This free punnett square worksheet teaches monohybrid crosses step by step. Students start with genetics vocabulary, then complete four blank Punnett squares for pea pods, rabbits and dogs, writing the gametes, filling the boxes and giving genotype and phenotype ratios.
They then turn ratios into percentages and expected numbers, work backwards from offspring to parents, plan a test cross, and use Punnett squares to predict the chances of Huntington disease and albinism.
- Grade 11 Biology
- 26 marks
- 40–50 minutes
- Full answer key
What's in This Punnett Square Worksheet
| Part | Skill practised | Marks |
|---|
| A | Genetics vocabulary | 4 |
| B | Complete four Punnett squares | 8 |
| C | Probability and expected numbers | 3 |
| D | Working backwards to the parents | 3 |
| E | The test cross | 2 |
| F | Human genetic disorders | 4 |
| G | Spot the mistake | 2 |
It supports the Ontario Grade 11 biology curriculum expectations in the Genetic Processes unit.
Genetics Vocabulary
Every Punnett square worksheet starts with the language of Mendelian genetics. Part A of the Punnett square worksheet matches these eight terms:
| Term | Meaning |
|---|
| Allele | A different version of the same gene |
| Dominant | An allele that shows its trait whenever it is present |
| Recessive | An allele that is hidden when a dominant allele is present |
| Genotype | The pair of alleles an organism has (GG, Gg or gg) |
| Phenotype | The observable trait, such as green pods |
| Homozygous | Having two identical alleles for a gene (GG or gg) |
| Heterozygous | Having two different alleles for a gene (Gg) |
| Gamete | A sex cell (egg or sperm) that carries one allele of each gene |
How to Complete a Punnett Square
Take the first cross on the Punnett square worksheet: in pea plants, green pods (G) are dominant to yellow pods (g), and we cross two heterozygous plants, Gg × Gg.
- Find the gametes. Each parent makes G and g gametes, because each gamete gets one allele (the law of segregation).
- Set up the grid. One parent's gametes go across the top and the other's go down the side.
- Fill the boxes. Each box gets the letter from its column and the letter from its row.
- Count. Each box is a 25% chance.
Genotypes: 1 GG : 2 Gg : 1 gg
Phenotypes: 3 green : 1 yellow
The other three crosses show the patterns worth memorizing: BB × bb gives all heterozygous offspring, Ww × ww gives a 1 : 1 ratio, and BB × Bb gives all dominant phenotypes but two different genotypes. For more background, see the Punnett square on Wikipedia.
Ratios and Probability
A Punnett square is a probability tool. In the Gg × Gg cross, 1 box in 4 is gg, so each offspring has a 25% chance of yellow pods. Multiply by the number of offspring to get expected numbers: in 600 pods, about 3/4 × 600 = 450 should be green. Part C of the Punnett square worksheet practises both. Real results vary, just as 10 coin flips rarely give exactly 5 heads. The genetics probability worksheet goes further with the product and sum rules.
Working Backwards and Test Crosses
Sometimes you know the offspring and need the parents. If a brown rabbit and a white rabbit have both brown and white babies, the brown parent must carry a hidden white allele, so it is Bb. A test cross uses this idea on purpose: cross an animal with the dominant phenotype with a homozygous recessive animal. All dominant offspring suggest it is BB; any recessive offspring prove it is Bb.
Part F of the Punnett square worksheet predicts inherited disorders. Huntington disease is caused by a dominant allele, so an Hh parent and an hh parent have a 50% chance of an affected child each time. Albinism is recessive, so a carrier (Aa) and an affected parent (aa) also have a 50% chance. When two genes are involved, move on to the dihybrid cross worksheet; for traits that don't follow simple dominance, try the codominance and incomplete dominance worksheet.
Punnett Square Worksheet Practice Questions
These questions match the Punnett square worksheet PDF, which has a blank Punnett square and ratio lines for every cross.
Part A – Genetics Vocabulary
- Allele
- Dominant
- Recessive
- Genotype
- Phenotype
- Homozygous
- Heterozygous
- Gamete
Part B – Complete the Punnett Squares
- Pea pods: green (G) is dominant to yellow (g). Cross Gg × Gg.
- Rabbits: brown fur (B) is dominant to white (b). Cross BB × bb.
- Dogs: wire hair (W) is dominant to smooth (w). Cross Ww × ww.
- Rabbits: cross a homozygous brown rabbit with a heterozygous brown rabbit.
Part C – Probability
- In question 9, what percentage of the offspring will have yellow pods?
- If the cross in question 9 produces 600 pods, about how many will be green?
- If the same cross produces 80 plants, about how many will be homozygous dominant (GG)?
Part D – Working Backwards
- A brown rabbit and a white rabbit have 7 brown and 6 white babies. What are the parents' genotypes?
- Two pea plants with green pods produce some offspring with yellow pods. What are the parents' genotypes?
- A wire-haired dog and a smooth-haired dog have 24 puppies over several litters. All are wire-haired. What are the parents' most likely genotypes?
Part E – The Test Cross
- A breeder has a brown rabbit but doesn't know if it is BB or Bb. What colour rabbit should it be crossed with in a test cross, and why?
- Describe the offspring you would expect if the brown rabbit is BB, and if it is Bb.
Part F – Human Genetic Disorders
- Huntington disease is caused by a dominant allele (H). A man who is Hh has children with a woman who is hh. What is the chance each child inherits the disease?
- Albinism is caused by a recessive allele (a). A carrier woman (Aa) has children with a man who has albinism (aa). What is the chance each child has albinism?
Part G – Spot the Mistake
- A student writes: “Dominant alleles are always the most common alleles in a population.” Explain the error and correct it.
- A student writes: “Gg is a recessive genotype because it contains the g allele.” Explain the error and correct it.
Punnett Square Worksheet Answer Key
Open each part of the Punnett square worksheet answers to check your work. The printable key, with every square filled in, is at the end of the PDF.
Part A answers (Q1–8)
- Allele: c (a different version of the same gene)
- Dominant: g (an allele that shows its trait whenever it is present)
- Recessive: d (an allele that is hidden when a dominant allele is present)
- Genotype: f (the pair of alleles an organism has (gg, gg or gg))
- Phenotype: a (the observable trait, such as green pods)
- Homozygous: h (having two identical alleles for a gene (gg or gg))
- Heterozygous: b (having two different alleles for a gene (gg))
- Gamete: e (a sex cell (egg or sperm) that carries one allele of each gene)
Part B answers (Q9–12)
- Genotypes 1 GG : 2 Gg : 1 gg; phenotypes 3 green : 1 yellow
- Genotypes all Bb; phenotypes all brown
- Genotypes 1 Ww : 1 ww; phenotypes 1 wire : 1 smooth
- Genotypes 1 BB : 1 Bb; phenotypes all brown
Part C answers (Q13–15)
- 1 out of 4 = 25%
- 3/4 × 600 = 450 green pods
- 1/4 × 80 = 20 plants
Part D answers (Q16–18)
- Bb × bb. White babies (bb) mean the brown parent must carry b; a 1 : 1 ratio matches Bb × bb.
- Gg × Gg. Each parent must carry a hidden g allele to produce gg offspring.
- WW × ww. If the wire parent were Ww, about half the puppies would be smooth.
Part E answers (Q19–20)
- A white (bb) rabbit. It can only pass on b, so any hidden b in the brown rabbit will show up as white babies.
- BB × bb: all brown. Bb × bb: about half brown, half white (1 : 1).
Part F answers (Q21–22)
- Genotypes 1 Hh : 1 hh; phenotypes 1 affected : 1 unaffected; 2/4 = 50%
- Genotypes 1 Aa : 1 aa; phenotypes 1 normal : 1 albino; 2/4 = 50%
Part G answers (Q23–24)
- Dominance only describes which allele is expressed, not how common it is. Huntington disease is dominant but rare.
- Gg is heterozygous. The dominant G is expressed, so the plant has green pods; only gg shows the recessive trait.
5 Common Mistakes on a Punnett Square Worksheet
- Putting both alleles in one gamete. A Gg parent makes G gametes and g gametes, never "Gg" gametes.
- Mixing up genotype and phenotype ratios. 1 GG : 2 Gg : 1 gg is the genotype ratio; 3 green : 1 yellow is the phenotype ratio.
- Calling Gg recessive. Heterozygotes show the dominant trait.
- Treating ratios as guarantees. Each offspring is an independent chance.
- Assuming dominant means common. Dominance describes expression, not frequency.
How to Use This Punnett Square Worksheet in Class
- Lesson 1: Parts A–C of the Punnett square worksheet. Model one cross with two coins, letting heads be the dominant allele.
- Lesson 2: Parts D–G, then extend to the pedigree worksheet to trace traits through families.
Review with the non-Mendelian genetics quiz once students are ready for exceptions to simple dominance.
Frequently Asked Questions
Is this punnett square worksheet free?
Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.
How do you fill in a Punnett square?
Write one parent's gametes across the top and the other parent's gametes down the side. Fill each box with the allele from its column and the allele from its row, then count the genotypes and phenotypes.
What ratio does a cross between two heterozygotes give?
Aa × Aa gives a genotype ratio of 1 AA : 2 Aa : 1 aa and a phenotype ratio of 3 dominant : 1 recessive.
What is a test cross?
A test cross mates an organism showing the dominant trait with a homozygous recessive organism. If any offspring show the recessive trait, the unknown parent must be heterozygous.
Does a 3 : 1 ratio mean exactly 3 out of 4 children will have the dominant trait?
No. Ratios are probabilities. Each child has a 3/4 chance of the dominant trait, so real families can vary, especially when they are small.
More Free Grade 11 Biology Resources
Ready to start? Download the free Punnett square worksheet PDF (use the Download button above), print it double-sided and keep the key for marking.