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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Codominance
Inheritance isn't always a simple case of dominant and recessive alleles. Sometimes, both alleles of a gene are fully expressed in the phenotype of heterozygous organisms. This pattern of inheritance is called codominance and it's a fascinating aspect of genetics that helps explain some of the variation we see in living things.
Key Definitions:
Complete Dominance: One allele (dominant) completely masks the other (recessive) in heterozygotes.
Incomplete Dominance: Heterozygotes show a phenotype that is intermediate between the two homozygotes.
Codominance: Both alleles are fully expressed in heterozygotes, resulting in a phenotype that shows characteristics of both alleles.
You can identify codominance when:
To really understand codominance, it helps to compare it with other inheritance patterns. Let's look at how codominance differs from complete dominance and incomplete dominance using flower colour as an example:
Red (R) is dominant to white (r)
RR = Red flowers
Rr = Red flowers
rr = White flowers
The heterozygote looks exactly like the dominant homozygote.
Red (R) and white (W) alleles
RR = Red flowers
RW = Pink flowers
WW = White flowers
The heterozygote shows a blend or intermediate phenotype.
Red (R) and white (W) alleles
RR = Red flowers
RW = Red AND white patches
WW = White flowers
The heterozygote shows both phenotypes distinctly.
One of the best examples of codominance in humans is the ABO blood group system. This system is controlled by a single gene with three alleles: IA, IB and i.
The IA and IB alleles are codominant to each other, but both are dominant to the i allele.
| Genotype | Blood Type (Phenotype) | Antigens Present |
|---|---|---|
| IAIA or IAi | A | A only |
| IBIB or IBi | B | B only |
| IAIB | AB | Both A and B |
| ii | O | Neither A nor B |
In people with blood type AB (genotype IAIB), both the A and B antigens are expressed on the surface of red blood cells. This is a perfect example of codominance - neither allele is masked and both are fully expressed.
Another classic example of codominance can be seen in the coat colours of certain animals, particularly in cattle breeds like the Shorthorn.
In Shorthorn cattle, the allele for red coat (R) and the allele for white coat (W) are codominant. When a red bull (RR) is crossed with a white cow (WW), all the offspring are roan (RW) - having both red and white hairs mixed together. The red and white hairs remain distinct; they don't blend to make pink cattle!
This is codominance in action - both the red and white phenotypes are expressed in the heterozygote, creating a distinctive roan pattern.
When working with codominant alleles, we can use Punnett squares to predict the outcomes of genetic crosses, just as we would with other inheritance patterns.
Let's consider a cross between a man with blood type A (genotype IAi) and a woman with blood type B (genotype IBi).
Step 1: Identify the possible gametes from each parent.
Man (IAi): IA or i
Woman (IBi): IB or i
Step 2: Set up a Punnett square.
| IA | i | |
|---|---|---|
| IB | IAIB | IBi |
| i | IAi | ii |
Step 3: Determine the phenotypes.
IAIB = Blood type AB (codominance)
IBi = Blood type B
IAi = Blood type A
ii = Blood type O
Step 4: Calculate the probabilities.
There is a 25% chance of having a child with each of the four blood types: A, B, AB and O.
When tackling codominance problems in your exams, follow these steps:
In exam questions, you might be asked to:
Remember that in codominance, both alleles are fully expressed in the heterozygote - neither is dominant over the other.
Understanding codominance helps us appreciate the complexity of inheritance patterns and explains some of the variation we see in living organisms. It's an important concept in genetics that builds on your knowledge of basic Mendelian inheritance.
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