Non-Mendelian Patterns, Co-dominance, Incomplete Dominance, Multiple Alleles
Karl Landsteiner identified human blood groups A, B, and O in 1900, then added the AB group in 1902, the first demonstration of codominance in humans. Before blood typing, approximately 50% of blood transfusions were fatal due to immune reactions. Landsteiner received the Nobel Prize in Physiology or Medicine in 1930. The ABO gene has three alleles (I^A, I^B, and i): I^A and I^B are codominant with each other, and both are dominant over i, producing four possible phenotypes from the three-allele system and demonstrating that both codominance and multiple alleles operate in a single human trait.
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
In 1900, doctors could observe that some blood transfusions killed patients while others did not, but without Karl Landsteiner's systematic blood typing they could not predict which. Landsteiner mixed blood from hospital staff and found that some samples clumped (agglutinated) when combined, while others did not. By 1902 he had identified groups A, B, O, and AB. The AB phenotype was the key anomaly: an individual with the I^A/I^B genotype expressed both the A antigen and the B antigen on their red blood cells simultaneously. Neither allele was silent. This codominance meant the simple 3:1 ratio of Mendelian inheritance did not apply, both allele products were present in the heterozygote.
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.
Simple Mendelian heterozygote → dominant phenotype → 3:1 ratio. Incomplete dominance → intermediate heterozygote → 1:2:1 ratio. Co-dominance → both allele products visible together (not intermediate). Not every cross gives 3:1.
Pause, copy the three heterozygote expression patterns and their expected ratios into your book.
When the heterozygote shows an intermediate phenotype, the pattern is called _____ dominance.
Intermediate phenotype · alleles still segregate
We just saw that incomplete dominance produces an intermediate heterozygote rather than the dominant phenotype. That raises a question: do the alleles permanently blend in incomplete dominance, or do they still segregate normally? This card answers it → the phenotype is intermediate, but the alleles still segregate unchanged in meiosis, they have not fused.
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.
Incomplete dominance: neither allele fully dominant; heterozygote is intermediate (e.g. pink from red × white). Genotype ratio 1:2:1 = phenotype ratio 1:2:1. Alleles still segregate normally in meiosis, they have NOT permanently merged.
Add the highlighted incomplete dominance definition and the important "alleles don't blend" point to your notes.
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.
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
We just saw that incomplete dominance gives an intermediate phenotype but the alleles still segregate normally. That raises a question: what if both alleles' products are fully expressed together in the heterozygote, not blended, not intermediate, but both present? This card answers it → co-dominance, where both allele products are visible simultaneously (e.g. AB blood type).
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.
Co-dominance: both allele products expressed simultaneously in the heterozygote; neither hides the other; phenotype is not intermediate. Standard example: AB blood type (IAIB), both A and B antigens present. Contrast: incomplete dominance = intermediate (pink); co-dominance = both separate products visible.
Pause, write the highlighted co-dominance definition and the AB blood type example into your book.
Which genotype gives blood group AB in the ABO system?
Population view · still two per individual
We just saw that the IA and IB alleles are co-dominant, both antigens appear in the AB blood type. That raises a question: how can a single gene produce four distinct blood groups (A, B, AB, O) if each person only has two alleles? This card answers it → multiple alleles: three allele forms in the population, but each individual still carries only two.
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.
A gene with three common allele forms in a population is an example of:
Interpretation · read the phenotype data
We just saw that multiple alleles can produce four phenotypes from three allele forms. That raises a question: how do we use phenotype ratio data from a cross to work out which inheritance pattern we're dealing with? This card answers it → the observed ratio directly tells you whether simple dominance, incomplete dominance, or co-dominance applies.
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.
3:1 phenotype ratio → simple dominant-recessive (heterozygote = dominant). 1:2:1 phenotype ratio → incomplete dominance or co-dominance (heterozygote has its own distinct phenotype). Read phenotype data first to identify the inheritance pattern before interpreting the Punnett square.
Pause, write the highlighted ratio-to-pattern table into your book.
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.
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
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).
Karl Landsteiner's 1900–1902 discovery that blood groups A, B, O, and AB follow a three-allele system, where I^A and I^B are codominant and both dominant over i, saved millions of lives by making safe transfusion possible, and it also demonstrated the limits of simple Mendelian genetics. The ABO system simultaneously shows codominance (I^A/I^B → blood type AB, not a blend or masked allele) and multiple alleles (three alleles at one locus produce four distinct phenotypes). The snapdragon example, red (R^1R^1) × white (R^2R^2) → pink (R^1R^2), shows incomplete dominance: the heterozygote is intermediate, not blended into a new allele, because half the normal level of pigment enzyme produces a halfway phenotype. Distinguishing these two patterns requires understanding what 'expression' means at the level of the protein product.