Genetics Probability Worksheet with Answers (Grade 11 Biology)

This free genetics probability worksheet teaches students to solve genetic crosses with probability instead of drawing ever-bigger Punnett squares. It starts with simple coin-and-gamete questions and builds through the product rule, the sum rule and crosses with three or four genes.

Students then apply probability to real families, such as the chance that at least one of three children inherits cystic fibrosis, and finish with a chi-square test on pea plant data. Every answer is given as an exact fraction and a percentage.

• Grade 11 Biology
• 24 marks
• 40–50 minutes
• Full answer key

What's in This Genetics Probability Worksheet

Part	Skill practised	Marks
A	Probability basics	3
B	The product rule (AND)	4
C	The sum rule (OR)	3
D	Crosses with three or four genes, without a Punnett square	4
E	Probability in families (multiple children)	4
F	Challenge: chi-square test	4
G	Spot the mistake	2

It supports the Ontario Grade 11 biology curriculum expectations in the Genetic Processes unit. Students should know basic Punnett squares first; the dihybrid cross worksheet covers them.

The Product Rule and the Sum Rule

Two rules solve almost every question on this genetics probability worksheet:

• Product rule (AND): multiply the probabilities of independent events. For AaBb × AaBb, P(AABB) = 1/4 × 1/4 = 1/16.
• Sum rule (OR): add the probabilities of outcomes that can't happen together. For Aa × Aa, P(dominant phenotype) = P(AA) + P(Aa) = 1/4 + 1/2 = 3/4.

A quick check: the word "and" usually means multiply, and "or" usually means add. If your answer is bigger than 1, you added when you should have multiplied.

Solving Crosses Without a Punnett Square

A trihybrid cross (AaBbCc × AaBbCc) would need a 64-box Punnett square. Instead, split it into three monohybrid crosses and multiply:

• P(A_B_C_) = 3/4 × 3/4 × 3/4 = 27/64
• P(aabbcc) = 1/4 × 1/4 × 1/4 = 1/64
• An AaBbCc individual makes 2³ = 8 kinds of gametes.

This works because genes on different chromosomes sort independently in meiosis. The meiosis worksheet shows why.

Probability in Families

For two carrier parents (Ff × Ff), each child has a 1/4 chance of cystic fibrosis, independent of their brothers and sisters. For a family with three children:

• All three unaffected: (3/4)³ = 27/64
• At least one affected: 1 − 27/64 = 37/64 (the complement rule)
• Exactly one affected: 3 × 1/4 × 3/4 × 3/4 = 27/64, because the affected child could be first, second or third

The "at least one" trick, subtracting the chance of none from 1, saves a lot of work on any genetics probability worksheet.

The Chi-Square Test

Real offspring counts never match expected ratios exactly. A chi-square test checks whether the difference is small enough to be chance. In Part F, a student counts 160 offspring from RrYy × RrYy:

Phenotype	Observed	Expected	(O − E)² ÷ E
Round yellow	95	90	0.28
Round green	28	30	0.13
Wrinkled yellow	27	30	0.30
Wrinkled green	10	10	0.00

χ² = 0.71. With 3 degrees of freedom the critical value is 7.81, so the results support the 9 : 3 : 3 : 1 ratio. Learn more about the chi-squared test on Wikipedia.

Genetics Probability Worksheet Practice Questions

These questions match the genetics probability worksheet PDF, which has ruled working space and the chi-square table.

Part A – Probability Basics

1. An Aa parent makes gametes. What is the probability that a gamete carries the a allele?
2. What is the probability that a baby is a boy?
3. Two coins are tossed. What is the probability of two heads? (Two Aa parents each passing on a works the same way.)

Part B – The Product Rule

4. Aa × Aa: what is the probability of an aa offspring?
5. AaBb × AaBb: what is the probability of an AABB offspring?
6. AaBbCc × AaBbCc: what is the probability of an aabbcc offspring?
7. AaBb × aabb: what is the probability of an Aabb offspring?

Part C – The Sum Rule

8. Aa × Aa: what is the probability of a heterozygous offspring?
9. Aa × Aa: what is the probability of an offspring with the dominant phenotype?
10. AaBb × AaBb: what is the probability that an offspring shows exactly one recessive trait?

Part D – Crosses Without a Punnett Square

11. AaBbCc × AaBbCc: what is the probability of an offspring with all three dominant traits (A_B_C_)?
12. AaBbCc × aaBbcc: what is the probability of an aaB_cc offspring?
13. AaBbCcDd × AaBbCcDd: what is the probability of an AABBCCDD offspring?
14. How many different kinds of gametes can an AaBbCc individual make?

Part E – Probability in Families

15. Cystic fibrosis is recessive. Two carrier parents (Ff × Ff) plan to have children. What is the probability that their first two children both have cystic fibrosis?
16. What is the probability that all three of their children are unaffected?
17. What is the probability that at least one of three children has cystic fibrosis?
18. What is the probability that exactly one of three children has cystic fibrosis?

Part F – Chi-Square Test

19. Calculate the expected number of offspring in each group and record it in the table.
20. Complete the (O − E)² ÷ E column.
21. Calculate χ² (the sum of the last column).
22. There are 3 degrees of freedom. Do the results support a 9 : 3 : 3 : 1 ratio? Explain.

Part G – Spot the Mistake

23. A student says: “A carrier couple has three children without albinism, so their fourth child is certain to have it.” Explain the error and correct it.
24. A student calculates P(A_B_) for AaBb × AaBb as 3/4 + 3/4. Explain the error and correct it.

Genetics Probability Worksheet Answer Key

Open each part of the genetics probability worksheet answers to check your work. The printable key is at the end of the PDF.

Part A answers (Q1–3)

1. Each gamete gets one allele: P(a) = 1/2 (50%)
2. The father’s sperm carries X or Y: P(boy) = 1/2 (50%)
3. 1/2 × 1/2 = 1/4 (25%)

Part B answers (Q4–7)

4. 1/2 × 1/2 = 1/4 (25%)
5. P(AA) × P(BB) = 1/4 × 1/4 = 1/16 (6.25%)
6. 1/4 × 1/4 × 1/4 = 1/64 (1.56%)
7. P(Aa) × P(bb) = 1/2 × 1/2 = 1/4 (25%)

Part C answers (Q8–10)

8. Aa can form two ways (A from mother, a from father or the reverse): 1/4 + 1/4 = 1/2 (50%)
9. P(AA) + P(Aa) = 1/4 + 1/2 = 3/4 (75%)
10. P(A_bb) + P(aaB_) = 3/16 + 3/16 = 3/8 (37.5%)

Part D answers (Q11–14)

11. 3/4 × 3/4 × 3/4 = 27/64 (42.19%)
12. P(aa) × P(B_) × P(cc) = 1/2 × 3/4 × 1/2 = 3/16 (18.75%)
13. (1/4)⁴ = 1/256 (0.39%)
14. 2³ = 8 kinds

Part E answers (Q15–18)

15. 1/4 × 1/4 = 1/16 (6.25%)
16. 3/4 × 3/4 × 3/4 = 27/64 (42.19%)
17. Use the complement: 1 − 27/64 = 37/64 (57.81%)
18. The affected child could be 1st, 2nd or 3rd: 3 × (1/4 × 3/4 × 3/4) = 27/64 (42.19%)

Part F answers (Q19–22)

19. 90, 30, 30 and 10 (160 × 9/16, 3/16, 3/16, 1/16)
20. 0.28, 0.13, 0.30, 0.00
21. χ² = 0.71
22. Yes. 0.71 is less than the critical value 7.81, so the differences are due to chance and the 9 : 3 : 3 : 1 hypothesis is accepted.

Part G answers (Q23–24)

23. Each child is an independent event. The fourth child still has a 1/4 chance of albinism.
24. “A and B” needs the product rule: 3/4 × 3/4 = 9/16. (Adding gives 1.5, which is impossible.)

5 Common Genetics Probability Mistakes

1. Adding instead of multiplying. "A and B" means multiply; answers above 1 are a giveaway.
2. Forgetting heterozygotes form two ways. P(Aa) from Aa × Aa is 1/2, not 1/4.
3. The gambler's fallacy. Previous children don't change the odds for the next one.
4. Missing the order in "exactly one". Multiply by the number of ways it can happen.
5. Reading the chi-square result backwards. A χ² below the critical value supports the expected ratio.

How to Use This Genetics Probability Worksheet in Class

• Warm-up: Toss two coins 20 times as a class to show how real results drift from 1/4, 1/2, 1/4.
• Main lesson: Parts A–E of the genetics probability worksheet.
• Extension: Part F, then have students run a chi-square test on their own coin data.

To see the same maths applied to whole populations, try the Hardy-Weinberg quiz. For blood types and sex-linked probability, use the codominance and incomplete dominance worksheet.

Frequently Asked Questions

Is this genetics probability worksheet free?

Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.

When do you multiply probabilities in genetics?

Multiply when you need two independent events to happen together (an AND question), such as an offspring being aa AND bb. For AaBb × AaBb, P(aabb) = 1/4 × 1/4 = 1/16.

When do you add probabilities in genetics?

Add when an outcome can happen in more than one way that can't happen at the same time (an OR question). For Aa × Aa, P(dominant phenotype) = P(AA) + P(Aa) = 1/4 + 1/2 = 3/4.

Why doesn't a 1 in 4 chance mean one of every four children?

Each fertilization is an independent event. A carrier couple has a 1/4 chance of an affected child every time, no matter how many unaffected children they already have.

What does a chi-square test show in genetics?

It checks whether the difference between observed and expected offspring counts is small enough to be due to chance. If χ² is below the critical value, the expected ratio is supported.

More Free Grade 11 Biology Resources

• Dihybrid Cross WorksheetPunnett squares
• Codominance and Incomplete Dominance WorksheetNon-Mendelian genetics
• Meiosis WorksheetStages and nondisjunction
• Non-Mendelian Genetics QuizGenetic processes
• All Grade 11 ResourcesWorksheets and quizzes

Ready to start? Download the free genetics probability worksheet PDF (use the Download button above), print it double-sided, and keep the answer key pages for marking.