Dihybrid Crosses Worksheet
10th Grade Biology Unit 4: Mendelian Genetics
Dihybrid Crosses Worksheet
Tracking Two Independent Traits & Applying Mendel's Laws
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Part 1: Gamete Determination (The FOIL Method)
Mendel's Law of Independent Assortment
During meiosis, alleles for two different traits segregate independently. For an individual with genotype AaBb, determine gametes using FOIL: First (AB), Outer (Ab), Inner (aB), Last (ab).
Genotype: RrYy → Gametes:
Genotype: TtPP → Gametes:
1 Problem 1: Classic Pea Plant Cross (Pisum sativum)
9 : 3 : 3 : 1 Pattern Analysis
In pea plants, seed shape is governed by the gene R (Round is dominant to wrinkled, r). Seed color is governed by the gene Y (Yellow is dominant to green, y). Cross two plants that are both heterozygous for both traits: RrYy × RrYy.
Complete the 16-box dihybrid grid: ♀ Gametes × ♂ Gametes
Phenotypic Tally Table:
| Phenotype | Count |
|---|
| Round, Yellow | / 16 |
| Round, Green | / 16 |
| Wrinkled, Yellow | / 16 |
| Wrinkled, Green | / 16 |
Phenotypic Ratio:
: : :
a) Probability of homozygous recessive (rryy) offspring:
Fraction:
Percentage:
b) Fraction of offspring showing at least one dominant trait:
High School Biology • Mendelian Inheritance Page 1 of 2
Dihybrid Crosses Practice | Part 2: Complex Scenarios & Mechanistic Reasoning
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2 Problem 2: Fruit Fly Cross (Drosophila melanogaster)
Non-Identical Parental Cross
In fruit flies, gray body color (G) is dominant to black body color (g), and normal wings (V) are dominant to vestigial wings (v). A male fly heterozygous for body color with vestigial wings (Ggvv) is crossed with a female fly that is heterozygous for both traits (GgVv).
Male (Ggvv) Gametes:
Female (GgVv) Gametes:
Construct the 16-box Punnett square:
Expected Phenotypes:
| Phenotype | Fraction |
|---|
| Gray body, Normal wings | / 16 |
| Gray body, Vestigial wings | / 16 |
| Black body, Normal wings | / 16 |
| Black body, Vestigial wings | / 16 |
What percent of offspring will be black with vestigial wings?
Answer:
Explain why the 9:3:3:1 ratio does not appear in this cross:
3 Problem 3: Genetic Mystery & Scientific Reasoning
Evidence-Based Claim
A botanist crosses two tomato plants of unknown genotypes. Plant height is governed by gene T (Tall is dominant to dwarf, t) and stem texture is governed by gene H (Hairy is dominant to hairless, h). Out of 400 total offspring, the botanist counts: 204 Tall & Hairy, 196 Tall & Hairless, 0 Dwarf & Hairy, and 0 Dwarf & Hairless.
A. Deduce the exact genotypes of both parent plants based on the phenotypic evidence:
Parent 1 Genotype:
Parent 2 Genotype:
B. Mechanistic Justification: State Mendel's Law of Independent Assortment and explain the cellular event in meiosis (anaphase I) that causes non-homologous chromosomes to distribute independently:
High School Biology • Dihybrid Inheritance & Probability Page 2 of 2
Dihybrid Crosses Answer Key
Teacher Resource 10th Grade Biology
Dihybrid Crosses Answer Key
Master Solutions, Punnett Grids, Ratios & Grading Guidance
Part 1: Gamete Determination Answers (The FOIL Method)
2 Pts Total
Alleles separate independently during meiosis. Verify students list all 4 combinations (order does not matter):
Genotype RrYy:
RY, Ry, rY, ry
Genotype TtPP:
TP, TP, tP, tP (or TP, tP)
1 Problem 1: Pea Plant Cross Solutions (RrYy × RrYy)
10 Pts Total
Completed 16-Box Grid: 4 pts for correct genotypes
| ♀ \ ♂ | RY | Ry | rY | ry |
|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
Phenotypic Tally Key:
| Phenotype | Count |
|---|
| Round, Yellow | 9 / 16 |
| Round, Green | 3 / 16 |
| Wrinkled, Yellow | 3 / 16 |
| Wrinkled, Green | 1 / 16 |
Expected Phenotypic Ratio:
9 : 3 : 3 : 1
a) Probability of homozygous recessive (rryy):
Fraction: 1/16 • Percent: 6.25%
b) At least one dominant trait:
15 / 16 (93.75%) (16 − 1 rryy)
Teacher Notes & Common Student Misconceptions
- Allele Grouping: Watch for students mixing traits like RYry. Remind them homologous gene pairs stay together: write the R's first (Rr), then the Y's (Yy).
- Capitalization Convention: Ensure students write capital (dominant) alleles first in each pair (Rr, not rR).
High School Biology • Teacher Answer Key Page 1 of 2
Answer Key Dihybrid Crosses Solutions
