HSC Exam Practice

Sample response. Co-dominance is an inheritance pattern in which both alleles in the heterozygote are fully and independently expressed, so the heterozygous phenotype contains both allele products visibly together (for example, both A and B antigens on the red blood cells of an IAIB individual).

Marking notes. 1 mark for "both alleles expressed in the heterozygote" / no masking; 1 mark for "both allele products visible together" or an equivalent example.

Sample response. In co-dominance both alleles in the heterozygote are fully expressed, with both allele products visible together — for example, blood group AB in humans (genotype IAIB) has both A and B antigens on the red blood cells. In incomplete dominance, neither allele is fully dominant in the heterozygote and the phenotype is intermediate between the two homozygotes — for example, the snapdragon cross CRCR (red) × CWCW (white) produces pink heterozygotes (CRCW).

Marking notes. 1 mark — distinguishing the heterozygote phenotypes (both products vs intermediate); 1 mark — one valid named example for co-dominance; 1 mark — one valid named example for incomplete dominance. Accept any biologically valid example (e.g. roan coats in cattle, MN blood group, four o'clock flowers).

Sample response. The three ABO alleles are IA, IB and i. IA and IB are co-dominant with each other (both are fully expressed in IAIB heterozygotes, giving blood group AB). Both IA and IB are fully dominant over i (the recessive O allele), so IAi heterozygotes are blood group A and IBi heterozygotes are blood group B.

Marking notes. 1 mark for naming all three alleles correctly; 1 mark for identifying IA/IB as co-dominant; 1 mark for identifying both as dominant over i.

Sample response. In a heterozygote × heterozygote cross (Aa × Aa), the underlying genotype ratio is always 1 AA : 2 Aa : 1 aa. Under simple Mendelian dominance, the Aa heterozygote looks identical to the AA homozygote, so the 1 AA + 2 Aa fall into one phenotype class and the 1 aa into the other, giving a 3:1 phenotype ratio. Under incomplete dominance, however, the Aa heterozygote has its own distinct intermediate phenotype, so each genotype maps to a different phenotype class — the 1:2:1 genotype ratio is observed directly as a 1:2:1 phenotype ratio.

Marking notes. 1 mark for identifying the 1:2:1 underlying genotype ratio; 1 mark for explaining that under Mendelian dominance the heterozygote phenotype matches one homozygote (collapsing the ratio to 3:1); 1 mark for explaining that under incomplete dominance the heterozygote has its own phenotype (so 1:2:1 phenotype mirrors the 1:2:1 genotype ratio).

Sample response. Multiple alleles means a gene exists in more than two allele forms across the population — for example, the ABO gene has three forms (IA, IB, i). A single diploid individual still carries only two alleles for the gene, one inherited from each parent.

Marking notes. 1 mark for "more than two allele forms in the population"; 1 mark for "each individual still carries only two alleles". Both required for full marks.

Sample response (a). Blood groups A and B can each be produced by two different genotypes (A: IAIA or IAi; B: IBIB or IBi), whereas O has only one (ii) and AB has only one (IAIB). This is consistent with simple dominance of IA and IB over i — both the homozygous and heterozygous forms produce the same phenotype because the i allele is masked when either dominant allele is present.

Sample response (b). The AB phenotype requires both IA and IB alleles in the same individual, but the i allele is by far the most common in the population (since 46% of donors are ii, the i allele frequency is high). The chance of inheriting one IA gamete and one IB gamete is therefore small, so IAIB individuals are rare. The dominance relationships also mean IA and IB are co-dominant — both must be present and both are fully expressed — so neither homozygote nor heterozygote of a single dominant allele can produce the AB phenotype.

Marking notes. Part (a) — 1 mark for identifying A and B as having two compatible genotypes each; 1 mark for explicit "IA and IB are dominant over i" reasoning. Part (b) — 1 mark for noting that AB requires both IA and IB in one individual; 1 mark for relating low AB frequency to low joint probability of inheriting both rare alleles; 1 mark for explicit reference to co-dominance / both alleles required.

Sample response. Mother (IAi) produces gametes IA and i; father (IBi) produces gametes IB and i. The Punnett square cells are: IAIB, IAi, IBi, ii. Genotype ratio 1 IAIB : 1 IAi : 1 IBi : 1 ii. Phenotype ratio 1 AB : 1 A : 1 B : 1 O. This single cross illustrates both co-dominance (because IA and IB are expressed together in the IAIB AB-blood-group offspring) and multiple alleles (because three different allele forms — IA, IB, i — appear in the parents and recombine in the offspring).

Marking notes. 1 mark for correctly filling the four Punnett cells; 1 mark for the 1:1:1:1 genotype ratio; 1 mark for the correct 1 AB : 1 A : 1 B : 1 O phenotype ratio; 1 mark for identifying co-dominance with reference to IAIB; 1 mark for identifying multiple alleles with reference to three allele forms.

Sample response. The claim is partly correct but largely flawed: incomplete dominance changes how alleles are expressed in the heterozygote, but it does not contradict Mendel's laws of segregation and independent assortment. In incomplete dominance neither allele is fully dominant, and the heterozygote shows an intermediate phenotype — a well-documented example is the snapdragon cross CRCR (red) × CWCW (white) producing pink heterozygotes (CRCW). If the alleles had genuinely "blended," the heterozygote would carry a permanently mixed allele form and pure red and pure white phenotypes could not reappear in later generations. They do reappear: a cross between two pink heterozygotes (CRCW × CRCW) yields a 1:2:1 phenotype ratio of red : pink : white, exactly mirroring the underlying 1:2:1 genotype ratio. This shows that the two alleles segregate normally during meiosis, are transmitted to gametes independently, and recombine in offspring in Mendelian proportions. Compare this with a simple Mendelian gene, where the heterozygote phenotype matches one homozygote and the 1:2:1 genotype ratio is observed as a 3:1 phenotype ratio; the difference between the two cases is at the phenotype-expression step, not at the inheritance step. The same logic applies to other non-Mendelian patterns such as co-dominance (e.g. the ABO blood group AB heterozygote) and multiple alleles (e.g. three ABO alleles in the population): the rules for transmitting alleles through gametes are still Mendelian, even when the rules for displaying alleles in the heterozygote are not "one allele fully masks the other." The claim therefore should be rejected. Incomplete dominance is best understood as an extension of Mendelian genetics, describing the expression of heterozygotes, rather than evidence against it.

Marking notes. 1 mark — clear evaluative judgement that the claim is partly correct but largely flawed. 1 mark — concedes the defensible element (incomplete dominance does produce an intermediate heterozygote and deviates from a 3:1 ratio). 1 mark — names a valid example of incomplete dominance with allele notation (e.g. snapdragon CRCW, four o'clock flowers). 1 mark — refutes "alleles blend" by noting that homozygous phenotypes reappear in later generations. 1 mark — correctly identifies the 1:2:1 phenotype ratio (and contrasts with 3:1) as evidence of normal allele segregation. 1 mark — distinguishes phenotype-level expression from genotype-level transmission; states that segregation/independent assortment still apply. 1 mark — integrated final reformulation that frames incomplete dominance as an extension of Mendel rather than a contradiction (extra credit for referencing co-dominance and/or multiple alleles as further examples of the same logic).