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# Why is every sperm and egg different?

Year 10 Biology · Genetics

<!-- Speaker note: Students model meiosis, explain how it halves the chromosome number and produces genetically different gametes, and contrast it with mitosis (AC9S10U01, node 3, a hinge node). About 20 minutes (Engage and Explore 7 min · Explain 8 min · Meiosis and mitosis 2 min · Evaluate 3 min · Wider adds about 2 min). -->

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## Today

**We are learning to** meiosis makes gametes. The chromosome pairs separate and each gamete gets one chromosome from each pair, chosen at random, so it has half the number and a different combination. With 23 pairs there are over 8 million combinations.

- I can explain that each gamete gets one chromosome from each pair.
- I can calculate the number of possible gametes for a small number of pairs.
- I can distinguish meiosis from mitosis.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace modelling meiosis with pipe cleaners or socks. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconception is that gametes are identical copies. The Explore board shows variety from one cell. Board notes: 3 pairs instead of 23; meiosis is shown as its outcome (pairs separating at random), without the stages or copying, and without crossing over. Pairs are labelled A, B and C (red, from the mother) and a, b and c (blue, from the father); the letters under each gamete name its combination. -->

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## Why it matters

> You and your brother or sister came from the same two parents. The chance that two of their sperm or eggs carry exactly the same chromosomes is tiny.

<!-- Speaker note: Chromosomes come in pairs, one from each parent. This lesson: how gametes get half, and why each is different. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**One person makes millions of sperm (or thousands of eggs). What do you predict about their chromosomes?**

- Each has a random number of chromosomes
- Each has half the chromosomes, but the combination of which ones differs from gamete to gamete
- Each has the full 46
- They are all identical

<!-- Speaker note: What happens: Each has 23, one from each pair, but which one of each pair is random, so almost every gamete is a different combination. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'all identical'. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-10%2Fmeiosis%2Fstep-2&code=widget%3A+cross%0Amode%3A+meiosis%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-10/meiosis/?full=step-2#step-2)

<!-- Speaker note: Red chromosomes came from this person's mother, blue from their father. Press **Make a gamete** many times. How many chromosomes does each gamete get? How many different combinations can you find? -->

<!-- Speaker note: Teacher: ask how many different combinations are possible and how they know they've found them all. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Each has 23, one from each pair, but which one of each pair is random, so almost every gamete is a different combination.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## One of each pair

**On the board, every gamete had three chromosomes. Which is true?**

- Each gamete gets the whole pair from one chromosome
- Each gamete gets exactly one chromosome from each pair
- Each gamete gets all the red ones or all the blue ones
- Each gamete gets any three chromosomes

* Answer: Each gamete gets exactly one chromosome from each pair

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the 2 × 2 × 2 question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Three pairs

**With 3 pairs, each with 2 choices, how many different gametes are possible?**

* Answer: 8

<!-- Speaker note: 2 × 2 × 2 = 8. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 23 pairs

**Humans have 23 pairs. About how many different combinations are possible?**

- 2 × 23 = 46: two choices for each of 23 pairs
- 23: one new combination for each pair
- 23 × 23 = 529
- 2 to the power 23: over 8 million

* Answer: 2 to the power 23: over 8 million

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: meiosis and mitosis

**Meiosis (making gametes) or mitosis (growth and repair)?**

Groups: Meiosis · Mitosis

Makes sperm and egg cells · New cells have 46 chromosomes · New cells have 23 chromosomes · New cells are genetically identical to the parent cell · New cells are all genetically different · Healing a cut

* Answer: Makes sperm and egg cells → Meiosis; New cells have 46 chromosomes → Mitosis; New cells have 23 chromosomes → Meiosis; New cells are genetically identical to the parent cell → Mitosis; New cells are all genetically different → Meiosis; Healing a cut → Mitosis

<!-- Speaker note: What this step is for: Contrasting meiosis with ordinary cell division stops students mixing them up. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: even more variety

**Each parent can make over 8 million different gametes. Ignoring crossing over, roughly how many different children could two parents have?**

- About 8 million × 8 million: around 70 trillion
- About 16 million
- 46
- About 8 million

* Answer: About 8 million × 8 million: around 70 trillion

<!-- Speaker note: Crossing over (pairs swapping pieces during meiosis) adds far more variety still. What this step is for: Adding random fertilisation (and crossing over) shows why siblings are almost never identical. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Meiosis** makes gametes: chromosome pairs separate; each gamete gets **one from each pair**, so it has **half** the number.
- Which of each pair goes into a gamete is **random**: 2 × 2 × 2 … for 23 pairs, over **8 million** combinations.
- Random fertilisation (and crossing over) multiply the variety.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Why does meiosis make every gamete different?**

* Each gamete gets one chromosome from each pair, and which one is random, so the combination differs.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Punnett squares (node 4), which track one gene through meiosis and fertilisation. In class: sock or pipe-cleaner meiosis, including crossing over. -->