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).