By the end of this lesson
- State human diploid and haploid chromosome numbers.
- Apply chromosome counts to meiosis and fertilisation.
- Distinguish chromosome pairs from chromatids.
BIG QUESTION
How do chromosome numbers remain constant across human generations?
Meiosis halves the number and fertilisation restores it.
Body cells are diploid
A typical human body cell contains 46 chromosomes, arranged as 23 pairs. One of each pair is usually inherited from each parent.
This is written 2n = 46.
Gametes are haploid
Human sperm and eggs contain 23 chromosomes, one from each pair. This haploid number is written n = 23.
Meiosis separates homologous pairs to produce the haploid cells.
Fertilisation restores the diploid number
Fusion of a 23-chromosome sperm nucleus and a 23-chromosome egg nucleus gives a 46-chromosome zygote.
Chromosome number is not the same as DNA amount; a replicated chromosome contains two chromatids but is still counted as one chromosome until chromatid separation.
Human chromosome numbers
Track ploidy across the life cycle.
| Cell | Chromosome number | Ploidy |
|---|---|---|
| Body cell before replication | 46. | Diploid. |
| Gamete | 23. | Haploid. |
| Zygote | 46. | Diploid. |
| Body cell after DNA replication | 46 chromosomes/92 chromatids. | Diploid. |
| Cell after mitosis | 46. | Diploid. |
Language that earns precision
- diploid
- Containing two sets of chromosomes.
- haploid
- Containing one set of chromosomes.
- zygote
- Diploid cell formed by fertilisation.
- chromatid
- One replicated copy of a chromosome before separation.
Common misconceptions
Claim: Gametes have 46 chromosomes each.
Correction: They normally have 23.
Claim: Replication makes 92 chromosomes.
Correction: It makes 92 chromatids while chromosome count remains 46.
Claim: A zygote has 23 chromosomes.
Correction: It receives 23 from each gamete, making 46.
Match the answer to the command word
Human somatic 46; human gamete 23.
23 + 23 = 46 at fertilisation.
Recall, apply, then justify
State the chromosome number in a typical human body cell, gamete and zygote.
Hint
Diploid, haploid, restored diploid.
Suggested marking points
- Body cell: 46.
- Gamete: 23.
- Zygote: 46.
Explain why chromosome number does not double at every generation.
Hint
Meiosis then fertilisation.
Suggested marking points
- Meiosis halves chromosome number in gametes.
- Gametes contain 23 chromosomes.
- Fertilisation restores 46 rather than doubling beyond it.
A species has 2n = 18. Predict chromosome numbers in a gamete, zygote and daughter cell after mitosis.
Planning hint
Halve for gametes; restore for zygote; maintain in mitosis.
What a strong design includes
- Gamete has 9 chromosomes.
- Zygote has 18 chromosomes.
- A daughter cell after mitosis has 18 chromosomes.
- Meiosis halves the diploid number.
- Fertilisation restores the diploid number.
- Mitosis maintains chromosome number.