Biology • Year 12 • Module 5 • Lesson 15

Non-Mendelian Patterns — Co-dominance, Incomplete Dominance, Multiple Alleles

Build HSC Band 5–6 extended-response technique on the three non-Mendelian patterns — using ABO blood-group data, snapdragon ratios and a misconception critique.

Master · Extended Response

1. Extended response — distinguish the three non-Mendelian patterns (Band 5–6)

7 marks Band 5–6

Q1. Compare and contrast co-dominance, incomplete dominance and multiple alleles as inheritance patterns. In your response you must:

Define each of the three terms.
Identify the typical phenotype ratio (where applicable) for a heterozygote × heterozygote cross under each pattern.
Use at least one named biological example per pattern (e.g. ABO blood group, MN blood group, snapdragon flower colour, Shorthorn cattle roan coat).
Explain how the ABO blood group simultaneously demonstrates two of these patterns.

Stuck? Plan first: define-define-define → ratios → named example × 3 → ABO does two at once → judgement on what these patterns share (heterozygote ≠ one homozygote).

2. Stimulus-based extended response — disputed paternity in an ABO maternity ward (Band 5–6)

8 marks Band 5–6

Stimulus. A hospital reports the following blood groups for one delivery suite:

Mother: blood group A (subsequently genotyped as IAi).
Putative father: blood group AB (genotype IAIB).
Their newborn baby: blood group O.

The hospital initially concludes that the babies must have been swapped at birth. A geneticist is called in. She explains that the conclusion can be drawn from inheritance reasoning alone — without DNA testing — by analysing the gametes each parent can produce and the offspring genotypes those gametes can generate.

Q2. Analyse and evaluate the geneticist's reasoning. Your response must:

Identify the possible gametes produced by each parent.
Draw or describe the Punnett square cross and list every possible offspring genotype and phenotype.
Explain, using the dominance relationships of IA, IB and i, why a blood-group-O child cannot arise from this couple.
Discuss one additional inheritance scenario (e.g. a rare Bombay phenotype, where the H antigen needed to display A and B antigens is absent) that could complicate this conclusion in real life.

Stuck? Use Card 4 (ABO summary) as the spine, the misconceptions box for the co-dominance rule, then add one nuance from beyond the lesson (Bombay phenotype) to lift the response into Band 6.

3. Evaluate this claim (Band 5–6)

6 marks Band 5–6

"Pink snapdragons prove that Mendelian genetics is wrong. The two alleles for flower colour have blended in the heterozygote, which means alleles don't always stay separate, and the offspring of two pink plants must therefore all be pink. This 'blending inheritance' is incomplete dominance, and it shows that Mendel's laws only apply when one allele is fully dominant."

Q3. Evaluate this claim. Identify which parts are correct, which are wrong, and reformulate the claim into a biologically defensible statement that explains both the pink phenotype and the 1:2:1 ratio in the next generation.

Stuck? Revisit lesson § Card 2 (alleles still segregate normally) and the misconceptions box (incomplete dominance ≠ blending of alleles).

Answers — Do not peek before attempting

Q1 — Sample Band 6 response (7 marks), annotated

Co-dominance is an inheritance pattern in which both alleles in a heterozygote are fully and independently expressed, so the heterozygote phenotype contains both allele products visibly side-by-side. [1 — co-dominance definition] Incomplete dominance is an inheritance pattern in which neither allele is fully dominant in the heterozygote, so the heterozygote shows a phenotype intermediate between the two homozygotes. [1 — incomplete dominance definition] Multiple alleles describes a gene that exists in more than two allele forms across the population; each diploid individual still carries only two alleles, but the population gene pool offers more than two possibilities. [1 — multiple alleles definition]

Heterozygote × heterozygote crosses under both co-dominance and incomplete dominance typically produce a 1:2:1 phenotype ratio because each of the three genotypes (homozygous A, heterozygous, homozygous B) maps to a visibly different phenotype, in contrast with the 3:1 ratio seen in simple Mendelian dominance where the heterozygote matches one homozygote. [1 — ratios with contrast]

Named examples include: co-dominance — Shorthorn cattle roan coats (red + white hairs co-occur) or human MN blood group; incomplete dominance — snapdragon flower colour (red × white → pink heterozygote); multiple alleles — human ABO blood group, with alleles IA, IB and i present in the population. [1 — three named examples]

The ABO blood group illustrates two of these patterns simultaneously. At the population level, three allele forms (IA, IB, i) exist, so ABO is a multiple-alleles system. At the heterozygote level, IAIB individuals are blood group AB because both A and B antigens are fully expressed on their red blood cells, which is co-dominance; IA and IB are nonetheless both fully dominant over i. [1 — ABO does two patterns at once]

The three patterns therefore share a common diagnostic feature: the heterozygote does not automatically match one homozygote phenotypically, so observed ratios deviating from 3:1 are a signal to consider non-Mendelian inheritance. Recognising whether the heterozygote shows both products (co-dominance), an intermediate (incomplete dominance), or whether more than two alleles exist (multiple alleles), is the key analytical skill when interpreting real-world data. [1 — integrated judgement linking back to data interpretation]

Q2 — Sample Band 6 response (8 marks), annotated

The mother (IAi) produces gametes carrying either IA or i. The putative father (IAIB) produces gametes carrying either IA or IB. [1 — gametes identified]

Combining these in a Punnett square gives four possible offspring genotypes — IAIA, IAIB, IAi, IBi [1 — four offspring genotypes] — with corresponding phenotypes A, AB, A, B (overall ratio 2 A : 1 AB : 1 B). [1 — phenotypes]

The crucial observation is that the father carries no i allele. Every gamete he can produce carries either IA or IB. [1 — no i in father's gametes] Because IA and IB are both fully dominant over i, every offspring of this couple inherits at least one A- or B-producing allele and so expresses an A, B or AB phenotype. A genotype of ii — necessary for blood-group O — is impossible from this cross, because at least one of the offspring's two alleles must come from the father, and his gametes never carry i. [1 — dominance reasoning to rule out O]

The geneticist's conclusion is therefore well supported by standard ABO inheritance rules: the putative father cannot be the biological father of this O-group baby. This is consistent with a hospital mix-up or with non-paternity; the inheritance reasoning alone cannot distinguish the two, so further DNA evidence would be needed before clinical or legal action. [1 — conclusion, with appropriate caution]

One genuine complication is the rare Bombay (hh) phenotype. In Bombay individuals, the H precursor antigen on which the A and B antigens are built is not produced, so even an IAIB person appears phenotypically as blood group O on standard tests. A baby could therefore be genotypically IAIB but phenotypically O — appearing inconsistent with both parents on the surface, even though inheritance is normal. [1 — Bombay phenotype nuance]

The final evaluation is therefore: under standard ABO inheritance and assuming no Bombay phenotype, the putative father is incompatible with the O baby and the geneticist's conclusion is correct. Before any clinical or legal action, hospital protocol should include additional testing — for example, screening for the Bombay phenotype and confirming parentage through STR DNA profiling — so the conclusion rests on more than the ABO system alone. [1 — integrated final evaluation]

Q3 — Sample Band 6 response (6 marks)

The claim is partly correct but largely flawed. [1 — judgement]

What is defensible: snapdragons do show incomplete dominance — the heterozygote (CRCW) has an intermediate pink phenotype that is not predicted by a simple 3:1 model. [1 — concedes correct element]

What is wrong:

"Alleles blend in the heterozygote." The alleles do not physically merge — they segregate normally during meiosis. The evidence: a cross between two pink plants regenerates pure red (CRCR) and pure white (CWCW) offspring, which would be impossible if the alleles had blended. [1 — refutes blending]
"Two pink plants must produce all pink offspring." A CRCW × CRCW cross gives a 1 red : 2 pink : 1 white phenotype ratio. Only about half the offspring are pink; a quarter are red and a quarter are white. [1 — refutes "all pink"]
"Mendel's laws only apply when one allele is fully dominant." Mendel's laws of segregation and independent assortment describe how alleles are transmitted through gametes, not how they are expressed. Under incomplete dominance, segregation still operates normally — only the heterozygote phenotype changes. [1 — refutes "Mendel doesn't apply"]

Defensible reformulation: "Snapdragons illustrate incomplete dominance, a phenotype-level expression pattern in which the heterozygote shows an intermediate phenotype (pink) rather than matching one homozygote. Underneath, the alleles still segregate independently and normally in meiosis, exactly as Mendel described — that is why crossing two pink plants regenerates a 1:2:1 phenotype ratio (1 red : 2 pink : 1 white) and recovers the pure parental phenotypes. Incomplete dominance modifies how dominance is expressed in the heterozygote, but it does not overturn Mendelian segregation." [1 — biologically defensible reformulation]