---
title: "Can you predict the offspring?"
subtitle: "Year 10 Biology · Genetics · Lesson 3"
---

# Can you predict the offspring?

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

> **Key question:** 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.


:::teacher-note

**In one sentence:** Students complete Punnett squares for monohybrid crosses, use genotype and phenotype vocabulary, and interpret ratios as probabilities (AC9S10U01, node 4, a hinge node).

**Students should be able to say:** 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.

**Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

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## Engage


:::keep-together

**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

My prediction, and why:

{{lines-3}}

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:::teacher-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.

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## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: genotype, phenotype, probability


:::keep-together

### Words

**Match each word to its meaning.**

A. Two identical alleles (PP or pp)

B. The characteristic you see, such as purple flowers

C. Shows only when there is no dominant allele

D. Two different alleles (Pp)

E. The pair of alleles, such as Pp

{{columns 60% 40%}}

{{rows 1}}

| Part | Letter |
|---|---|
| `Genotype` | |
| `Phenotype` | |
| `Homozygous` | |
| `Heterozygous` | |
| `Recessive` | |

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:::teacher-note

**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

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:::keep-together

### 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%

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:::teacher-note

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

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:::keep-together

### 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

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:::teacher-note

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

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:::keep-together

### 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

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:::teacher-note

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

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## a different cross


:::keep-together

### White offspring

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

☐ 1 in 2

☐ 1 in 4

☐ None

☐ All

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:::teacher-note

**Answer:** 1 in 2

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:::typical

## Elaborate: is it hiding a b?

Use the board to try BB × bb and Bb × bb before you answer.

:::keep-together

**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

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:::teacher-note

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

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:::wider

## Elaborate: a family tree


:::keep-together

**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

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:::teacher-note

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

Each pregnancy is independent: the chance stays 1 in 4 every time.

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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.

## Exit ticket

:::keep-together

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

{{lines-3}}

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:::teacher-note

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

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