Co-dominance and multiple alleles
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of key inheritance terms and monohybrid crosses. You can test your knowledge on these below.
What is an allele?
An allele is a version of a gene.
What is meant by homozygous and heterozygous?
- Homozygous: both alleles are the same (e.g., AA or aa)
- Heterozygous: alleles are different (e.g., Aa)
What is meant by phenotype and genotype?
- Genotype: the alleles an organism has for a particular gene or genes.
- Phenotype: the observable characteristics of an organism, determined by genotype and environment.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains co-dominance and multiple alleles or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Codominance Inheritance
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Codominance occurs when both alleles are expressed equally in the phenotype, neither allele is recessive. The phenotype shows features of both alleles or blending between them.
Example: Flower colour in snapdragons
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Red allele (CR), White allele (CR)
CRCR = Red, CWCW = White, CRCW = Pink (both red and white pigment expressed in the flower)
When writing symbols for co-dominant alleles we use one letter for the gene, such as C for colour, then superscript letters for the different alleles. If they are co-dominant the alleles will both have capital letters.
Multiple Allele Inheritance
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Some genes have more than two alleles in the population (called multiple alleles).
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An individual can only possess two alleles, one from each parent, but more than two alleles may exist in the gene pool.
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Classic example: ABO blood group system.
ABO Blood Group System
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The Immunoglobin gene codes for the expression of antigens on the surface of red blood cells.
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The I gene has three alleles in the gene pool:
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IA (codes for antigen A)
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IB (codes for antigen B)
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IO (codes for no antigen)
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IA and IB are codominant; IO is recessive.
|
Genotype |
Phenotype (Blood type/antigens expressed) |
|
IAIA or IAIO |
A |
|
IBIB or IBIO |
B |
|
IAIB |
AB (codominant) |
|
IOIO |
O |
Key Terms
- Allele: A version of a gene.
- Codominance: Both alleles in a heterozygote are expressed in the phenotype
- Multiple alleles: More than two alleles in the population for a gene
No answer provided.
Exam Tips
Don't confuse dominance with codominance: In codominance, both alleles are expressed equally.
Multiple alleles can be present in the population, but remember, a diploid organism will only have 2 alleles for a gene.
No answer provided.
Punnett Squares & Inheritance Examples
Example 1: Blood Group Inheritance
Parents: IA IO × IB IO
|
IA |
IO |
|
|
IB |
IAIB (AB) |
IB IO (B) |
|
IO |
IA IO (A) |
IOIO (O) |
Offspring Phenotypes:
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25% AB
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25% B
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25% A
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25% O
Example 2: Codominance in Cattle
Parents: CRCR × CRCW
|
CR |
CR |
|
|
CR |
CRCR (Red) |
CRCR (Red) |
|
CW |
CRCW (Pink) |
CRCW (Pink) |
Offspring Phenotypes:
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50% Red
-
50% Pink
In humans, the gene for blood group has three alleles: IA , IB and IO. IA and IB are codominant. IO is recessive. A man has blood group A and a woman has blood group B. Their child has blood group O.
Explain the genotypes of both parents and show how this is possible. (4 marks)
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Blood group O has genotype IOIO.
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The child must inherit one IO allele from each parent.
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Therefore, the man must have genotype IA IO (blood group A).
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The woman must have genotype IB IO (blood group B).
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!