This free codominance and incomplete dominance worksheet covers the inheritance patterns that don't follow Mendel's simple dominant-recessive rules. Students first name the pattern behind six real examples, then complete Punnett squares for pink snapdragons and roan cattle.
The second half moves on to ABO blood types, with two paternity-style questions, and X-linked traits such as colour blindness and haemophilia. The answer key shows every completed square with correct allele notation.
- Grade 11 Biology
- 23 marks
- 40–50 minutes
- Full answer key
What's in This Codominance and Incomplete Dominance Worksheet
| Part | Skill practised | Marks |
|---|
| A | Name the inheritance pattern (6 scenarios) | 3 |
| B | Incomplete dominance crosses (snapdragons) | 4 |
| C | Codominance crosses (roan cattle) | 4 |
| D | Blood types and multiple alleles | 6 |
| E | Sex-linked inheritance | 4 |
| F | Spot the mistake | 2 |
It supports the Ontario Grade 11 biology curriculum expectations in the Genetic Processes unit. Students should be comfortable with basic Punnett squares first; the dihybrid cross worksheet covers those.
Incomplete Dominance: The Blend
In incomplete dominance, neither allele is fully dominant, so the heterozygote shows an intermediate phenotype. Snapdragons are the classic example: CRCR is red, CWCW is white and CRCW is pink. Crossing two pink flowers gives:
That gives 1 red : 2 pink : 1 white. Notice that the phenotype ratio matches the genotype ratio, because every genotype looks different. Red and white flowers come back in the next generation, which proves the alleles never blended.
Codominance: Both Show
In codominance, both alleles are fully expressed side by side. A roan cow (CRCW) has red hairs and white hairs, not pink ones. Crossing two roan cattle gives:
That gives 1 red : 2 roan : 1 white. The ratio looks just like incomplete dominance, so on any codominance and incomplete dominance worksheet the question to ask is: does the heterozygote look like a mix (codominance) or a blend (incomplete dominance)? For more background, see dominance in genetics on Wikipedia.
Blood Types and Multiple Alleles
Human ABO blood type is controlled by three alleles. IA and IB are codominant with each other, and both are dominant to i.
| Blood type | Possible genotypes |
|---|
| Type A | IAIA or IAi |
| Type B | IBIB or IBi |
| Type AB | IAIB |
| Type O | ii |
Two parents with types A and B can have a child of any blood type if both are heterozygous:
Result: 1 AB : 1 A : 1 B : 1 O. Part D of the codominance and incomplete dominance worksheet uses this logic for two paternity questions. For how blood itself works, see the circulatory system quiz.
Sex-Linked Inheritance
Genes on the X chromosome are sex-linked. A carrier mother (XBXb) and a father with normal vision (XBY) give this cross for colour blindness:
Each son has a 1/2 chance of colour blindness, while no daughters are colour-blind and half are carriers. That is why X-linked recessive disorders appear far more often in males.
Codominance and Incomplete Dominance Worksheet Questions
These questions match the codominance and incomplete dominance worksheet PDF, which includes blank Punnett squares and ruled answer lines.
Part A – Name the Inheritance Pattern
- A red snapdragon is crossed with a white snapdragon. All offspring are pink.
- A red cow and a white bull have a roan calf with both red hairs and white hairs.
- A tall pea plant crossed with a short pea plant gives only tall offspring.
- Parents with type A and type B blood have a child with type AB blood.
- Two parents with normal colour vision have a colour-blind son.
- A black chicken and a white chicken produce chicks with black and white speckled feathers.
Parts B and C – Punnett Squares
- Snapdragon flower colour shows incomplete dominance: CRCR = red, CRCW = pink, CWCW = white. Cross two pink snapdragons.
- Cross a pink snapdragon with a white snapdragon.
- Coat colour in cattle is codominant: CRCR = red, CRCW = roan, CWCW = white. Cross two roan cattle.
- Cross a roan bull with a red cow.
Part D – Blood Types
- Give the possible genotype(s) for blood types A, B, AB and O.
- A woman with genotype IAi has children with a man who is IBi. Complete the Punnett square and give the blood type ratio. (2 marks)
- A type A mother and a type B father have a type O child. What are the genotypes of both parents? Explain.
- Could a man with type AB blood be the father of a type O child? Explain.
Part E – Sex-Linked Inheritance
- A carrier woman (XBXb) and a man with normal vision (XBY) have children. What is the chance that a son is colour-blind?
- Explain why X-linked recessive disorders such as colour blindness are more common in males than in females.
- A man with haemophilia (XhY) has children with a woman who is not a carrier (XHXH). What fraction of their daughters will be carriers, and what fraction of their sons will have haemophilia?
Part F – Spot the Mistake
- A student says: “Pink snapdragons prove that the red and white alleles blend together permanently.” Explain the error and correct it.
- A student says: “A roan cow has pink hairs because red and white are codominant.” Explain the error and correct it.
Codominance and Incomplete Dominance Worksheet Answer Key
Open each part of the codominance and incomplete dominance worksheet answers to check your work. The printable key with completed squares is at the end of the PDF.
Part A answers (Q1–6)
- Incomplete dominance
- Codominance
- Complete dominance
- Codominance (multiple alleles)
- Sex-linked (X-linked recessive)
- Codominance
Parts B and C answers (Q7–10)
- CRCW × CRCW: 1 red : 2 pink : 1 white
- CRCW × CWCW: 1 pink : 1 white
- CRCW × CRCW: 1 red : 2 roan : 1 white
- CRCW × CRCR: 1 red : 1 roan
Part D answers (Q11–14)
- Type A: IAIA or IAi; Type B: IBIB or IBi; Type AB: IAIB; Type O: ii
- IAi × IBi: 1 AB : 1 A : 1 B : 1 O
- Mother IAi and father IBi. The type O child is ii, so each parent must carry an i allele.
- No. An AB man (IAIB) always passes on IA or IB, so his child cannot be ii.
Part E answers (Q15–17)
- XBXb × XBY: 1 normal daughter : 1 carrier daughter : 1 unaffected son : 1 affected son. Chance a son is colour-blind: 1/2.
- Males (XY) have only one X chromosome, so one recessive allele causes the trait. Females need two copies, one on each X.
- All daughters are carriers (XHXh); no sons have haemophilia (they get Y from the father).
Part F answers (Q18–19)
- Crossing two pink flowers gives red, pink and white offspring again, so the alleles stay separate. Only the phenotype is intermediate.
- In codominance both phenotypes appear separately: a roan cow has a mix of red hairs and white hairs, not pink ones.
4 Common Mistakes With Non-Mendelian Inheritance
- Mixing up the two patterns. Blend = incomplete dominance; both visible = codominance.
- Using capital and lowercase letters. For codominance and incomplete dominance, use superscripts such as CR and CW, since neither allele is recessive.
- Forgetting the Y chromosome. In sex-linked crosses, fathers pass their X to daughters and their Y to sons.
- Assuming type A means IAIA. A type A person can be IAi, which is how two A parents can have a type O child.
How to Use This Codominance and Incomplete Dominance Worksheet
- Lesson 1: Parts A–C on the codominance and incomplete dominance worksheet, ideally with photos of snapdragons and roan cattle.
- Lesson 2: Parts D–F on blood types and sex-linked traits.
- Extension: Have students design their own pedigree for a family with colour blindness.
Test understanding with the non-Mendelian genetics quiz, then look at allele frequencies in populations with the Hardy-Weinberg quiz.
Frequently Asked Questions
Is this codominance and incomplete dominance worksheet free?
Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.
What is the difference between codominance and incomplete dominance?
In incomplete dominance the heterozygote is a blend in between the two parents, such as pink snapdragons from red and white parents. In codominance both traits show fully and separately, such as roan cattle with red and white hairs.
Is blood type an example of codominance?
Yes. The IA and IB alleles are codominant, so a person with both has type AB blood. Blood type also shows multiple alleles, because there are three alleles (IA, IB and i) in the population.
Why are sex-linked traits more common in males?
Males have one X chromosome and one Y. A single recessive allele on the X is enough to cause the trait in a male, while a female needs two copies.
What grade is this codominance and incomplete dominance worksheet for?
It is written for Grade 11 biology (the Genetic Processes unit) and also works as review for Grade 12 biology.
More Free Grade 11 Biology Resources
Ready to start? Download the free codominance and incomplete dominance worksheet PDF (use the Download button above), print it double-sided, and keep the answer key pages for marking.