Codominance, multiple alleles & incomplete dominance (Biology HL)

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 (I^A, I^B 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. C^R, C^W) 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:

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 C^R (pink) and half do not (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:

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.

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 I^A and no I^B).

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 I^A and I^B are codominant.