Explain variation from random fertilisation
Explain how random fusion of genetically different gametes creates genetically different individuals.
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 random fertilisation combine?
What you will learn
- Explain that fertilisation randomly combines gametes
- Relate genetically different gametes to variation among offspring
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Normal precursor | n = 20; diploid complement 40 |
| Normal gametes | 20 in each haploid nucleus |
| Normal fertilisation | 20 + 20 = 40 in one zygote nucleus |
| Counterfactual unreduced fusion | 40 + 40 = 80; no viability inference |
Build the genetic explanation
Meiosis can produce genetically different gametes through independent assortment and crossing over. At fertilisation, any one genetically different gamete may fuse with any compatible gamete from another parent. The random combination produces a new diploid genotype, adding variation to that created during meiosis. It does not guarantee that every possible genotype is equally likely if gamete production or survival differs.
Check the evidence and limits
Random fertilisation is a source of genetic variation, not a predictable one-to-one pairing of matching gametes. Probability describes expected chances over many events; it does not identify the genotype of a particular offspring.
Pause and explain
If each of two parents can produce many genetically different gametes, explain why the number of possible offspring genotypes can be large.
Worked example
Why can two siblings from the same parents have different allele combinations?
Show the worked solution
- Each parent produces genetically varied gametes.
- Fertilisation randomly combines one gamete from each parent.
Answer Different genetically varied gametes can fuse at fertilisation, giving siblings different combinations of parental alleles.
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.
Explain how independent assortment and random fertilisation contribute to offspring variation.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Meiosis produces different gamete combinations.
- Fertilisation combines one gamete from each parent at random.
Independent assortment produces gametes with different combinations of parental homologues. Random fertilisation combines one gamete from each parent, creating new diploid allele combinations.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A couple has three children with different genotypes. Does this contradict Mendelian probability? Explain.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Each fertilisation is a separate random event.
- Probabilities do not prescribe a fixed sequence of outcomes.
No. Genetic probabilities describe expected proportions over many independent events, not a fixed order or guaranteed composition of a small family.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
If each of two parents can produce many genetically different gametes, explain why the number of possible offspring genotypes can be large.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- Each gamete can combine with many possible gametes from the other parent.
- Different combinations produce different zygote genotypes.
Each of many gamete types from one parent can fuse with many types from the other, producing numerous possible zygote genotypes.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Claiming siblings inherit identical gametes
- Treating probability ratios as guaranteed family outcomes
What can you explain now?
What does random fertilisation combine?
Compare with the explanation
One gamete from each parent
The combination restores diploidy and creates a new genotype.
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 random fertilisation add to the variation generated by meiosis?
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.