The big idea: In simple Mendelian genetics a dominant allele fully masks a recessive one, so the heterozygote looks exactly like the dominant homozygote.
But not every gene works that way. In non-Mendelian inheritance the heterozygote looks different — it shows a blend, or it shows both alleles at once.
This micro covers three of these patterns: incomplete dominance, codominance, and multiple alleles (the ABO blood-group system).
- Incomplete dominance
- Neither allele is fully dominant. The heterozygote shows a NEW, intermediate (blended) phenotype — e.g. red × white flowers → pink.
- Codominance
- Both alleles are fully expressed in the heterozygote at the same time — you can see BOTH phenotypes together (not blended).
- Multiple alleles
- A gene that has more than two possible alleles in the population — e.g. the ABO gene has three (IA, IB and i). Any one individual still carries only two.
- Allele symbols
- For non-Mendelian genes we write both letters as capitals with superscripts (e.g. CR, CW) so neither looks 'dominant'.
Why the notation changes: With simple dominance we write B (dominant) and b (recessive).
With incomplete dominance and codominance neither allele is recessive, so we use two capital letters with superscripts — like C^R (red) and C^W (white). Writing one as a lower-case letter would wrongly suggest it is masked.
Incomplete dominance — the heterozygote blends: In four-o'clock flowers, C^R gives red and C^W gives white. Neither masks the other, so the heterozygote C^R C^W is pink — an in-between blend.
Cross two pink flowers (C^R C^W × C^R C^W) and you get three phenotypes, all visible:
| Gametes | CR (from parent 2) | CW (from parent 2) |
|---|---|---|
| CR (from parent 1) | CR CR — red | CR CW — pink |
| CW (from parent 1) | CR CW — pink | CW CW — white |
Read the grid
- 1 C^R C^R → red
- 2 C^R C^W → pink (the blend)
- 1 C^W C^W → white
- Phenotype ratio = 1 red : 2 pink : 1 white (this is the F2 from a pink F1)
The genotype and phenotype ratios MATCH: In simple dominance the 1 : 2 : 1 genotype ratio collapses to a 3 : 1 phenotype ratio, because the two heterozygotes look like the dominant homozygote.
In incomplete dominance every genotype looks different, so the phenotype ratio is also 1 : 2 : 1. Three visible types, not two.
A test-style cross of pink × white (C^R C^W × C^W C^W) gives a 1 : 1 ratio — half the offspring inherit a CR (pink) and half do not (white):
| Gametes | CW (from white parent) | CW (from white parent) |
|---|---|---|
| CR (from pink parent) | CR CW — pink | CR CW — pink |
| CW (from pink parent) | CW CW — white | CW CW — white |
Codominance — both alleles show at once: In some chickens, C^B gives black feathers and C^W gives white feathers. In the heterozygote C^B C^W you do not get grey — you get a bird with both black and white feathers together (a 'blue' or roan/speckled bird). Both alleles are fully expressed.
Cross a black bird with a white bird and all the offspring are this third, blue type:
| Gametes | CW (from white parent) | CW (from white parent) |
|---|---|---|
| CB (from black parent) | CB CW — blue (black + white feathers) | CB CW — blue (black + white feathers) |
| CB (from black parent) | CB CW — blue (black + white feathers) | CB CW — blue (black + white feathers) |
The trap: blended vs both-shown: Incomplete dominance = a new blended colour (red + white → pink).
Codominance = both colours visible at the same time (black feathers AND white feathers on the same bird).
If an exam describes a heterozygote that is a smooth mix, say incomplete dominance. If it shows both parental features side by side, say codominance.
| Feature | Incomplete dominance | Codominance |
|---|---|---|
| What the heterozygote looks like | A NEW, in-between (blended) phenotype | BOTH alleles' phenotypes shown TOGETHER |
| Flower-colour example | Red × white → pink (a mix) | (not blended — would be red AND white patches) |
| Animal example | — | Black × white feathers → blue/roan (black AND white feathers seen) |
| Are the alleles 'mixed'? | Yes — the phenotype is intermediate | No — each allele is fully expressed, side by side |
| F2 ratio from two heterozygotes | 1 : 2 : 1 (and 3 phenotypes, all visible) | 1 : 2 : 1 (and 3 phenotypes, all visible) |
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How this is tested: On Paper 1A (1 mark) you are asked to predict a ratio or phenotypes for an incomplete-dominance flower cross, or to identify the pattern and the reason for red × white → pink, or to explain a codominant feather cross (blue/black/white).
On Paper 1B / Paper 2 a longer Explain can ask you to account for how ABO blood groups are inherited as discrete variation — that needs multiple alleles AND codominance.
ABO = multiple alleles + codominance: One gene controls ABO blood group, but the population has three alleles: I^A, I^B and i.
I^A and I^B are codominant (a person with both has group AB).
Both I^A and I^B are dominant to i.
Group O only appears when someone is i i (no IA and no IB).
| Blood group (phenotype) | Possible genotypes | Why |
|---|---|---|
| A | IA IA or IA i | IA is dominant to i |
| B | IB IB or IB i | IB is dominant to i |
| AB | IA IB | IA and IB are CODOMINANT — both shown |
| O | i i | i is recessive — only shows when there is no IA or IB |
IB-style question — account for ABO inheritance as discrete variation
Human ABO blood group is an example of discrete variation. Account for how the four ABO blood groups are inherited. [3]
How to score all three marks
- Multiple alleles. The gene has three alleles in the population — I^A, I^B and i — but each person inherits only two (one from each parent).
- Dominance relationships. I^A and I^B are codominant (both are expressed, giving group AB); both are dominant to i, which is recessive (group O is i i).
- Why it is discrete. Because each genotype maps to one of only four distinct groups (A, B, AB, O) with no in-betweens, the variation falls into separate categories — i.e. discrete (not continuous) variation. (Award 1 mark per distinct point, max 3.)
Final answer
Three alleles (IA, IB, i) but two per person; IA and IB are codominant and both dominant to i; this gives exactly four distinct groups (A, B, AB, O), so the variation is discrete.
✓ Why this scores full marks: It hits the three ideas the markscheme wants — multiple alleles, the codominance + recessive-i dominance pattern, and the link to discrete variation (four distinct categories). A common 1-mark loss is naming the alleles but never saying IA and IB are codominant.