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# Can you predict the offspring?

Year 10 Biology · Genetics

<!-- Speaker note: Students complete Punnett squares for monohybrid crosses, use genotype and phenotype vocabulary, and interpret ratios as probabilities (AC9S10U01, node 4, a hinge node). About 25 minutes (Engage and Explore 8 min · Explain 7 min · Practise 4 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** a Punnett square shows every way two parents' gametes can combine for one gene. It gives the probability of each genotype and phenotype (such as 3 in 4 showing the dominant trait for Pp × Pp), not a guaranteed count.

- I can complete a Punnett square for one gene.
- I can use the words genotype, phenotype, dominant, recessive, homozygous and heterozygous.
- I can explain a ratio as a probability, not a fixed count.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconceptions are that a 3 : 1 ratio is a guaranteed count, and that offspring must show a parent's phenotype. Pea flower colour is used because it really is controlled by one gene with complete dominance; human eye colour is not, so it is avoided. Board notes: students click to cycle each gamete and cell; "Breed 100 offspring" draws random gametes, so the percentage varies around the prediction. The typical tier's board lets students choose the parents. -->

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## Why it matters

> Two purple-flowered pea plants can have white-flowered offspring. Gregor Mendel counted thousands of them in the 1860s and found the same ratio every time.

<!-- Speaker note: Gametes get one of each pair at random. This lesson: following one gene from parents to offspring. -->

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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

**Two purple-flowered pea plants, both Pp, are crossed. Purple (P) is dominant, white (p) recessive. What do you predict?**

- Mostly purple, about 1 in 4 white
- All pale purple (a blend)
- Half purple, half white
- All offspring purple

<!-- Speaker note: What happens: Mostly purple, about 1 in 4 white. Each parent passes on p half the time; a pp offspring (1 chance in 4) is white. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'all purple' or 'a blend'. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-10%2Fpunnett%2Fstep-2&code=widget%3A+cross%0Amode%3A+punnett%0Agene%3A+P+%7C+purple+flowers+%7C+p+%7C+white+flowers%0Aparents%3A+Pp+x+Pp%0A&fresh=1}}

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

<!-- Speaker note: Click each gamete box to choose the allele it carries, then each grid cell for the offspring's genotype. When it's right, press **Breed 100 offspring** a few times and compare the percentage with the square. -->

<!-- Speaker note: Teacher: collect the class's breeding percentages on the board: they cluster near 75% but vary. -->

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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?

* Mostly purple, about 1 in 4 white. Each parent passes on p half the time; a pp offspring (1 chance in 4) is white.

<!-- 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>

## Words

**Match each word to its meaning.**

| | |
|---|---|
| `Genotype` | A. Two identical alleles (PP or pp) |
| `Phenotype` | B. The characteristic you see, such as purple flowers |
| `Homozygous` | C. Shows only when there is no dominant allele |
| `Heterozygous` | D. Two different alleles (Pp) |
| `Recessive` | E. The pair of alleles, such as Pp |

* Answer: Genotype → E. The pair of alleles, such as Pp; Phenotype → B. The characteristic you see, such as purple flowers; Homozygous → A. Two identical alleles (PP or pp); Heterozygous → D. Two different alleles (Pp); Recessive → C. Shows only when there is no dominant allele

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the 'exactly 3 and 1?' 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>

## Probability

**From Pp × Pp, a gardener grows 4 seeds. Are exactly 3 purple and 1 white?**

- No, it's always half and half
- Yes, always exactly 3 and 1
- No: the square only works for animals
- Not necessarily: each seed has a 3 in 4 chance of purple, so small numbers vary; large numbers get close to 75%

* Answer: Not necessarily: each seed has a 3 in 4 chance of purple, so small numbers vary; large numbers get close to 75%

<!-- 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>

## Hidden alleles

**Why can two purple parents have a white offspring?**

- White came from a mutation every time
- One parent was really white
- The environment made it white
- Both carry a hidden recessive p allele, and the offspring got p from each

* Answer: Both carry a hidden recessive p allele, and the offspring got p from each

<!-- 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>

## Family tree

**In a family tree (pedigree), shaded shapes show people with a recessive condition. Two unshaded parents have a shaded child. What are the parents' genotypes?**

- Both Ff: each carries one hidden f allele
- Both FF
- Both ff
- One is FF and the other ff

* Answer: Both Ff: each carries one hidden f allele

<!-- Speaker note:  -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## a different cross

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-10%2Fpunnett%2Fstep-4&code=widget%3A+cross%0Amode%3A+punnett%0Agene%3A+B+%7C+black+fur+%7C+b+%7C+white+fur%0Aparents%3A+Bb+x+bb%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-10/punnett/?full=step-4#step-4)

<!-- Speaker note: What this step is for: A second cross with a homozygous recessive parent gives a 1 : 1 ratio, so students can't just memorise 3 : 1. -->

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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>

## White offspring

**What fraction of the offspring do you expect to be white?**

- 1 in 2
- 1 in 4
- None
- All

* Answer: 1 in 2

<!-- Speaker note:  -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: is it hiding a b?

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-10%2Fpunnett%2Fstep-5&code=widget%3A+cross%0Amode%3A+punnett%0Agene%3A+B+%7C+black+fur+%7C+b+%7C+white+fur%0Aparents%3A+choose%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-10/punnett/?full=step-5#step-5)

<!-- Speaker note: Use the board to try BB × bb and Bb × bb before you answer. What this step is for: Choosing the cross that reveals a hidden allele (a test cross) uses the square in reverse. -->

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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: is it hiding a b?

**A black guinea pig could be BB or Bb. You cross it with a white (bb) one and get several white babies. What was its genotype?**

- bb
- Bb: a white baby needs a b from each parent
- BB
- You can't tell

* Answer: Bb: a white baby needs a b from each parent

<!-- Speaker note: What this step is for: Choosing the cross that reveals a hidden allele (a test cross) uses the square in reverse. -->

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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: a family tree

**Cystic fibrosis is caused by a recessive allele (f). Two parents without it have a child who has it. What is the chance their next child has it?**

- 1 in 4: both parents must be Ff carriers
- 0: it can't happen twice
- 1 in 2
- 1 in 1

* Answer: 1 in 4: both parents must be Ff carriers

<!-- Speaker note: Each pregnancy is independent: the chance stays 1 in 4 every time. What this step is for: Reasoning about carriers in a human context applies the square to a real recessive condition. -->

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## Evaluate: look back

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

**Revision summary**

- **Genotype:** the alleles (PP, Pp, pp). **Phenotype:** the characteristic seen.
- A **dominant** allele shows when present; a **recessive** one only in **homozygous** form (pp).
- A **Punnett square** gives the **probability** of each offspring: Pp × Pp gives 1 PP : 2 Pp : 1 pp, so 3 : 1 purple to white; Bb × bb gives 1 : 1.
- Real numbers vary around the prediction, especially when there are few offspring.

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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

**Complete Bb × Bb. What fraction of offspring are expected to show the recessive trait?**

* BB, Bb, Bb, bb: 1 in 4 (25%).

<!-- 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 unit: variation and natural selection (10U02). In class: coin-toss crosses (each parent a coin, heads = P); pedigree analysis. -->