Non-Mendelian Patterns — Co-dominance, Incomplete Dominance, Multiple Alleles
Not every inheritance pattern fits a simple dominant-recessive model. Some traits show both alleles together, some show an intermediate phenotype, and some genes have more than two alleles in the population.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A student says, "If red flower colour is dominant and white flower colour is recessive, then crossing a red flower with a white flower should always give red offspring. So pink flowers prove the experiment went wrong."
Before reading on, explain why that reasoning is weak. What inheritance pattern could produce pink offspring, and why does that not mean the alleles have disappeared or blended permanently?
Know
- What co-dominance, incomplete dominance and multiple alleles mean.
- That ABO blood groups are the standard HSC example of co-dominance plus multiple alleles.
Understand
- Why some phenotype ratios are not the classic 3:1 Mendelian ratio.
- Why intermediate phenotype does not mean the alleles have merged permanently.
Can Do
- Interpret Punnett squares for non-Mendelian patterns.
- Distinguish co-dominance from incomplete dominance in examples and data.
Core Content
Beyond simple dominance · expression patterns
Simple dominant-recessive inheritance is only one inheritance pattern. The effect of two alleles in a heterozygote depends on how the gene product is expressed.
In Lesson 14, the heterozygous genotype often showed the dominant phenotype. In this lesson, the heterozygous phenotype may be intermediate, or it may show both allele products together. That is why some crosses do not produce the expected 3:1 phenotypic ratio.
Simple Mendelian
One allele masks the other in the heterozygote, often producing a 3:1 phenotype ratio.
Incomplete dominance
The heterozygote shows an intermediate phenotype, often producing a 1:2:1 phenotype ratio.
Co-dominance
Both alleles are expressed in the heterozygote, so the phenotype includes both.
What to write in your book
- The heterozygote's appearance depends on how the gene product is expressed.
- Simple Mendelian → often 3:1; incomplete dominance → often 1:2:1.
- Co-dominance → both products shown together.
- Not every cross gives 3:1.
When the heterozygote shows an intermediate phenotype, the pattern is called _____ dominance.
Intermediate phenotype · alleles still segregate
In incomplete dominance, neither allele is fully dominant in the heterozygote. A common example is red-flowered and white-flowered snapdragons producing pink heterozygous offspring.
In incomplete dominance, the 1:2:1 genotype ratio often matches the 1:2:1 phenotype ratio.
The key idea is that the alleles still segregate normally in meiosis. They have not permanently mixed into one new allele. The intermediate phenotype appears because of the expression pattern in the heterozygote.
What to write in your book
- Incomplete dominance: neither allele fully dominant; heterozygote intermediate (e.g. pink).
- Genotype ratio 1:2:1 = phenotype ratio 1:2:1 (red:pink:white).
- Alleles still segregate normally in meiosis.
- They have NOT permanently merged into a new allele.
In incomplete dominance, the two alleles have permanently fused into one new allele.
In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes.
Co-dominance means one allele is completely dominant over the other in the heterozygote.
Both alleles visible · the ABO example
In co-dominance, the heterozygous phenotype shows both allele products. Neither allele hides the other, and the phenotype is not simply halfway between them.
Co-dominance
- Both alleles are expressed in the heterozygote.
- The phenotype shows both forms together.
- AB blood type is the standard human example.
Incomplete dominance
- The heterozygote has an intermediate phenotype.
- It does not show both separate products clearly together.
- Pink flowers are the standard school example.
What to write in your book
- Co-dominance: both allele products expressed; neither hides the other; not halfway.
- Standard example: AB blood type (IAIB).
- Contrast with incomplete dominance (intermediate, e.g. pink).
- IA and IB are co-dominant; both antigens present.
Which genotype gives blood group AB in the ABO system?
Population view · still two per individual
Each individual still has only two alleles for an autosomal gene, one inherited from each parent. However, the gene may exist in more than two forms across the population. This is called multiple alleles.
ABO alleles
The ABO blood group has three common alleles: IA, IB and i.
Dominance relationships
IA and IB are co-dominant with each other, and both are dominant over i.
Possible blood groups
Phenotypes are A, B, AB and O, depending on which two alleles the individual has.
ABO shows both multiple alleles in the population and co-dominance between IA and IB.
What to write in your book
- Each individual has only two alleles, but the gene can have more than two forms in the population.
- ABO: three common alleles — IA, IB, i.
- IA and IB co-dominant; both dominant over i.
- Phenotypes: A, B, AB, O.
A gene with three common allele forms in a population is an example of:
Interpretation · read the phenotype data
If the heterozygote shows the dominant phenotype, a monohybrid cross often gives a 3:1 phenotype ratio. If the heterozygote is intermediate, the same cross may give a 1:2:1 phenotype ratio. That is why phenotype data matters in inheritance questions.
When you see 3:1
- Think simple dominant-recessive inheritance.
- The heterozygote likely matches one homozygote phenotypically.
When you see 1:2:1
- Consider incomplete dominance or co-dominance.
- The heterozygote likely has its own distinct phenotype.
What to write in your book
- 3:1 ratio → simple dominant-recessive.
- 1:2:1 ratio → incomplete dominance or co-dominance (distinct heterozygote).
- Phenotype data tells you the inheritance pattern.
- Don't assume 3:1 every time — check the heterozygote first.
Activities
Flower Colour Cross
In a plant species, red flowers and white flowers show incomplete dominance. Heterozygous plants are pink. Cross two pink plants. Write the Punnett square, then state the genotype and phenotype ratios.
ABO Reasoning
One parent has genotype IAi and the other has genotype IBi. List all possible offspring genotypes and identify the possible blood group phenotypes. Explain why this example shows both multiple alleles and co-dominance.
Co-dominance
- Both alleles are fully expressed in the heterozygous phenotype, such as IAIB giving blood group AB.
Incomplete dominance
- The heterozygous phenotype is intermediate between the two homozygous phenotypes, such as red and white flowers producing pink offspring.
Multiple alleles
- More than two allele forms exist in the population, even though each individual still has only two alleles for the gene.
Ratios
- Non-Mendelian inheritance patterns often produce phenotype ratios other than 3:1 because the heterozygote has its own distinct phenotype.
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ApplyBand 4(3 marks) 1. Explain the difference between co-dominance and incomplete dominance using one example for each.
ApplyBand 5(4 marks) 2. Two pink flowers are crossed in a species showing incomplete dominance. Red flowers are CRCR, white flowers are CWCW, and pink flowers are CRCW. Determine the genotype ratio and phenotype ratio of the offspring.
AnalyseBand 5–6(5 marks) 3. In the ABO blood group system, one parent has genotype IAi and the other has genotype IBi. Use Punnett square reasoning to identify all possible offspring genotypes and phenotypes, and explain why this system demonstrates both co-dominance and multiple alleles.
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Multiple choice
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Short Answer 1
Co-dominance means both alleles are fully expressed in the heterozygote, such as IAIB producing blood group AB. Incomplete dominance means the heterozygote shows an intermediate phenotype, such as pink flowers from a red and white flower cross.
Short Answer 2
The cross is CRCW × CRCW. Each parent produces gametes CR and CW. Offspring genotypes are CRCR, CRCW, CRCW and CWCW. Genotype ratio = 1 CRCR : 2 CRCW : 1 CWCW. Phenotype ratio = 1 red : 2 pink : 1 white.
Short Answer 3
The first parent produces gametes IA and i. The second parent produces gametes IB and i. Possible offspring genotypes are IAIB, IAi, IBi and ii. Their phenotypes are AB, A, B and O. This shows co-dominance because IA and IB are both expressed together in blood group AB. It shows multiple alleles because the ABO gene exists as three common alleles in the population: IA, IB and i.
Co-dominance
The heterozygote expresses both allele products, as in blood group AB.
Incomplete dominance
The heterozygote has an intermediate phenotype, as in pink flowers from red and white parents.
Multiple alleles
A gene can have more than two allele forms in the population even though each individual still carries only two alleles.
Rapid-fire questions on co-dominance, incomplete dominance, multiple alleles and the ABO system. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Return to the pink-flower claim from the start of the lesson. Rewrite it using the correct inheritance language.