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💡 Halving to keep the balance
If both parents gave a full set of chromosomes, every generation would carry double. Meiosis solves this by handing each gamete exactly half.
What is meiosis?
Meiosis
A cell division that forms gametes, halving the chromosome number from diploid to haploid. Humans: 46 → 23
- A special type of cell division that forms gametes .
- It reduces the chromosome number of a parent cell ( diploid ) to half ( haploid ) in the daughter cells.
- Humans have 23 pairs (46 chromosomes); each human gamete carries only 23 .
What are gametes?
Gametes
Male gamete — Female gamete — sperm (animals) or egg, held in the pollen (plants) ovule in plants
- The haploid cells used only for reproduction are called gametes .
- In animals, the male gamete is sperm and the female gamete is the egg .
- In plants, the pollen grain carries the male gametes and delivers them to the ovule , which contains the female gamete (egg).
Activity 11.4 — Let us explore
In this Activity, we will explore how meiosis creates variation by making combinations from three pairs of coloured beads.
Activity 11.4 — Bead Combinations
Step 1
Take three pairs of coloured beads
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Step 2
Each pair is a contrasting character
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Step 3
Pick one bead from each pair at random
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Step 4
Count the possible combinations
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Three pairs give 8 combinations → variation
- Take three pairs of beads of different colours, each pair standing for two contrasting characters (Fig. 11.9).
- Pair 1 (green): blonde vs black hair; Pair 2 (blue): straight vs curly hair; Pair 3 (red): brown vs black eyes.
- Make a combination by randomly picking one bead from each pair.
- Write your combination, e.g. ‘light green, light blue, light red’.
- Count how many combinations are possible — with three pairs, eight combinations arise; imagine the number with 23 pairs.
What we observe
Just three pairs give
eight
different combinations — showing how meiosis produces enormous variation, making each child genetically unique.
How does meiosis create variation?
- During meiosis, the chromosomes of each pair separate randomly into the gametes.
- Each gamete receives a unique mix of characters from the many chromosomes.
- Fusion of two such gametes makes children genetically different from their parents and from their siblings — variation that helps species adapt and evolve.
Mitosis vs Meiosis
Mitosis
Meiosis
Produces body cells and asexual offspring
Produces gametes for sexual reproduction
Chromosome number stays the same
Chromosome number is halved
Daughter cells are identical to the parent
Gametes carry a unique mix (variation)
✅ Test Yourself
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How many chromosomes does a human body cell have?
Show Answer
46 — that is 23 pairs. -
How many does a sperm or an egg have?
Show Answer
23 — meiosis halves the number. -
Why must gametes be haploid?
Show Answer
So the zygote gets back 46, and the number does not double every generation. -
What does the random mixing of chromosomes create?
Show Answer
Variation — children differ from their parents and from each other.
Important Definitions
- Chromosome — a thread-like structure in the nucleus that carries genetic information.
- Meiosis — the special cell division that forms gametes and halves the chromosome number.
- Diploid — a cell carrying the full set of chromosome pairs — 46 in humans.
- Haploid — a cell carrying only one chromosome of each pair — 23 in a human gamete.
- Gamete — a haploid reproductive cell that fuses with another gamete to start a new individual.
- Sperm — the small, motile male gamete in animals.
- Egg — the large female gamete in animals, carrying stored nutrients.
- Pollen grain — the structure that carries and delivers the male gametes in a flowering plant.
- Ovule — the structure inside the ovary that holds the female gamete (egg cell).
