Codominance and multiple alleles

Biology· 17.3 Inheritance· 15 min read

1. Codominance★★☆☆☆⏱ 5 min

📘 Definition

Codominance

A pattern of inheritance where both alleles are fully and simultaneously expressed in the phenotype of heterozygous individuals. Neither allele is dominant or recessive to the other.

Example:

Roan coat colour in cattle, sickle cell trait in humans, AB blood group

Codominance differs from complete dominance, where only the dominant allele is visible in heterozygotes, and incomplete dominance where heterozygotes show a blended intermediate phenotype. For codominant alleles, we usually use superscript notation to show both alleles are equally expressed.

📐 Worked Example

In cattle, coat colour is controlled by codominant alleles: $C^{R}$ codes for red coat, $C^{W}$ codes for white coat. Heterozygotes have roan (red and white spotted) coats. Cross a homozygous red cow with a roan bull. What is the expected genotypic and phenotypic ratio of offspring?

1
1. Identify parental genotypes using correct notation:
2
$\text{Homozygous red cow} = C^R C^R \\ \text{Roan bull (heterozygous)} = C^R C^W$
3
1. Identify gametes produced by each parent during meiosis:
4
- The homozygous cow only produces gametes carrying the $C^{R}$ allele
- The heterozygous bull produces 50% $C^{R}$ gametes and 50% $C^{W}$ gametes
5
1. Complete a Punnett square to find all possible offspring combinations:
6
Gamete: $C^{R}$ (cow) Gamete: $C^{R}$ (cow)
Gamete: $C^{R}$ (bull) $C^{R} C^{R}$ $C^{R} C^{R}$
Gamete: $C^{W}$ (bull) $C^{R} C^{W}$ $C^{R} C^{W}$
7
1. Summarize the resulting ratios:
8
- Genotypic ratio: $1 C^{R} C^{R} : 1 C^{R} C^{W}$
- Phenotypic ratio: 1 red coat : 1 roan coat

	Gamete: $C^{R}$ (cow)	Gamete: $C^{R}$ (cow)
Gamete: $C^{R}$ (bull)	$C^{R} C^{R}$	$C^{R} C^{R}$
Gamete: $C^{W}$ (bull)	$C^{R} C^{W}$	$C^{R} C^{W}$

Exam tip:

Always use superscript notation for codominant alleles in your exam answers to avoid losing marks for incorrect notation.

2. Multiple Alleles★★★☆☆⏱ 6 min

📘 Definition

Multiple Alleles

When a single gene has more than two different alleles present in a population. Diploid individuals only carry two alleles (one from each parent) regardless of how many alleles exist in the wider population.

Example:

ABO blood group system in humans

The most common example of multiple alleles in humans is the ABO blood group gene, which has three alleles: $I^{A}$ , $I^{B}$ , and $i$ . $I^{A}$ codes for the A antigen, $I^{B}$ codes for the B antigen, and $i$ codes for no antigen. $I^{A}$ and $I^{B}$ are codominant to each other, and both are completely dominant to $i$ .

📐 Worked Example

A man has blood group A, and a woman has blood group B. They have a first child with blood group O. What is the probability their second child will have blood group AB?

1
1. Recall ABO allele rules and possible genotypes:
2
- Blood group A: $I^{A} I^{A}$ or $I^{A} i$
- Blood group B: $I^{B} I^{B}$ or $I^{B} i$
- Blood group O: only $ii$ (homozygous recessive)
3
1. Deduce parental genotypes from the child's blood group:
4
The child is $ii$ , so they inherited one $i$ allele from each parent. This means both parents must carry the $i$ allele. So father = $I^{A} i$ , mother = $I^{B} i$ .
5
1. Identify gametes and complete the Punnett square:
6
$I^{A}$ (father) $i$ (father)
$I^{B}$ (mother) $I^{A} I^{B}$ (AB) $I^{B} i$ (B)
$i$ (mother) $I^{A} i$ (A) $ii$ (O)
7
1. Calculate probability: Only 1 out of 4 possible offspring genotypes is $I^{A} I^{B}$ (blood group AB). So the probability is 25% or $\frac{1}{4}$ .

	$I^{A}$ (father)	$i$ (father)
$I^{B}$ (mother)	$I^{A} I^{B}$ (AB)	$I^{B} i$ (B)
$i$ (mother)	$I^{A} i$ (A)	$ii$ (O)

3. Problem Solving Method★★★☆☆⏱ 4 min

Identify all alleles for the gene, and note all dominance/codominance relationships
Use correct notation (usually superscripts for codominant/multiple alleles)
Deduce parental genotypes from given phenotypes: use offspring phenotypes to rule out impossible genotypes
Construct a Punnett square to show all possible offspring combinations
Calculate the requested ratio or probability, showing all working

✓ Quick check

Test your understanding with this quick question:

A person with blood group AB has a child with a person with blood group O. What is the probability the child has blood group A?
- 0%
- 25%
- 50%
- 100%
Reveal answer
50% —
Correct! The AB parent is $I^{A} I^{B}$ , and the O parent is homozygous $ii$ . Half the offspring will inherit $I^{A}$ from the AB parent and $i$ from the O parent, giving genotype $I^{A} i$ (blood group A).

Exam tip:

Always show your working (including the Punnett square) for genetic cross questions. Examiner's award marks for correct method even if your final answer has a small error.

4. Common Pitfalls

Wrong move:

Confusing codominance with incomplete dominance

Why:

Both produce different heterozygote phenotypes, so they are often mixed up, but they have distinct expression patterns

Correct move:

Remember: codominance = both traits fully expressed; incomplete dominance = blended intermediate phenotype

Wrong move:

Using lowercase notation for codominant alleles, implying one is recessive

Why:

Incorrect notation suggests you do not understand codominance, and examiners will deduct marks

Correct move:

Use different capital letters or superscripts for all codominant alleles, e.g. $C^{R}$ and $C^{W}$ , not $R$ and $w$

Wrong move:

Assuming an individual can carry more than two alleles for a gene when multiple alleles exist

Why:

Multiple alleles refers to variation across the whole population, not per individual

Correct move:

Remember: diploid organisms always carry exactly two alleles per gene, regardless of how many alleles exist in the population

Wrong move:

Failing to use offspring phenotypes to deduce unknown parental genotypes

Why:

Many problems give ambiguous parental phenotypes (e.g. blood group A can be two genotypes) and require you to work backwards

Correct move:

If you have information about a child's phenotype, always use this to confirm the parental genotype before calculating ratios

5. Quick Reference Cheatsheet

Concept	Key Features	Example
Codominance	Both alleles fully expressed in heterozygotes	Roan cattle, AB blood group
Multiple alleles	2 alleles in population; 2 per individual	ABO blood group system
Incomplete dominance	Blended intermediate phenotype	Pink snapdragons
ABO genotypes	$I^{A}, I^{B}$ codominant, both dominant to $i$	A: $I^{A} I^{A} / I^{A} i$ , B: $I^{B} I^{B} / I^{B} i$ , AB: $I^{A} I^{B}$ , O: $ii$

6. Frequently Asked

What is the difference between codominance and incomplete dominance?

Codominance means both alleles are fully expressed simultaneously (e.g. AB blood group has both A and B antigens on red blood cells). Incomplete dominance produces a blended intermediate phenotype in heterozygotes (e.g. pink snapdragon flowers from red and white parents).

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 22
ABO blood group genetic cross
2021 · 12
Codominance in cattle coat colour
2023 · 23
Multiple allele inheritance problem

Going deeper

syllabusOfficial CIE 9700 SyllabusTopic reference page 48

What's Next

Codominance and multiple alleles are core concepts that underpin all more complex patterns of inheritance in CIE A-Level Biology. These topics are regularly tested in both multiple choice and extended response questions, often combined with pedigree analysis and sex linkage. Mastering the method for solving genetic crosses here will give you a strong foundation for tackling harder inheritance problems that combine multiple concepts. You are now ready to learn about other non-Mendelian patterns of inheritance.