fertilisation.
Show the worked solution
- List four parental-class pairings under the equal-choice model.
- Combine A/a gametes as AA, Aa, aA and aa, grouping the middle two as Aa.
Answer Both models have four stated combination positions, but parental pairings and gamete pairings are different steps; Aa × Aa gives three genotype categories in a 1:2:1 prediction.
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.
List the four possible parental pairings in the stipulated P1/P2 with Q1/Q2 equal-choice model.
Give me a hint
Pair each P class with each Q class.
Compare my reasoning
- P1 can pair with Q1 or Q2.
- P2 can pair with Q1 or Q2.
P1–Q1, P1–Q2, P2–Q1 and P2–Q2; one quarter each under the supplied assumptions.
Look for these in your work
- All four parental pairs
- Assumption-limited probabilities
For Aa × Aa with equal A/a gametes, give genotype probabilities and expected counts in 80 offspring.
Give me a hint
Group Aa and aA as the same genotype.
Compare my reasoning
- AA occupies 1/4, Aa 2/4 and aa 1/4 of equal-probability positions.
- Multiply 80 by those fractions.
AA 1/4, Aa 1/2, aa 1/4; expected counts 20, 40 and 20. These are predictions, not guaranteed observations.
Look for these in your work
- Correct categories and counts
- Expected scope
A student says random mating, fertilisation and every Aa genotype all show a newly mutated allele. Correct the inference.
Give me a hint
Combination of existing versions differs from an altered version.
Compare my reasoning
- Mating supplies parental combinations; fertilisation supplies gamete combinations.
- The Aa model retains pre-existing A and a with no supplied sequence alteration.
These steps can create different combinations of existing information. A new allele by mutation is not established by the parental pairing or Aa genotype alone.
Look for these in your work
- Mating/fertilisation distinction
- New allele not invented
Common mistakes
- Treating mate classes as allele symbols
- Counting Aa and aA as different genotypes
- Assuming real mating is always equal and random
What can you explain now?
How many genotype categories arise from AA, Aa, aA and aa?
Compare with the explanation
Three
Aa and aA name the same allele pair.
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Make it stick
Without your notes, separate mate choice and gamete combination and use bounded probability models without implying new allele formation. 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.