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.
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.
Gametes get one of each pair at random. This lesson: following one gene from parents to offspring.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: where does white come from?
ExploreTry it on the board and compare what happens with your prediction.
Explore: fill the square
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.
Try it yourselfYour edits are saved in this browser.
Fill in the gametes and the grid correctly, then breed some offspring.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: genotype, phenotype, probability
In your book: draw the Pp × Pp square and write the genotype ratio and the phenotype ratio.
Match each word to its meaning.
Genotype
Phenotype
Homozygous
Heterozygous
Recessive
PractiseA few on your own, to check you have it.
A different cross
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Fill in the square for a heterozygous black guinea pig crossed with a white one.
Use the board. This ticks when you get there.
Elaborate TypicalUse the same idea in a new context.
Elaborate: is it hiding a b?
Use the board to try BB × bb and Bb × bb before you answer.
Try it yourselfYour edits are saved in this browser.
Elaborate WiderUse the same idea in a new context.
Elaborate: a family tree
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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.
I can use the words genotype, phenotype, dominant, recessive, homozygous and heterozygous.
I can explain a ratio as a probability, not a fixed count.
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.
Running the lesson
Engage: count how many chose 'all purple' or 'a blend'.
Explore: collect the class's breeding percentages on the board: they cluster near 75% but vary.
Explain: the 'exactly 3 and 1?' question is the checkpoint.
The hard part (on purpose)
Reading the square as chances, not a count. When a student says three children out of four must show the trait, ask whether the next child knows what the last one got.
When students get stuck
A student puts PP in every cell. Ask: Which allele does this gamete carry? And this one? Fix: Each cell combines one gamete from the top and one from the side.
A student says the next child of carriers can't have cystic fibrosis because one already does. Ask: Does the egg or sperm know about the last child? Fix: No: each child is a new 1 in 4 chance.
Stretch and support
Stretch: Why did Mendel need to grow thousands of plants to see a clear 3 : 1?
Support: Give a blank Punnett square with the gamete headers already filled.
Where it leads
Next unit: variation and natural selection (10U02).
In class: coin-toss crosses (each parent a coin, heads = P); pedigree analysis.
What each step is for
Engage: where does white come from?: Students who think offspring must look like a parent can't explain white from purple × purple; the square shows the hidden recessive allele.
Explore: fill the square: Filling the gametes and grid by hand, then breeding 100 offspring, links the square to real counts that vary around 75%.
Explain: genotype, phenotype, probability: Using the vocabulary and reading the square as a probability is the hinge.
a different cross: A second cross with a homozygous recessive parent gives a 1 : 1 ratio, so students can't just memorise 3 : 1.
Elaborate: is it hiding a b?: Choosing the cross that reveals a hidden allele (a test cross) uses the square in reverse.
Elaborate: a family tree: Reasoning about carriers in a human context applies the square to a real recessive condition.
Now you can
Complete a Punnett square for one gene.
Use the words genotype, phenotype, dominant, recessive, homozygous and heterozygous.
Explain a ratio as a probability, not a fixed count.