Biology • Year 12 • Module 5 • Lesson 15
Non-Mendelian Patterns — Co-dominance, Incomplete Dominance, Multiple Alleles
Lock in the vocabulary, the three non-Mendelian patterns, the ABO blood-group system, and the rule that the heterozygote does not always look like one homozygote.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: co-dominance, incomplete dominance, multiple alleles, homozygous, heterozygous, allele, phenotype, genotype, ABO blood group, phenotypic ratio. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | An inheritance pattern in which both alleles are fully and independently expressed in the heterozygote. | |
| 1.2 | An inheritance pattern in which the heterozygous phenotype is intermediate between the two homozygous phenotypes. | |
| 1.3 | Describes a gene that exists in more than two allele forms within the population. | |
| 1.4 | Having two identical alleles for a particular gene. | |
| 1.5 | Having two different alleles for a particular gene. | |
| 1.6 | One of the alternative forms of a gene at a particular locus. | |
| 1.7 | The observable trait of an organism, produced by the interaction of genotype and environment. | |
| 1.8 | The combination of alleles that an organism carries for a particular gene. | |
| 1.9 | A human blood group system that demonstrates both co-dominance and multiple alleles. | |
| 1.10 | The relative frequency of observable traits among the offspring of a cross. |
2. Cloze — fill the blanks
Complete the passage using terms from the word bank. Each term is used once. 10 marks
Word bank: co-dominant · incomplete dominance · 1:2:1 · 3:1 · multiple alleles · intermediate · heterozygote · IA · IB · i
In simple Mendelian inheritance, a cross between two heterozygotes produces a 2.1 _______________ phenotype ratio because one allele masks the other. When the heterozygote shows an 2.2 _______________ phenotype, as in red × white snapdragons producing pink offspring, the pattern is called 2.3 _______________. The cross between two pink plants instead gives a 2.4 _______________ phenotype ratio.
In the ABO blood group, the alleles 2.5 _______________ and 2.6 _______________ are 2.7 _______________ with each other, and both are dominant over 2.8 _______________. Because three allele forms exist in the population, ABO is also an example of 2.9 _______________. Each individual still carries only two alleles, but the 2.10 _______________ can show both alleles' products together (e.g. blood group AB).
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 8 marks (1 for T/F, 1 for the correction where needed)
3.1 Co-dominance and incomplete dominance describe the same inheritance pattern. T / F
3.2 In incomplete dominance, the alleles fuse permanently into one new allele in the heterozygote. T / F
3.3 The ABO gene has three common alleles in the population, but each person carries only two of them. T / F
3.4 A cross between two heterozygotes that yields a 1:2:1 phenotype ratio is consistent with the heterozygote having its own distinct phenotype. T / F
4. Complete the Punnett squares
Fill in the empty cells of each HTML Punnett square, then state the genotype and phenotype ratios. Use the snapdragon allele symbols from the lesson: CR = red, CW = white, with pink heterozygotes CRCW. 12 marks (4 per cross)
4.1 Cross A — pink × white (CRCW × CWCW)
CW |
CW |
|
|---|---|---|
CR | ||
CW |
Genotype ratio:
Phenotype ratio:
4.2 Cross B — pink × pink (CRCW × CRCW)
CR |
CW |
|
|---|---|---|
CR | ||
CW |
Genotype ratio:
Phenotype ratio:
4.3 Cross C — blood group A heterozygote × blood group B heterozygote (IAi × IBi)
IB |
i |
|
|---|---|---|
IA | ||
i |
Offspring genotypes:
Offspring phenotypes (blood groups):
IA and IB are co-dominant; both are dominant over i.
5. Compare the three non-Mendelian patterns
Complete the comparison grid using lesson terminology. Each empty cell is worth 1 mark. 9 marks
| Feature | Co-dominance | Incomplete dominance | Multiple alleles |
|---|---|---|---|
| How is the heterozygote expressed? | (Not defined by heterozygote alone — describe what the gene shows at the population level instead) | ||
| Typical phenotype ratio for a heterozygote × heterozygote cross | (Depends on which two of the three alleles each parent carries — give one possible ratio.) | ||
| Named biological example from the lesson |
Q1 — Term–definition matches (10 marks)
1.1 co-dominance • 1.2 incomplete dominance • 1.3 multiple alleles • 1.4 homozygous • 1.5 heterozygous • 1.6 allele • 1.7 phenotype • 1.8 genotype • 1.9 ABO blood group • 1.10 phenotypic ratio.
Q2 — Cloze (10 marks, 1 each)
2.1 3:1 • 2.2 intermediate • 2.3 incomplete dominance • 2.4 1:2:1 • 2.5 IA (or IB) • 2.6 IB (or IA — accept either order) • 2.7 co-dominant • 2.8 i • 2.9 multiple alleles • 2.10 heterozygote.
Q3 — True / false with correction (8 marks)
3.1 False. Correction: co-dominance and incomplete dominance are different patterns — co-dominance shows both allele products visible together (e.g. AB blood type), while incomplete dominance shows an intermediate phenotype (e.g. pink flowers).
3.2 False. Correction: the alleles do not fuse — they still segregate normally in meiosis. The intermediate phenotype is produced because of the expression pattern in the heterozygote, not because the DNA has merged.
3.3 True.
3.4 True.
Q4 — Punnett squares (12 marks, 4 per cross)
4.1 Pink × white (CRCW × CWCW). Cells: CRCW, CRCW, CWCW, CWCW. Genotype ratio 1 CRCW : 1 CWCW. Phenotype ratio 1 pink : 1 white.
4.2 Pink × pink (CRCW × CRCW). Cells: CRCR, CRCW, CRCW, CWCW. Genotype ratio 1 CRCR : 2 CRCW : 1 CWCW. Phenotype ratio 1 red : 2 pink : 1 white.
4.3 ABO heterozygotes (IAi × IBi). Cells: IAIB, IAi, IBi, ii. Offspring genotypes (1 each): IAIB, IAi, IBi, ii. Offspring phenotypes (1 each): AB, A, B, O.
Q5 — Comparison grid (9 marks)
Co-dominance / heterozygote: both alleles are fully and independently expressed; the phenotype shows both forms together (e.g. both A and B antigens). Co-dominance / ratio: 1:2:1 (e.g. 1 AA-type : 2 AB-type : 1 BB-type when crossing two heterozygotes). Co-dominance / example: ABO blood group AB (or human MN blood group; cattle roan coat colour).
Incomplete dominance / heterozygote: phenotype is intermediate between the two homozygotes; both allele products contribute partially. Incomplete dominance / ratio: 1:2:1 (1 homozygous-dominant : 2 intermediate : 1 homozygous-recessive). Incomplete dominance / example: snapdragon / four o'clock flower colour — red × white → pink heterozygotes.
Multiple alleles / population view: the gene exists in more than two allele forms across the population, even though each individual still carries only two. Multiple alleles / one possible ratio: e.g. IAi × IBi gives 1 AB : 1 A : 1 B : 1 O. Multiple alleles / example: ABO blood group (three alleles IA, IB, i).