Biology • Year 12 • Module 5 • Lesson 15
Non-Mendelian Patterns — Co-dominance, Incomplete Dominance, Multiple Alleles
Apply the three non-Mendelian patterns to ABO blood-group crosses, real cattle-coat data, a colour-mixing co-dominance scenario, and a student diagram critique.
1. ABO blood-group paternity scenario
A hospital is investigating a possible identification mix-up. A baby has blood group O. The mother has blood group A and her genotype is IAi. Two candidate fathers come forward: Father 1 has blood group AB; Father 2 has blood group B with genotype IBi. 8 marks
1.1 Complete the Punnett square for the cross between Mother (IAi) and Father 1 (genotype IAIB). State the offspring genotypes and their blood-group phenotypes. 3 marks
IA |
IB |
|
|---|---|---|
IA | ||
i |
Offspring genotypes / phenotypes:
1.2 Complete the Punnett square for the cross between Mother (IAi) and Father 2 (IBi). State the offspring genotypes and their blood-group phenotypes. 3 marks
IB |
i |
|
|---|---|---|
IA | ||
i |
Offspring genotypes / phenotypes:
1.3 Using your two Punnett squares, identify which candidate father cannot be the biological father, and which could be. Justify each conclusion using the dominance relationships of IA, IB and i. 2 marks
ii child (group O) can only inherit an i allele from each parent. Which fathers carry an i allele?
2. Data table — Shorthorn cattle coat colour
In Shorthorn cattle, coat colour is controlled by a single gene with two co-dominant alleles. CR = red coat allele; CW = white coat allele. Heterozygotes (CRCW) display a roan coat in which patches of red and white hairs grow side by side — the two allele products are visible together. A breeder records three matings and their offspring over five seasons. 8 marks
| Cross | Sire | Dam | Red calves | Roan calves | White calves |
|---|---|---|---|---|---|
| P | red (CRCR) | white (CWCW) | 0 | 48 | 0 |
| Q | roan (CRCW) | roan (CRCW) | 14 | 27 | 13 |
| R | roan (CRCW) | red (CRCR) | 22 | 21 | 0 |
2.1 Identify the inheritance pattern shown by Cross P. Justify your answer with reference to the heterozygote phenotype. 2 marks
2.2 Calculate the phenotype ratio observed in Cross Q (round to whole numbers). Does this ratio match the expected Mendelian / non-Mendelian prediction? 2 marks
2.3 Predict the expected phenotype ratio for Cross R and explain why no white calves were observed. 2 marks
2.4 Explain why roan is best classified as co-dominance and not incomplete dominance, even though the roan coat could superficially be mistaken for "pink-ish". 2 marks
3. Colour-mixing co-dominance — speckled fish
In a tropical fish species, scale colour is controlled by a single gene with two co-dominant alleles. SY = yellow scale allele; SB = blue scale allele. Homozygotes (SYSY) have pure yellow scales; homozygotes (SBSB) have pure blue scales; heterozygotes (SYSB) have speckled scales — visibly half yellow, half blue, with no green tones (the two pigment products co-occur without blending). 6 marks
3.1 Two heterozygous speckled fish are crossed. Complete the Punnett square below, then state the genotype and phenotype ratios. 3 marks
SY |
SB |
|
|---|---|---|
SY | ||
SB |
Genotype ratio / phenotype ratio:
3.2 A different geneticist proposes that this gene instead shows incomplete dominance, with the heterozygote being green. State one phenotype-level observation that would let you distinguish co-dominance from incomplete dominance in this species. 2 marks
3.3 A breeder wants only speckled offspring from a single cross. State the two parental genotypes that would guarantee this and justify briefly. 1 mark
4. Diagram critique — what's wrong with this student's diagram?
A Year 12 student has drawn the diagram below to explain ABO inheritance. There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
4.1 Error 1: What is wrong?
Correction:
4.2 Error 2: What is wrong?
Correction:
4.3 Error 3: What is wrong?
Correction:
5. Predict-and-justify — pink snapdragons in a field
A horticulturalist has 200 pink snapdragons (CRCW) and lets them cross-pollinate freely among themselves. She predicts that next year's seedlings will be "mostly pink, like the parents." 4 marks
5.1 Predict the phenotype ratio you would actually expect among the seedlings, and state the underlying genotype ratio. 2 marks
5.2 Explain why the horticulturalist's intuition ("mostly pink, like the parents") is wrong, with reference to allele segregation and incomplete dominance. 2 marks
Q1 — ABO paternity (8 marks)
1.1 Mother IAi × Father 1 IAIB. Punnett cells: IAIA, IAIB, IAi, IBi. Phenotypes: A, AB, A, B. Ratio 2 A : 1 AB : 1 B. No child of blood group O is possible. [3 marks: 1 cells, 1 phenotypes, 1 explicit "no O possible" point]
1.2 Mother IAi × Father 2 IBi. Punnett cells: IAIB, IAi, IBi, ii. Phenotypes: AB, A, B, O. Ratio 1 AB : 1 A : 1 B : 1 O. [3 marks: 1 cells, 1 phenotypes, 1 explicit "O is possible" point]
1.3 Father 1 (genotype IAIB) cannot be the biological father because he carries no i allele — every gamete he produces carries either IA or IB, so no ii (group O) child is possible [1]. Father 2 (IBi) could be the biological father because he carries an i allele; when his i gamete combines with the mother's i gamete, an ii (group O) child results [1].
Q2 — Shorthorn coat colour (8 marks)
2.1 Co-dominance [1]. The heterozygote (CRCW) expresses both allele products — patches of red and patches of white hair are visible together rather than a blended pink, so neither allele masks the other [1].
2.2 Observed counts 14 : 27 : 13. Dividing by ~13.5 gives roughly 1 red : 2 roan : 1 white, which matches the expected 1:2:1 ratio for a heterozygote × heterozygote cross under co-dominance [2].
2.3 Expected ratio: 1 red : 1 roan : 0 white [1]. No white calves are possible because every offspring inherits at least one CR allele from the homozygous red sire, so the genotype CWCW cannot arise from this cross [1]. The observed 22 : 21 : 0 fits this prediction well.
2.4 In roan, individual hairs are either pure red or pure white — the two pigment products are both visible, patch-by-patch, in the same animal [1]. In incomplete dominance the heterozygote shows a single intermediate pigment (e.g. uniformly pink snapdragon petals), not two distinct pigments coexisting; the observable pattern in roan therefore matches the definition of co-dominance [1].
Q3 — Speckled fish (6 marks)
3.1 Punnett cells: SYSY, SYSB, SYSB, SBSB. Genotype ratio 1 SYSY : 2 SYSB : 1 SBSB. Phenotype ratio 1 pure yellow : 2 speckled : 1 pure blue [3].
3.2 Look at the heterozygote phenotype directly: in co-dominance the heterozygote shows both pigments visibly distinct (yellow patches AND blue patches on the same fish) [1]; in incomplete dominance the heterozygote shows a single intermediate colour (uniform green) with no separate yellow or blue areas [1]. Accept also: examine the scales under magnification — co-dominance gives some scales fully yellow and others fully blue, incomplete dominance gives every scale the same intermediate colour.
3.3 Cross a pure yellow homozygote (SYSY) with a pure blue homozygote (SBSB) — every offspring is SYSB and therefore speckled [1].
Q4 — Diagram critique (6 marks)
4.1 Error 1 (genotype-phenotype mapping swapped): IAi gives blood group A (not AB) — because IA is dominant over i. IAIB gives blood group AB (not A) — because IA and IB are co-dominant. Correction: redraw the mapping so IAi → A and IAIB → AB. [1 + 1]
4.2 Error 2 ("blended group, intermediate between A and B"): The IAIB heterozygote is not intermediate — both A and B antigens are fully expressed on the red blood cells. This is co-dominance, not incomplete dominance. Correction: relabel the IAIB cell as "blood group AB — both A and B antigens fully expressed on the red blood cells." [1 + 1]
4.3 Error 3 ("Each person carries one of the three ABO alleles"): Each person carries two alleles (one inherited from each parent), drawn from the population pool of three. Correction: replace the caption with "Each person carries two ABO alleles, chosen from the three forms IA, IB and i in the population." [1 + 1]
Q5 — Pink snapdragons in a field (4 marks)
5.1 Predicted seedling ratio: 1 red : 2 pink : 1 white, from an underlying genotype ratio of 1 CRCR : 2 CRCW : 1 CWCW [2]. Only about half the seedlings should be pink — a quarter should be red and a quarter white.
5.2 Each pink parent (CRCW) produces gametes CR and CW in equal proportions because alleles segregate independently at meiosis — they have not fused into a single "pink" allele [1]. When two pink plants cross-pollinate, all four gametic combinations occur with equal probability, producing the 1:2:1 ratio rather than mostly pink offspring [1].