BiologyChapter 12: Inheritance
Dihybrid Cross
Two genes, independent assortment (different chromosomes in the school model).
Learning objectives
- Explain dihybrid cross in clear biological language.
- Distinguish 1. gametes from 2. why 9-3-3-1.
- Use the key terms to answer a short Federal Board exam question.
Definition
Two genes, independent assortment (different chromosomes in the school model). RrYy x RrYy (round yellow peas school) gives 9 : 3 : 3 : 1 phenotypes if both show complete dominance and assort independently. 16 Punnett boxes. Parental types more common; recombinants from assortment. Dihybrid crosses test independent assortment: two genes on different chromosomes assort independently, giving the 9:3:3:1 ratio when both parents are doubly heterozygous.
Important terms
- DIHYBRID CROSS
- Two genes, independent assortment (different chromosomes in the school model).
- 1. GAMETES
- A heterozygote RrYy makes RY, Ry, rY, ry (equal if independent).
- 2. WHY 9-3-3-1
- 9: both dominant phenotypes.
Overview
Two letters, sixteen boxes:
Gametes
A heterozygote RrYy makes RY, Ry, rY, ry (equal if independent). FOIL-like list. If genes are linked, the ratio breaks (next chapter hint). RrYy heterozygote produces four gamete types: RY, Ry, rY, ry (equal proportions if genes on different chromosomes). 4 × 4 = 16-box Punnett for dihybrid.
Examples 16 boxes · 9 both dominant · 1 double recessive
Why 9-3-3-1
9: both dominant phenotypes. 3: first dominant second recessive. 3: the other way. 1: both recessive. Each 3 is a recombinant look. 916: both dominant traits. 316: first dominant, second recessive. 316: reversed. 116: both recessive. Each 3 category is a recombinant combination.
Examples R_Y_ is 9/16 · rryy is 1/16 · Not 3:1 of a monohybrid
WHAT — DIHYBRID CROSS
Two genes, independent assortment (different chromosomes in the school model). RrYy x RrYy (round yellow peas school) gives 9 : 3 : 3 : 1 phenotypes if both show complete dominance and assort independently. 16 Punnett boxes. Parental types more common; recombinants from assortment. Dihybrid crosses test independent assortment: two genes on different chromosomes assort independently, giving the 9:3:3:1 ratio when both parents are doubly heterozygous.
WHY — Why this topic matters
DIHYBRID CROSS is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.
HOW — How it works
GAMETES: A heterozygote RrYy makes RY, Ry, rY, ry (equal if independent). FOIL-like list. If genes are linked, the ratio breaks (next chapter hint). RrYy heterozygote produces four gamete types: RY, Ry, rY, ry (equal proportions if genes on different chromosomes). 4 × 4 = 16-box Punnett for dihybrid. Examples: 16 boxes; 9 both dominant; 1 double recessive. WHY 9-3-3-1: 9: both dominant phenotypes. 3: first dominant second recessive. 3: the other way. 1: both recessive. Each 3 is a recombinant look. 916: both dominant traits. 316: first dominant, second recessive. 316: reversed. 116: both recessive. Each 3 category is a recombinant combination. Examples: RY is 916; rryy is 116; Not 3:1 of a monohybrid. Worked phenotype fractions (R round, Y yellow). Round yellow: 916 Round green: 316 Wrinkled yellow: 316 Wrinkled green: 116
Worked example
- Worked phenotype fractions (R round, Y yellow).
- Round yellow: 916
- Round green: 316
- Wrinkled yellow: 316
- Wrinkled green: 116
In simple words
Independent assortment assumed. Linked genes wait for Ch 13.
Key points
- 9 : 3 : 3 : 1
- independent assortment
- four gamete types
- count boxes
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Dihybrid Cross
Sticky Notes · Quick Revision
Core Idea
- Two genes, independent assortment (different chromosomes in the school model). RrYy x RrYy (round yellow peas school) gives 9 : 3 : 3 : 1 phenotypes if both show complete dominance and assort independently. 16 Punnett boxes. Parental types more common; recombinants from assortment. Dihybrid crosses test independent assortment: two genes on different chromosomes assort independently, giving the 9:3:3:1 ratio when both parents are doubly heterozygous.
1. GAMETES
- A heterozygote RrYy makes RY, Ry, rY, ry (equal if independent). FOIL-like list. If genes are linked, the ratio breaks (next chapter hint). RrYy heterozygote produces four gamete types: RY, Ry, rY, ry (equal proportions if genes on different chromosomes). 4 × 4 = 16-box Punnett for dihybrid.
2. WHY 9-3-3-1
- 9: both dominant phenotypes. 3: first dominant second recessive. 3: the other way. 1: both recessive. Each 3 is a recombinant look. 916: both dominant traits. 316: first dominant, second recessive. 316: reversed. 116: both recessive. Each 3 category is a recombinant combination.
Exam Example
- Worked phenotype fractions (R round, Y yellow).
- Round yellow: 916
- Round green: 316
- Wrinkled yellow: 316
- Wrinkled green: 116
Common Misconception
- Do not treat 1. GAMETES and 2. WHY 9-3-3-1 as the same idea; each explains a different part of the topic.
FBISE Exam Check
- Define dihybrid cross, explain the two key parts, and use the correct biological terms.
- Exam reminder: Worked phenotype fractions (R round, Y yellow).
60-second revision
- Two genes, independent assortment (different chromosomes in the school model).
- 1. GAMETES: A heterozygote RrYy makes RY, Ry, rY, ry (equal if independent).
- 2. WHY 9-3-3-1: 9: both dominant phenotypes.
- Exam reminder: Worked phenotype fractions (R round, Y yellow).
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.