This free dihybrid cross worksheet builds Punnett square skills step by step. Students start with genotypes and phenotypes, complete four monohybrid crosses, and then work through a full 4 × 4 dihybrid cross to reach Mendel's famous 9 : 3 : 3 : 1 ratio.
It finishes with test crosses, the product-rule shortcut and two classic misconceptions. Blank Punnett squares are printed right on the sheet, and the answer key shows every completed square and ratio, all checked by computer.
- Grade 11 Biology
- 26 marks
- 45–60 minutes
- Full answer key
What's in This Dihybrid Cross Worksheet
| Part | Skill practised | Marks |
|---|
| A | Genotypes and phenotypes | 3 |
| B | Four monohybrid Punnett squares (incl. a test cross and cystic fibrosis) | 8 |
| C | Probability from crosses | 3 |
| D | Full 4 × 4 dihybrid cross | 5 |
| E | Test cross and product-rule problems | 5 |
| F | Spot the mistake | 2 |
It supports the Ontario Grade 11 biology curriculum expectations in the Genetic Processes unit.
Monohybrid Crosses First
A monohybrid cross follows one gene. Crossing two heterozygous tall pea plants (Tt × Tt) gives this square:
Genotype ratio 1 TT : 2 Tt : 1 tt; phenotype ratio 3 tall : 1 short. Part B of the dihybrid cross worksheet builds up to this with three more crosses, including a test cross to find an unknown genotype and a cystic fibrosis carrier problem.
How to Solve a Dihybrid Cross
Round (R) is dominant to wrinkled (r) and yellow (Y) is dominant to green (y). To cross RrYy × RrYy:
- Find the gametes. Each gamete gets one allele of each gene: RY, Ry, rY, ry.
- Set up a 4 × 4 square with one parent's gametes across the top and the other's down the side.
- Fill in the 16 boxes by combining the row and column alleles.
- Count the phenotypes. Any box with at least one R is round; any box with at least one Y is yellow.
| RY | Ry | rY | ry |
|---|
| RY | RRYY | RRYy | RrYY | RrYy |
|---|
| Ry | RRYy | RRyy | RrYy | Rryy |
|---|
| rY | RrYY | RrYy | rrYY | rrYy |
|---|
| ry | RrYy | Rryy | rrYy | rryy |
|---|
The result is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, the 9 : 3 : 3 : 1 ratio that Mendel used to propose the law of independent assortment. Read more about the dihybrid cross on Wikipedia. Independent assortment comes from how chromosomes line up in metaphase I, which is covered in the meiosis worksheet.
The Product-Rule Shortcut
You don't always need 16 boxes. Because the two genes sort independently, split the cross into two monohybrid crosses and multiply. For RRYy × rrYy: all offspring are round (Rr), and Yy × Yy gives 3/4 yellow, so the ratio is 3 round yellow : 1 round green. For AaBb × AaBb, the chance of aabb is 1/4 × 1/4 = 1/16. Part E of the dihybrid cross worksheet practises both methods.
Dihybrid Cross Worksheet Practice Questions
These questions match the dihybrid cross worksheet PDF, which includes blank Punnett squares and ruled answer lines.
Part A – Genotypes and Phenotypes
Questions 1–6: for TT, Tt, tt, rr, RR and Rr, state whether each is homozygous dominant, homozygous recessive or heterozygous, and give the phenotype.
Part B – Monohybrid Crosses
- In pea plants, tall (T) is dominant to short (t). Cross two heterozygous tall plants.
- Purple flowers (P) are dominant to white flowers (p). Cross a heterozygous purple plant with a white plant.
- In guinea pigs, black fur (B) is dominant to white fur (b). A black guinea pig and a white guinea pig have some white babies. What is the black parent’s genotype? Show the cross.
- Cystic fibrosis is a recessive disorder (f). Two parents are both carriers (Ff). What is the chance that their child has cystic fibrosis?
Part C – Probability
- In question 7, the cross produces 200 offspring. About how many are short?
- In question 7, what fraction of the tall offspring are heterozygous?
- The carrier couple in question 10 already has one child with cystic fibrosis. What is the chance their next child has it?
Part D – Dihybrid Cross
- List the four possible gametes of an RrYy plant.
- Complete the 4 × 4 Punnett square for RrYy × RrYy.
- Give the phenotype ratio of the offspring.
Part E – More Dihybrid Problems
- Cross RrYy × rryy (a test cross). Give the phenotype ratio. (2 marks)
- Cross RRYy × rrYy. Give the phenotype ratio. (2 marks)
- For AaBb × AaBb, use the product rule to find the probability of an aabb offspring.
Part F – Spot the Mistake
- A student says: “A 3 : 1 ratio means that if a couple has four children, exactly three will show the dominant trait.” Explain the error and correct it.
- A student writes the gametes of an RrYy plant as Rr and Yy. Explain the error and correct it.
Dihybrid Cross Worksheet Answer Key
Open each part of the dihybrid cross worksheet answers to check your work. The printable key with completed squares is at the end of the PDF.
Part A answers (Q1–6)
- TT: Homozygous dominant, tall
- Tt: Heterozygous, tall
- tt: Homozygous recessive, short
- rr: Homozygous recessive, wrinkled
- RR: Homozygous dominant, round
- Rr: Heterozygous, round
Part B answers (Q7–10)
- Square: TT / Tt, Tt / tt. Genotypes 1 TT : 2 Tt : 1 tt; phenotypes 3 tall : 1 short
- Square: Pp / Pp, pp / pp. Genotypes 1 Pp : 1 pp; phenotypes 1 purple : 1 white
- Square: Bb / Bb, bb / bb. Genotypes 1 Bb : 1 bb; phenotypes 1 black : 1 white. Black parent: Bb (it must carry b to have bb babies)
- Square: FF / Ff, Ff / ff. Genotypes 1 FF : 2 Ff : 1 ff; phenotypes 3 unaffected : 1 cystic fibrosis. Chance of CF: 1/4 = 25% (and 1/2 = 50% chance of being a carrier)
Part C answers (Q11–13)
- 1/4 × 200 = 50 short plants
- Tall = TT + 2 Tt, so 2/3 are Tt
- Still 1/4. Each child is an independent event.
Part D answers (Q14–16)
- RY, Ry, rY, ry
- See the completed 4 × 4 square above.
- 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
Part E answers (Q17–19)
- Gametes RY, Ry, rY, ry × ry. Offspring: 1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green
- All offspring are Rr (round); Yy × Yy gives 3 yellow : 1 green. Ratio: 3 round yellow : 1 round green
- P(aa) × P(bb) = 1/4 × 1/4 = 1/16
Part F answers (Q20–21)
- Ratios are probabilities. Each child independently has a 3/4 chance of the dominant trait, so real families can differ.
- Each gamete carries one allele of each gene: RY, Ry, rY and ry.
5 Common Punnett Square Mistakes
- Writing both alleles of one gene in a gamete. RrYy makes RY, Ry, rY and ry, never Rr or Yy.
- Mixing up genotype and phenotype ratios. 1 : 2 : 1 is genotypes; 3 : 1 is phenotypes.
- Treating ratios as guarantees. A 3 : 1 ratio is a probability for each offspring, not an exact count.
- Miscounting the 9 : 3 : 3 : 1. Check that your four groups add up to 16.
- Forgetting the test cross logic. A dominant individual crossed with a recessive one reveals its genotype through the offspring.
How to Use This Dihybrid Cross Worksheet in Class
- Lesson 1: Parts A–C on monohybrid crosses and probability.
- Lesson 2: Parts D–F on the dihybrid cross worksheet problems, then compare students' 9 : 3 : 3 : 1 results with Mendel's actual pea counts.
- Extension: Coin-flip simulation: two coins per parent show how real results drift from expected ratios.
When students are ready for traits that break Mendel's rules, move on to the non-Mendelian genetics quiz, or apply allele frequencies with the Hardy-Weinberg quiz.
Frequently Asked Questions
Is this dihybrid cross worksheet free?
Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.
What is a dihybrid cross?
A dihybrid cross follows two genes at once, such as seed shape and seed colour. Crossing two plants that are heterozygous for both genes (RrYy × RrYy) gives a 4 × 4 Punnett square and a 9 : 3 : 3 : 1 phenotype ratio.
How do you find the gametes for a dihybrid cross?
Each gamete gets one allele of each gene. For RrYy, combine each R allele with each Y allele: RY, Ry, rY and ry. A quick check is the FOIL method from algebra.
Is there a shortcut for dihybrid crosses?
Yes. Treat each gene as its own monohybrid cross and multiply the probabilities (the product rule). For AaBb × AaBb, P(aabb) = 1/4 × 1/4 = 1/16.
What grade is this dihybrid cross worksheet for?
It is written for Grade 11 biology (the Genetic Processes unit) and starts with monohybrid crosses, so it also works for Grade 10 extension.
More Free Grade 11 Biology Resources
Ready to start? Download the free dihybrid cross worksheet PDF (use the Download button above), print it double-sided, and keep the answer key pages for marking.