Biology • Year 12 • Module 5 • Lesson 14b
Dihybrid Crosses and Independent Assortment
Lock in the vocabulary of two-gene crosses, the structure of a 4×4 Punnett square, and the 9:3:3:1 phenotypic ratio. By the end you should be able to set up an AaBb × AaBb grid from scratch.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: dihybrid cross, monohybrid cross, independent assortment, 9:3:3:1 ratio, linked genes, bivalent, phenotypic class, gamete, homozygous, heterozygous. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A genetic cross that tracks the inheritance of two different genes at the same time. | |
| 1.2 | The expected phenotypic ratio from an AaBb × AaBb cross when both genes assort independently. | |
| 1.3 | A paired homologous chromosome structure formed during meiosis I whose random orientation produces independent assortment. | |
| 1.4 | Mendel's second law — alleles of different genes segregate independently of each other during gamete formation. | |
| 1.5 | A group of offspring sharing the same observable combination of traits. | |
| 1.6 | Genes located on the same chromosome that tend to be inherited together, violating independent assortment. | |
| 1.7 | A haploid sex cell carrying one allele from each gene pair. | |
| 1.8 | A genetic cross that tracks the inheritance of only one gene. | |
| 1.9 | An organism carrying two identical alleles at a gene locus (e.g. AA or aa). | |
| 1.10 | An organism carrying two different alleles at a gene locus (e.g. Aa). |
2. Identify the gametes from each parent
For each parent genotype below, list all the unique gamete types it can produce, assuming independent assortment. Write each gamete with two letters (one allele from each gene). 8 marks (1 mark per correct gamete set)
| Parent genotype | Possible gamete types |
|---|---|
| 2.1 AABB | |
| 2.2 AAbb | |
| 2.3 AaBB | |
| 2.4 AaBb | |
| 2.5 aabb | |
| 2.6 YYRr | |
| 2.7 YyRR | |
| 2.8 YyRr |
3. Complete the 4×4 Punnett square — AaBb × AaBb
Fill in the empty cells of the 4×4 Punnett square below. The gamete labels on the top and the left have been provided. Each cell should contain a two-gene genotype (e.g. AaBb). 16 marks (1 per cell)
| ♀ / ♂ gametes | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | ||||
| Ab | ||||
| aB | ||||
| ab |
3.17 After completing the grid, how many of the 16 cells contain the genotype AaBb (in any allele order)?
4. Count the four phenotypic classes
Using the completed Punnett square from Section 3, count how many of the 16 cells fall into each phenotypic class. A_ means "at least one dominant A allele"; bb means "homozygous recessive for gene B". 5 marks (1 per row + 1 for the ratio)
| Phenotypic class | What it means (in plain English) | Cell count | Fraction of 16 |
|---|---|---|---|
| A_B_ | Dominant phenotype for both genes | ||
| A_bb | Dominant for A, recessive for B | ||
| aaB_ | Recessive for A, dominant for B | ||
| aabb | Recessive for both genes | ||
| Total | All offspring | 16 | 16/16 |
4.5 Write the phenotypic ratio in the simplest whole-number form:
A_B_ : A_bb : aaB_ : aabb = ____ : ____ : ____ : ____
5. True or false — with correction
For each statement, circle T or F. If false, write the corrected version. 8 marks (1 for T/F, 1 for the correction where needed)
5.1 An AaBb parent produces four gamete types in equal frequency when the two genes assort independently. T / F
5.2 The 9:3:3:1 ratio is a genotypic ratio. T / F
5.3 Independent assortment requires the two genes to be on the same chromosome. T / F
5.4 A dihybrid cross of two heterozygotes produces four distinct phenotypic classes when both genes show simple dominance. T / F
6. Cloze — independent assortment paragraph
Fill in the blanks using the word bank: bivalents, metaphase, independent assortment, different, 9:3:3:1, four, random, linked. 8 marks (1 per blank)
Mendel's law of (6.1) _______________ states that the alleles of two different genes segregate independently of each other into gametes. The cellular mechanism is the (6.2) _______________ orientation of paired homologous chromosomes ((6.3) _______________) on the (6.4) _______________ plate during meiosis I.
When two gene pairs are on (6.5) _______________ chromosomes, the way one pair aligns has no effect on how the other aligns. This produces (6.6) _______________ equally frequent gamete types from a dihybrid parent, and an F2 phenotypic ratio of (6.7) _______________. Genes that are on the same chromosome are described as (6.8) _______________ and do not produce this ratio.
Q1 — Term–definition matches
1.1 dihybrid cross • 1.2 9:3:3:1 ratio • 1.3 bivalent • 1.4 independent assortment • 1.5 phenotypic class • 1.6 linked genes • 1.7 gamete • 1.8 monohybrid cross • 1.9 homozygous • 1.10 heterozygous.
Q2 — Gametes from each parent
2.1 AB (only).
2.2 Ab (only).
2.3 AB, aB.
2.4 AB, Ab, aB, ab (all four in equal frequency).
2.5 ab (only).
2.6 YR, Yr.
2.7 YR, yR.
2.8 YR, Yr, yR, yr.
Q3 — Completed 4×4 Punnett square
| AB | Ab | aB | ab | |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
3.17 Four cells contain AaBb — the (AB,ab), (Ab,aB), (aB,Ab) and (ab,AB) intersections.
Q4 — Phenotypic class counts
A_B_ = 9 cells (9/16) • A_bb = 3 cells (3/16) • aaB_ = 3 cells (3/16) • aabb = 1 cell (1/16). Total 16/16.
4.5 Ratio = 9 : 3 : 3 : 1.
Q5 — True / false with correction
5.1 True.
5.2 False. Correction: 9:3:3:1 is the phenotypic ratio. The corresponding genotypic ratio is 1:2:2:4:1:2:1:2:1.
5.3 False. Correction: independent assortment requires the two genes to be on different chromosomes. Genes on the same chromosome are linked and do not assort independently.
5.4 True.
Q6 — Cloze paragraph
6.1 independent assortment • 6.2 random • 6.3 bivalents • 6.4 metaphase • 6.5 different • 6.6 four • 6.7 9:3:3:1 • 6.8 linked.