State haploid gamete and diploid zygote nuclei
Count complete chromosome sets rather than individual shapes.
Section 16.2 · Sexual reproduction
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 DNA, inherited characteristics, nuclei and the distinction between growth and reproduction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does haploid mean?
What you will learn
- Define chromosome sets
- Apply the terms to fertilisation
State haploid gamete and diploid zygote nuclei
| Feature | Meaning |
|---|---|
| Haploid | One set, n |
| Diploid | Two sets, 2n |
| Gamete nuclei | One set each |
| Zygote nucleus | Two sets after fusion |
Define chromosome sets
A haploid nucleus contains one complete set of chromosomes, written n. A diploid nucleus contains two sets, written 2n. One set can include many individual chromosomes; haploid does not mean a nucleus contains only one chromosome. The number in a set depends on the organism, so the human chromosome count is not a universal rule for all plants and animals.
In the syllabus fertilisation model, gamete nuclei are haploid and the nucleus of the zygote is diploid. Each gamete contributes one set. When their nuclei fuse, the resulting zygote has two sets rather than an endlessly doubling chromosome number at every generation. Keep the set count separate from the number of nuclei or cells shown in a sequence.
Apply the terms to fertilisation
For a stipulated organism with six chromosomes in one complete set, a haploid gamete nucleus has six and a diploid zygote nucleus has twelve. The word haploid describes the set organisation even though six chromosomes are present. For another organism, the counts may differ. Always use the species or model information supplied before doing the arithmetic.
This foundation states the ploidy relationship; it is not a complete account of the stages of meiosis or of every biological life cycle. The later inheritance pathway will examine chromosome reduction in more detail. Here, explain one set plus one set forming the two-set zygote nucleus, and do not infer genotype identity simply because two nuclei contain the same number of chromosomes.
Pause and explain
What is the syllabus zygote nucleus after two haploid gamete nuclei fuse?
Worked example
A model gamete nucleus contains a complete set of four chromosomes. What are its ploidy and the ordinary zygote count?
Show the worked solution
- One complete set is haploid.
- Fusion of two such gamete nuclei gives two sets.
Answer Haploid gamete; diploid zygote with eight chromosomes.
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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.
State the ploidy of gamete nuclei and the zygote nucleus in the syllabus model.
Give me a hint
Use one set and two sets.
Compare my reasoning
- Gamete nuclei have one set.
- The zygote nucleus has two sets.
Gamete nuclei are haploid; the zygote nucleus is diploid.
Look for these in your work
- Name haploid gametes
- Name diploid zygote
A complete set contains six chromosomes. Explain why a six-chromosome gamete is haploid.
Give me a hint
Haploid refers to complete sets.
Compare my reasoning
- The six chromosomes make one complete set.
- One set is haploid regardless of having several members.
It is haploid because the six chromosomes form one complete set, not because haploid means one chromosome.
Look for these in your work
- Count sets
- Distinguish set and chromosome
Two nuclei each contain six chromosomes. Does matching chromosome number prove identical inherited information?
Give me a hint
Count and information content are different.
Compare my reasoning
- The same number can occur in different genetic combinations.
- The record does not test inherited identity.
No. Matching chromosome counts alone do not establish identical inherited information or genotype.
Look for these in your work
- Retain the count observation
- Reject count-only identity proof
Common mistakes
- Treating haploid as one chromosome.
- Using a human count for every organism.
What can you explain now?
What is the syllabus zygote nucleus after two haploid gamete nuclei fuse?
Compare with the explanation
Diploid
It has two chromosome sets.
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Make it stick
Define haploid and diploid and explain why one set can contain many chromosomes.
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