Explain reduction from diploid to haploid chromosome sets
Reduction halves the complement per nucleus: two complete sets become one.
Section B16.2 · Cell division
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 does diploid mean?
What you will learn
- Define haploid and diploid using complete sets
- Calculate the number per nucleus after reduction
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Reference | n = 3 means one complete set of three types |
| Diploid precursor | 2n = 6 chromosomes in one nucleus |
| Normal haploid product | n = 3 chromosomes in each product nucleus |
| Across separate products | A combined total is not the complement of one nucleus |
Define haploid and diploid using complete sets
Diploid means two complete chromosome sets; haploid means one complete set. A set includes one chromosome of each required type. If a species has three chromosome types in its haploid complement, n is three. A normal diploid precursor contains six chromosomes, written 2n. A normal haploid product contains three, written n. The letters describe sets, not the number of cells in a drawing.
Meiosis is a reduction division: the chromosome number per product nucleus is halved from the diploid starting complement to the haploid complement. In the stipulated six-to-three example, six divided by two is three. The word halved concerns each product nucleus, not a total obtained by adding chromosomes from several cells. A four-product illustration therefore does not mean divide the starting number by four.
Calculate the number per nucleus after reduction
The haploid number need not be even. A species with n equal to fifteen has a diploid complement of thirty, and normal reduction gives fifteen per haploid nucleus. Fifteen is odd but still one complete set. Ploidy is defined relative to the species complement: the same count could describe one set in one species and two sets in a different species.
Check identity as well as arithmetic when a model gives chromosome types. Three objects labelled type one, type one and type two do not form a complete three-type set if type three is missing. A numerical count alone does not establish a normal haploid complement, gamete function or genetic sequence. Use the reference set and explicitly stated normal-division assumptions before drawing those conclusions.
Sources for this lesson
Pause and explain
A species has n = 7. What is the normal haploid product count?
Worked example
A stipulated species has n = 5. Interpret its precursor and four normal final product nuclei.
Show the worked solution
- Two sets give 2 × 5 = 10 chromosomes in the diploid precursor.
- One complete set gives five in each haploid product.
- Four separate product nuclei contain 4 × 5 = 20 in total.
Answer Ten becomes five per nucleus; twenty is only the combined count across four nuclei, not the complement in one nucleus.
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.
A model species has n = 3. Calculate the diploid precursor count and the count per normal haploid product nucleus.
Give me a hint
Use two sets before reduction and one after.
Compare my reasoning
- Calculate 2 × 3 for the precursor.
- Halve six to obtain the product count.
Six chromosomes in the diploid precursor and three per haploid product nucleus.
Look for these in your work
- Precursor count is six
- Each product count is three
An animal has a normal diploid complement of thirty chromosomes. Calculate n and explain whether its odd value prevents haploid status.
Give me a hint
Ploidy depends on complete sets, not evenness.
Compare my reasoning
- Divide thirty by two.
- Interpret fifteen as one complete set.
n is fifteen; an odd haploid number is possible because fifteen is the complete single-set complement for this animal.
Look for these in your work
- Calculates fifteen
- Rejects the evenness rule
For a species with n = 5, four separate haploid nuclei each contain five chromosomes. Find the combined count and correct the claim that each nucleus is now diploid because the combined total is twenty.
Give me a hint
Keep the four boundaries separate.
Compare my reasoning
- Add 4 × 5 across the nuclei.
- Compare each nucleus with n = 5.
- Do not transfer the combined count into one nucleus.
Twenty chromosomes are counted across four nuclei. Each nucleus remains haploid with five chromosomes, one complete set; adding separate counts does not change its ploidy.
Look for these in your work
- Combined count twenty
- Each nucleus remains haploid with five
Common mistakes
- Dividing by four because four products are drawn
- Treating an odd chromosome count as impossible
- Using the count without the reference species complement
What can you explain now?
A species has n = 7. What is the normal haploid product count?
Compare with the explanation
Seven per nucleus
Reduction changes two sets, fourteen, to one set, seven.
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Make it stick
Without your notes, define haploid and diploid using complete sets and calculate the number per nucleus after reduction. 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.