Dihybrid Cross

Dihybrid Cross

What is it?

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.

In simple words

• 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).

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.

What you should learn

• 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.

Examples

• 16 boxes
• 9 both dominant
• 1 double recessive

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.

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.

Full school-level explanation

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.

Two letters, sixteen boxes:

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

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.

Remember

• four gamete types

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.

Remember

• count boxes

Examples

• R_Y is 916
• rryy is 116
• Not 3:1 of a monohybrid

Remember

• 9 : 3 : 3 : 1
• independent assortment
• 916, 316, 316, 116
• independent only
• four gamete types
• count boxes
• 9 : 3 : 3 : 1
• independent assortment

How the ideas connect

Worked phenotype fractions (R round, Y yellow). Round yellow: 916 Round green: 316 Wrinkled yellow: 316 Wrinkled green: 116

Deeper understanding

Go beyond memorising Dihybrid Cross. Connect the definition to reasons, relationships, exceptions, and exam wording.

How the ideas connect

Worked phenotype fractions (R round, Y yellow). Round yellow: 916 Round green: 316 Wrinkled yellow: 316 Wrinkled green: 116

Common mistakes

Exam focus

Core Idea

1. GAMETES

2. WHY 9-3-3-1

Exam Example

Exam focus

Quick revision

• 9 : 3 : 3 : 1
• independent assortment
• 916, 316, 316, 116
• independent only
• 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).

Independent assortment assumed. Linked genes wait for Ch 13.

For long answers on Dihybrid Cross: define clearly → explain the main parts → give one example → state one exam caution.