What is a Punnett square?

A grid that shows all the ways two parents' alleles can combine for one trait. It predicts the chance of each genotype and phenotype in the offspring.

What is the difference between genotype and phenotype?

The genotype is the pair of alleles, such as Yy. The phenotype is the trait you can see, such as yellow peas.

What does dominant mean?

A dominant allele shows its trait even when there is only one copy. A recessive allele only shows when both copies are recessive.

Why did Mendel use peas?

Peas grow quickly, produce lots of seeds and have traits that come in two clear forms, like yellow or green and round or wrinkled. That made the patterns easy to count.

Grades 7 to 10 (Years 8 to 11 in England)

Punnett square simulator

This Punnett square tool lets you choose the alleles of two parent pea plants for one trait, fill in the square yourself and see the genotype and phenotype ratios. Start with two Yy parents for pea color, fill in all four boxes and press Check.

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What this tool shows

Genes come in versions called alleles. For each trait a pea plant has two alleles, one from each parent. A Punnett square shows every way the parents' alleles can combine: one parent's alleles go across the top, the other's down the side, and each box gets one from each. This tool uses traits Gregor Mendel studied in pea plants, such as yellow (Y) and green (y) peas. Yellow is dominant, so a plant with YY or Yy has yellow peas; green is recessive and only shows in yy plants.

Students choose the parents, fill in each box and press Check. Once the square is right, the tool shows the genotype ratio, such as 1 YY : 2 Yy : 1 yy, and the phenotype ratio, such as 3 yellow : 1 green. Grow 20 offspring picks alleles at random, so the counts come close to the square's prediction but are rarely exact.

Why filling in the square helps

Watching a computer fill in a square teaches very little. Choosing the pair for each box makes students think about where each allele comes from. Growing offspring connects the square to probability: it predicts chances, not exact numbers. Mendel's own large counts came close to 3 : 1, for example 6,022 yellow and 2,001 green peas, while small samples wobble much more.

Common misconceptions

Dominant does not mean more common or better. It only means one copy is enough for the trait to show.
A 3 : 1 ratio does not mean a family of four will have exactly three with the trait. Each offspring is a separate chance.
Two yellow parents can have green offspring if both carry the recessive allele.
Most human traits, like height or eye color, depend on many genes, so a simple Punnett square can't predict them.

A 10-minute genetics activity

Set both parents to Yy and ask the student to predict the colors of the offspring.
Let them fill in the square and check it.
Change one parent to yy and repeat. Ask what changed and why.
Grow 20 offspring several times and record the numbers of yellow and green.
Finish with two Challenge targets.

Where this fits in school

US, grades 6 to 8: NGSS MS-LS3-2 asks students to model why sexual reproduction produces offspring with genetic variation, and Punnett squares are a common way to show it.
US, high school biology: NGSS HS-LS3-3 asks students to use probability to explain the variation and distribution of traits in a population.
England, Key Stage 3: students meet heredity and a simple model of chromosomes, genes and DNA.
England, GCSE (Years 10 and 11): students use Punnett squares and genetic diagrams to predict the results of single-gene crosses.

Learning more

Science is part of Eurekami's skill map, with lessons that draw science diagrams live and narrate them in English or Spanish. When a question is tricky, the AI tutor asks guiding questions so the student does the reasoning. For older students, Exam Prep includes GCSE Combined Science practice papers.

Questions people ask

What is a Punnett square?: A grid that shows all the ways two parents' alleles can combine for one trait. It predicts the chance of each genotype and phenotype in the offspring.
What is the difference between genotype and phenotype?: The genotype is the pair of alleles, such as Yy. The phenotype is the trait you can see, such as yellow peas.
What does dominant mean?: A dominant allele shows its trait even when there is only one copy. A recessive allele only shows when both copies are recessive.
Why did Mendel use peas?: Peas grow quickly, produce lots of seeds and have traits that come in two clear forms, like yellow or green and round or wrinkled. That made the patterns easy to count.

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