Investigate the inheritance patterns including but not limited to: codominance, incomplete dominance, multiple alleles

What this dot point is asking

NESA wants you to explain non-Mendelian inheritance patterns: codominance, incomplete dominance, and multiple alleles. Each is a real deviation from the simple dominant-recessive model Mendel described. The most common worked exam example is the ABO blood group system, which combines codominance and multiple alleles.

The answer

Codominance

In codominance, both alleles in a heterozygote are fully and simultaneously expressed. The phenotype shows both traits side by side, NOT blended.

Notation. Use uppercase letters with superscripts. For ABO blood groups: $I^A$ and $I^B$ are codominant.

Standard worked example: ABO blood groups.

An individual with $I^A I^B$ produces both the A antigen and the B antigen on their red blood cells, so they have blood type AB. Both alleles are expressed; neither dominates.

Another classic example is the MN blood group, where heterozygotes have both M and N antigens.

Incomplete dominance

In incomplete dominance, the heterozygote shows an intermediate phenotype between the two homozygotes, as if the alleles had been blended.

Notation. Use uppercase letters or different letter pairs.

Standard worked example: snapdragon flower colour.

Red snapdragons ($RR$) crossed with white snapdragons ($rr$) produce all pink ($Rr$) heterozygotes in the F1 generation. The pink colour is intermediate between red and white. Neither allele dominates fully.

If you cross two pink heterozygotes ($Rr \times Rr$), the F2 ratio is 1 red : 2 pink : 1 white (genotypic and phenotypic ratios are the same here, because each genotype produces a distinct phenotype).

Codominance vs incomplete dominance at a glance

Multiple alleles

Most genes in textbooks have just two alleles (e.g. A and a). In reality, many genes have multiple alleles in the population.

Worked example: ABO blood groups.

There are three alleles for the ABO gene: $I^A$, $I^B$, and $i$.

• IMATH_12 produces the A antigen.
• IMATH_13 produces the B antigen.
• IMATH_14 produces no antigen.

$I^A$ and $I^B$ are codominant with each other. Both $I^A$ and $I^B$ are **dominant over $i$**.

The six possible genotypes and four possible phenotypes:

Individual people still only carry two alleles (one from each parent). The "multiple alleles" refers to the variety within the population.

Worked ABO cross

Father type A heterozygous ($I^A i$) × Mother type B heterozygous ($I^B i$).

Genotypes: 1 $I^A I^B$ : 1 $I^B i$ : 1 $I^A i$ : 1 $ii$.
Phenotypes: 1 AB : 1 B : 1 A : 1 O.

All four blood types are possible offspring in this cross.

Common traps

Confusing codominance with incomplete dominance. Codominance = both visible (AB blood, both antigens). Incomplete dominance = blended intermediate (pink, between red and white).

Wrong allele notation for ABO. Use $I^A$, $I^B$, and $i$ (lowercase i for recessive O). Writing A, B, O directly without the I superscript loses marks.

Forgetting that each person has only two alleles. Multiple alleles exist in the population, but each individual still inherits one allele from each parent (total two).

Assuming all O offspring need O parents. Two heterozygous parents ($I^A i$ × $I^B i$, for example) can produce an O child even though neither parent is O.

In one sentence

Codominance produces a heterozygote that expresses both alleles simultaneously (e.g. AB blood type), incomplete dominance produces an intermediate heterozygote (e.g. pink snapdragons from red and white parents), and multiple alleles describe genes like the ABO blood group that have three or more alleles in the population even though each individual still carries only two.