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 $i$.

• $I^A$ produces the A antigen.
• $I^B$ produces the B antigen.
• $i$ 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.

Examples in context

Example 1. Roan coat colour in Australian Shorthorn cattle. Shorthorn cattle exhibited at the Sydney Royal Easter Show often carry the classic codominance gene for coat colour. A red bull ($C^R C^R$) crossed with a white cow ($C^W C^W$) produces all roan ($C^R C^W$) calves. Roan animals do not have pink coats; they have individual red and white hairs intermingled across the body, both alleles expressed in different patches of follicle cells. NSW DPI breeding records show this pattern is fully predictable: roan-to-roan crosses give a 1 red : 2 roan : 1 white ratio in the calves. Stud breeders use this to plan their show entries years in advance.

Example 2. Blood transfusion compatibility at NSW pathology. When a patient at Royal Prince Alfred Hospital needs a transfusion, the pathology lab cross-matches ABO and Rh blood types because the codominance of $I^A$ and $I^B$ creates real clinical risk. A type O patient ($ii$) has anti-A and anti-B antibodies in their plasma; transfusing type A or B blood would trigger antigen-antibody clumping and a potentially fatal haemolytic reaction. Type AB patients ($I^A I^B$) are "universal recipients" because they make neither anti-A nor anti-B. Type O blood (donor) is "universal" because its red cells lack both A and B antigens. The multiple-allele system explains why blood drives target O-negative donors most aggressively.

Try this

Q1. In horses, the gene for coat colour shows incomplete dominance. Chestnut ($CC$) crossed with cremello ($cc$) produces palomino ($Cc$) foals. A breeder crosses two palomino horses. Predict the genotypic and phenotypic ratios of the offspring. [3 marks]

• Cue. $Cc \times Cc$ gives 1 CC : 2 Cc : 1 cc, which is 1 chestnut : 2 palomino : 1 cremello. Genotypic and phenotypic ratios match because each genotype gives a distinct colour.

Q2. A woman with blood type O has a child with blood type AB. The mother claims a particular man is the father. The man has blood type A (genotype $I^A i$). Use a Punnett square to determine whether he could be the biological father. [3 marks]

• Cue. Mother ($ii$) can only pass $i$. The child must receive $I^B$ from the father. A man with $I^A i$ cannot pass $I^B$, so he is excluded.

Q3. Sickle cell trait shows codominance at the molecular level. (a) Define codominance. (b) An $Hb^A Hb^S$ heterozygote produces both normal and sickle haemoglobin in red blood cells - explain how this is consistent with codominance. (c) Why is this heterozygote partially protected against malaria? [1+2+2 marks]

• Cue. (a) Both alleles fully expressed in the heterozygote. (b) Each allele transcribes its own polypeptide, so red cells contain a mixture of both forms. (c) Sickled cells are inhospitable to Plasmodium parasites; heterozygote advantage.