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LAUNCH · GCSE Biology

Build the cross. Explain the chance.

Autumn 2 · Week 5 · Explore · 40 minutes

Learning intention

I can construct a Punnett square and explain what its probabilities predict.

Success criteria

  • Place one allele in each gamete position.
  • Combine the top and side alleles in all four cells.
  • State a phenotype probability without treating it as a guarantee.

Use Next to begin. Ask for word help or your usual support when needed.

Arrival · 2 min

Read the plant key

Which is an allele, and which is a genotype?

Heterozygous means two different alleles, such as Pp.

Alleles: P and p.

Genotypes: PP, Pp and pp.

In our model, PP and Pp give purple flowers; pp gives white.

Choose the heterozygous genotype.

A P allele from one parent and a p allele from the other combine as Pp.One allele from each parentPpPpPp = two alleles in the offspring genotype.P and p = different forms of the same gene.
A simplified single-gene inheritance model. Probability describes possibilities, not a guaranteed family pattern.
Starter · 3 min

One allele per gamete

A parent has genotype Pp. What can a gamete carry for this gene?

Choose one allele possibility.

Then find the other possibility.

A gamete carries one allele for this gene, not the whole pair.

Which pair of possibilities belongs around the grid for a Pp parent?

I do · 5 min

Combine one from each parent

Watch the cross Pp × Pp grow one cell at a time.

Put P and p across the top.

Put P and p down the side.

Each cell takes one top allele and one side allele.

Keep both Pp cells: they are two different pairings.

  1. Parents.
  2. Gametes.
  3. Four combinations.
  4. Phenotypes.

Pp × Pp · Reveal one cell at a time

GametesPp
P
P + PPP
p + PPp
p
P + pPp
p + ppp

Each box is one equally likely pairing in this model. Both Pp boxes count.

We do · 6 min

Build all four outcomes

Construct Pp × Pp with the digital grid.

Enter the gametes first.

Fill each cell by following its row and column.

Check your combination before moving on.

When all four cells are complete, count the purple and white possibilities.

Paper or spoken route

Print two blank 2×2 grids with parent labels and the phenotype key; use P/p letter tiles or pupil-directed adult writing. Retain each repeated cell and count all four.

I do · 4 min

Count probability, not a promise

What does the completed grid predict?

PP, Pp and Pp give purple flowers.

One cell, pp, gives white flowers.

White: 1 out of 4 = 1/4 = 25%.

Each new offspring has this chance in the model.

A P allele from one parent and a p allele from the other combine as Pp.One allele from each parentPpPpPp = two alleles in the offspring genotype.P and p = different forms of the same gene.
A simplified single-gene inheritance model. Probability describes possibilities, not a guaranteed family pattern.
We do · 5 min

Can four all be purple?

A model run gives four purple offspring. Does that break the prediction?

The prediction is a chance for each offspring.

It does not control the order.

Small groups can differ from the expected ratio.

Decide what chance the next offspring has of being white.

Constructed model run

Offspring 1 purple; 2 purple; 3 purple; 4 purple. This is an authored possible sequence, not an observed breeding result.

Constructed teaching example; not pupil or field results

What is the chance that the next offspring is white in Pp × Pp?

Independent · 12 min

A new cross: Pp × pp

Use the same flower-colour key. Build the new cross yourself.

Show both parent genotypes and their gametes.

Complete all four cells.

State the probability of white flowers as a fraction and percentage.

Explain whether two white flowers are guaranteed in four offspring.

Return with Next or the slide controls. Your grid stays in place.

Independent · Pp × pp

Gametes______
___
___

Chance of white: ____

Supported

Complete Pp×pp with parent labels and gamete positions provided. Trace one row and column per cell, then count the white possibilities and state the chance.

Offer PP/Pp/pp letter tiles and the phenotype key. Adult reads and points to the row/column only when needed; the pupil chooses both alleles.

No name is needed. Save your written notes before closing or reloading.

Exit · 3 min

Find and repair the claim

“Pp × pp must give two white flowers in every four.” Repair this claim.

Use your completed grid.

Give the chance, then explain its limit.

Make one edit or re-attempt after your adult responds.

Lundy Loop

Choose how to share. Your adult will listen and say what happens next.

Complete

Lesson complete

Show one thing you learned.

Tell your adult what helped or what needs another look.

Save any written notes you want to keep. Your teacher will agree the next step.

Use Print for pupil sheets or Back to revisit a stage.

Slide 1

TA Brief

Current stage

LAUNCH · Build the cross. Explain the chance.

Autumn 2 · Week 5 · Explore

SoW

LAUNCH Weekly - Autumn!B43, C43

  • Complete a genetic cross (digital Punnett square).

Aut2·W5; current calendar absolute W13. Retains the digital cross requirement with an actual four-cell construction. Paper or pupil-directed adult operation preserves the reasoning when access requires it. GCSE Biology 1BI0 points 3.13–3.16 provide the subject context; no exam-board approval claim.

Prior: Previous Introduce lesson: P and p are alleles of one gene; Pp is heterozygous. In this simple plant model, PP and Pp flowers are purple; pp flowers are white. No personal or family traits are used.

Next: In the Do lesson, compare predicted probabilities with outcome counts from larger model samples and evaluate the difference between chance and guarantee.

Preparation

Care and safety

Teacher response guidance

Evidence of learning

Check parents→gametes→four combinations→phenotype→probability. Distinguish a gamete error from a combining error or probability-language error. Evidence must include a fresh independent cross; a category sort alone cannot prove the outcome.

Access and independence

Keep GCSE vocabulary with plain definitions. Use large allele tiles, row/column tracing, spoken reasoning or exact scribe. Preserve the complete cross and probability reasoning across modes. Give a prompt to the next step rather than supplying a missing genotype.

Lundy Loop

The adult listens to the pupil’s chosen cell explanation or probability claim, names the exact step needing attention and asks for an edit on the existing grid or sentence. The pupil confirms or changes it; do not turn an adult-written answer into pupil authorship.

Arrival · Read the plant key

Expected Pp because the two alleles differ. Keep capitals large and distinct; provide explicit upper/lowercase labels rather than relying on colour. Recap supplied key for an absent pupil. Do not infer a genotype from a purple phenotype alone.

Starter · One allele per gamete

Expected P or p. A pupil selecting Pp is carrying a body-cell pair into a gamete; ask them to separate the pair. In this model gametes carrying P and p are equally likely.

I do · Combine one from each parent

Model top-left P+P=PP; top-right p+P=Pp; bottom-left P+p=Pp; bottom-right p+p=pp. Write dominant letter first by convention, but pP contains the same alleles as Pp. The grid is a probability model for one gene with complete dominance and independent fertilisation.

We do · Build all four outcomes

Do not accept three genotype categories as a completed square. Inspect all four cells and allow calm revision. Use row/column feedback for the exact wrong cell. Two Pp entries are required. The phenotype key remains visible; adult should not supply cell content.

I do · Count probability, not a promise

Work the conversion visibly: number of white cells divided by all four equally likely cells, multiplied by 100. Genotype ratio is 1 PP:2 Pp:1 pp; phenotype ratio is 3 purple:1 white. Distinguish the two; dominant does not mean most common in a population.

We do · Can four all be purple?

All four purple is possible; the next has 1/4 chance of white under the same Pp×Pp model. Do not say a white is due. Optional stretch: 3/4×3/4×3/4×3/4=81/256 for four purple, but this calculation is not needed for the core objective.

Independent · A new cross: Pp × pp

This is fresh independent application. Keep the worked Pp×Pp answer hidden where possible while retaining the allele/phenotype key. The pp parent gives p only; show p in both row positions so the four equal pairings remain explicit. A repeated p is not a new allele type. Optional stretch compares PP×pp.

Exit · Find and repair the claim

Expected: “Pp×pp gives a 1/2 or 50% chance of white for each offspring; a group of four need not contain exactly two white.” If the grid is wrong, feedback addresses one row/column combination before demanding the explanation. LAUNCH close is an actual edit/re-attempt on the existing work, not another form.

Misconceptions

A gamete carries Pp. Separate the parent’s allele pair; a gamete carries one allele for this gene.

The two Pp outcomes can be merged into one box. They arise from different row/column pairings. Keep both to count the equally likely outcomes.

Dominant means stronger or automatically more common. It describes expression in the heterozygous genotype in this model.

One in four guarantees one white in each group of four. The model states a chance per offspring. A small observed group can differ.

Sources

Pearson Edexcel · GCSE Biology 1BI0 specification, Issue 4
Points 3.13–3.16: inheritance terms, monohybrid genetic diagrams and calculations of probabilities, ratios and percentages.

Arizona State University · Ask A Biologist, Punnett Squares
A Punnett square combines parental allele possibilities and uses its cells to express an offspring probability.

Review trace

Science_Teesside/Launch/W8-W13_2026-27/SCI_L_W13L2_Punnett_Square_Explore.html

Separate review copy. No live lesson or award record has been changed.

Word help

allele: a version of a gene

gamete: a sex cell; it has one allele for this gene

genotype: the allele combination, such as Pp

phenotype: the feature expressed, such as flower colour

dominant: expressed in the heterozygous genotype in this model

recessive: expressed only when both copies are recessive in this model

heterozygous: Two different alleles of a gene, such as Pp.

homozygous: Two matching alleles of a gene, such as PP or pp.

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