Explain how reduction and fertilisation maintain the diploid complement
Reduction before fusion allows the usual two-set complement to be restored.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chromosomes, genes and alleles, complete chromosome sets, mitosis and fusion of gamete nuclei.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens when two normal haploid gamete nuclei fuse in this diploid animal model?
What you will learn
- Trace haploid gamete contributions into a diploid zygote
- Explain the consequence of omitting reduction in a stated model
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 |
Trace haploid gamete contributions into a diploid zygote
In the normal diploid animal model, meiosis is involved in producing haploid gametes. Each gamete nucleus contributes one complete chromosome set. At fertilisation the gamete nuclei fuse and their two contributions form a diploid zygote nucleus. Reduction and fusion therefore have opposite effects on the set number in this route: two sets become one before fertilisation, and one plus one becomes two at fertilisation.
For a stipulated animal with n equal to twenty, the diploid complement is forty. Normal gamete nuclei contain twenty each. Fusion gives twenty plus twenty, or forty, in the zygote nucleus. Later normal mitosis maintains that diploid complement in the nucleated descendants of the model. Do not halve the zygote again merely because it divides during growth.
Explain the consequence of omitting reduction in a stated model
Consider a counterfactual model in which gametes retain the unreduced diploid complement of forty. Fusion of two such nuclei gives eighty chromosomes in the resulting nucleus, twice the stipulated usual diploid count. This illustrates why reduction is important in the normal route. It is not a claim that every abnormal fertilisation produces a viable organism or that all real organisms have the same life cycle.
Keep sequence and event types clear: reduction occurs during the gamete-production pathway, fusion creates the zygote, and mitosis supports subsequent cell production. Fertilisation also combines genetic contributions from two gametes, but that combination does not mean meiosis takes place during fusion. Use the supplied complete sets and count per nucleus; a combined count from gametes before fusion describes separate boundaries.
Sources for this lesson
Pause and explain
A stipulated animal has n = 12. What is the normal zygote complement after two gamete nuclei fuse?
Worked example
Use n = 20 to compare normal fusion with counterfactual unreduced fusion.
Show the worked solution
- A diploid precursor has forty chromosomes; a normal haploid gamete has twenty.
- Normal fusion gives 20 + 20 = 40 in the zygote nucleus.
- Two unreduced forty-chromosome contributions would give eighty.
Answer Normal reduction and later fusion restore the usual two-set count. The counterfactual arithmetic does not prove viable development.
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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.
In an n = 20 animal model, state the diploid precursor count, normal gamete count and normal zygote count after fusion.
Give me a hint
Use two sets, then one set per gamete, then two contributions.
Compare my reasoning
- Calculate 2 × 20 for the precursor.
- Use twenty per haploid gamete nucleus.
- Add twenty plus twenty at fusion.
Forty in the diploid precursor, twenty in each normal gamete nucleus and forty in the zygote nucleus.
Look for these in your work
- Forty precursor and twenty gamete
- Forty restored zygote
The same model counterfactually has unreduced gametes with forty chromosomes each. Calculate the fusion result and compare it with the usual diploid count of forty.
Give me a hint
Add two unreduced contributions.
Compare my reasoning
- Calculate 40 + 40.
- Compare eighty with forty.
- Keep viability outside the numerical conclusion.
Eighty chromosomes, twice the stipulated usual diploid count. The arithmetic illustrates the consequence of missing reduction; it does not prove viable development.
Look for these in your work
- Eighty and twice usual count
- Does not claim viability
A normal n = 7 model produces a zygote with fourteen chromosomes. It then divides by normal mitosis. Correct a prediction that each daughter nucleus will have seven.
Give me a hint
Distinguish maintenance division from reduction division.
Compare my reasoning
- Identify the zygote as diploid.
- Apply mitotic maintenance of the complement.
- Reserve seven for normal haploid products in the model.
Each normal mitotic daughter nucleus retains fourteen chromosomes, two sets. Seven is the haploid complement after reduction during gamete production, not the result of normal zygote mitosis.
Look for these in your work
- Fourteen in each daughter
- Distinguishes mitosis from reduction
Common mistakes
- Calling fertilisation a reduction division
- Halving the zygote complement during normal mitotic growth
- Treating a counterfactual count as proof of viability
What can you explain now?
A stipulated animal has n = 12. What is the normal zygote complement after two gamete nuclei fuse?
Compare with the explanation
Twenty-four
Twelve plus twelve restores two complete sets.
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Make it stick
Without your notes, trace haploid gamete contributions into a diploid zygote and explain the consequence of omitting reduction in a stated model. State the chromosome number per nucleus and distinguish chromosome-set reduction from fertilisation.
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.