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Punnett Square Practice: Free Printable Worksheet for Year 9
- ForYear 9
- StageKey Stage 3
- CostFree
- FormatPrintable PDF
Punnett Square Practice is a free printable science worksheet for Year 9 (ages 13 to 14), in Key Stage 3. Children use Punnett squares to work out the chance of an inherited trait from simple single gene crosses. It takes about 20 minutes, needs only a pencil and scrap paper, and page 2 gives the grown up notes, help, a stretch idea and the answers.
It comes from the Mini Minds Hub, the free library of printables from Jaques of London, the British games and toy company that has been making games for families since 1795. Print both pages, give page 1 to your child and keep page 2 for yourself. Black and white printing works well.
What is on the sheet
Children work with crosses between two parents, written as pairs of letters such as Bb. They draw a Punnett square on scrap paper, fill it in, then read it. Some questions ask for a fraction of the offspring. Some ask for a genotype. Others give a number of offspring and ask how many are expected to show a particular genotype. There is space to write each answer.
- Bb x Bb: what fraction of offspring are bb?
- Bb x Bb: what fraction show the dominant trait?
- BB x bb: what is the genotype of all the offspring?
- Bb x bb: what fraction are Bb?
- Bb x Bb with 20 offspring: how many are expected to be bb?
- Bb x bb with 8 offspring: how many are expected to be bb?
Worked example: a different cross
TT x Tt with 12 offspring: how many are expected to be TT?
This is a different cross from the sheet, with different letters and a different number of offspring. It shows two ways to reach the same prediction.
Method 1: draw the square- Write the TT parent's alleles across the top: T and T.
- Write the Tt parent's alleles down the side: T and t.
- Fill each box with one allele from the top and one from the side. The four boxes are TT, Tt, TT and Tt.
- Two of the four boxes are TT, so half of the offspring are expected to be TT.
- Half of 12 is 6.
- The TT parent can only pass on T.
- The Tt parent passes on T half the time and t half the time.
- An offspring is TT when the second T arrives, which happens half the time.
- Half of 12 is 6.
How the sheet builds the skill
Label the square
Children write each parent's alleles along the top and down the side. Getting this right first makes everything after it easier.
Fill in the boxes
Each box combines one allele from each parent. Children copy them carefully and keep the capital letter first.
Read the result
Children count boxes to find a fraction of the offspring, then say what the genotype or trait is.
Scale up to numbers
Children turn a fraction into an expected number of offspring. This links genetics to fractions and prediction.
Why Punnett squares are useful
A gene is a section of DNA that helps decide a characteristic. Different versions of a gene are called alleles. Offspring inherit one allele for each gene from each parent. A Punnett square is a simple grid that shows the different ways the parents' alleles can combine. It is named after Reginald Punnett, a British geneticist.
Gregor Mendel, who bred pea plants, is often called the father of genetics. His work showed that traits are passed on in predictable patterns. A Punnett square turns that idea into a quick drawing. It gives a chance, not a certainty. Real families and real experiments can vary, especially when there are only a few offspring.
Quick genetics facts
- Reginald Punnett was a British geneticist, and the Punnett square is named after him.
- Gregor Mendel studied pea plants to learn how traits are passed on.
- An allele is one version of a gene.
- Offspring get one allele for each gene from each parent.
- Scientists use the word offspring for the young of any living thing, from plants to people.
Skills and curriculum link
- Using Punnett squares
- Genotype and phenotype
- Fractions of a whole
- Predicting outcomes
- Scientific vocabulary
Facts that help with Punnett squares
- Two alleles per parentEach parent has two alleles for the gene. Each offspring gets one from each parent.
- Capital and lower case lettersA capital letter stands for the dominant allele. A lower case letter stands for the recessive allele.
- Equal boxesEach box in the square is equally likely, so counting boxes gives a fraction.
- From fraction to numberTo find an expected number, multiply the fraction by the number of offspring.
Key words
- AlleleOne version of a gene.
- GenotypeThe pair of alleles an organism has, such as Bb.
- PhenotypeThe trait you can see or measure, such as flower colour.
- DominantAn allele that shows in the phenotype even when only one copy is present.
- RecessiveAn allele that only shows when two copies are present.
- OffspringThe young produced by parents.
How to use it
- Print both pages. Page 1 is for your child. Keep page 2 for the grown up.
- Use the sheet after alleles, genotype and phenotype have been taught. Have scrap paper ready for the squares. Allow about 20 minutes.
- Work through the first question together. Ask your child to draw the square and say what each letter means.
- Let them try the rest alone, then check against the answers on page 2. Ask, "How did you work that out?"
- Come back to it another day. Repeating a familiar activity builds confidence.
Help, stretch and game ideas
If it feels too hard
Draw the first square together. Label the top and side with the parent alleles, then let your child fill in the boxes. Say each box out loud as you go. Once one square feels easy, ask them to draw the next one alone.
To stretch confident children
Ask your child to explain why the expected numbers may not match real results when there are only a few offspring. They could give an example in their own words, then say what would happen with many more offspring.
Make it a game
Flip two coins to stand for two Bb parents. Heads is B and tails is b. Each pair of flips makes one offspring. Tally 20 offspring, then compare your results with the prediction from the Punnett square.
Mistakes to watch for
Watch forA child mixes up genotype and phenotype.
Try thisSay it together: genotype is the letters, phenotype is what you can see. Ask them to label each answer as one or the other.
Watch forA child writes Bb and bB as two different results.
Try thisExplain that both have one of each allele. Agree to write the capital letter first every time.
Watch forA child gives a fraction when the question asks for a number of offspring.
Try thisUnderline the question word. Remind them to multiply the fraction by the number of offspring.
Watch forA child puts the wrong alleles on the top or side of the square.
Try thisAsk them to say which parent is which, then tick each parent's letters on the sheet before drawing the grid.
Talk about it
- What does each letter in a pair like Bb stand for?
- Why do we write a capital letter for one allele and a lower case letter for the other?
- What does a box in the Punnett square tell us?
- Why is an expected number only a prediction?
- How is genotype different from phenotype?
- Which part of the Punnett square did you find easiest, and why?
Take it off the page
Bead bag crosses
Put two colours of beads in each of two bags to stand for alleles. Draw one from each bag without looking. Record the pair and repeat, then compare with a Punnett square.
Card cross
Make cards labelled B and b. Shuffle two small piles, one for each parent, and turn over one card from each. Tally the results over many rounds.
Teach someone else
Ask your child to explain a Punnett square to a parent or younger sibling using a drawing. Explaining a method shows how well they understand it.
Who it is for
Parents
Sit with your child for the first question, then let them lead. Short bursts work well, and the sheet takes about 20 minutes. You do not need to know genetics. The notes and answers on page 2 will guide you.
Teachers
Use it as a starter, plenary, early finisher task, homework or table activity after teaching alleles, genotype and phenotype. It needs no preparation beyond printing. Page 2 gives help and stretch ideas for mixed ability groups.
Home educators
Print it as part of a wider science plan. It pairs well with a bead or coin experiment at the table. Reuse it for brothers and sisters, or for revision a few weeks later.
Where this fits
Before this sheet, children learn that characteristics are passed from parents to offspring, and meet the words gene, allele, genotype and phenotype. Fractions work in maths also helps, because each box in a square is a part of a whole.
After this, children can move on to more varied crosses, probability in genetics and reading family trees. Later study builds on the same square, so a secure start with simple crosses pays off.
Questions parents and teachers ask
- What age is Punnett Square Practice for?
It is written for Year 9 (ages 13 to 14), in Key Stage 3. It suits children who have been taught alleles, genotype and phenotype and want practice applying them.
- What will children practise?
They practise using Punnett squares to predict genotypes and ratios from simple single gene crosses. They also turn fractions into expected numbers of offspring, which links science to maths.
- How long does it take?
It takes about 20 minutes. Children need scrap paper to draw the squares. Work through the first question together, then let them try the rest alone.
- What if it feels too hard?
Draw the first square together. Label the top and side with the parent alleles, then let your child fill in the rest. Repeat on the next question with less help each time.
- Is it free?
Yes. It is free to print for home and classroom use. Print it again for brothers, sisters or revision.
- Can teachers use it in class?
Yes. It works as a starter, plenary, early finisher task, homework or table activity. It needs no preparation beyond printing, and page 2 has help and stretch ideas.
- Do I need to print in colour?
No. It prints well in black and white on plain A4 paper. It is a screen free activity that needs only paper and a pencil.
Preview the full PDF
Made by Jaques of London, making games for families since 1795. A printable supports classroom learning and does not replace a teacher.