IPA Kelas 9 : Pewarisan Sifat III (Hukum 2 Mendel : Persilangan Dihibrid)

Introduction

This tutorial focuses on the principles of dihybrid crosses as explained in the video "Pewarisan Sifat III (Hukum 2 Mendel : Persilangan Dihibrid)." Understanding dihybrid crosses is essential for mastering genetics and Mendelian inheritance. This guide will walk you through the key concepts and steps involved in performing and analyzing dihybrid crosses.

Step 1: Understand Key Terms

Before diving into dihybrid crosses, familiarize yourself with the following terms:

• Genes: Units of heredity that determine traits.
• Alleles: Different forms of a gene.
• Homozygous: Having two identical alleles for a trait.
• Heterozygous: Having two different alleles for a trait.
• Phenotype: The observable traits of an organism.
• Genotype: The genetic makeup of an organism.

Step 2: Review Mendel’s Laws

Mendel's laws of inheritance form the foundation of genetics. Pay special attention to:

• Law of Segregation: During gamete formation, alleles for a trait separate so that each gamete carries only one allele for each gene.
• Law of Independent Assortment: Genes for different traits are inherited independently of each other.

Step 3: Set Up a Dihybrid Cross

A dihybrid cross involves two traits. Follow these steps to set up your cross:

1. Choose Two Traits: For example, consider seed shape (round vs. wrinkled) and seed color (yellow vs. green).
2. Identify Alleles: Assign alleles for each trait:
   • Round (R) is dominant to wrinkled (r).
   • Yellow (Y) is dominant to green (y).
3. Determine Parental Genotypes: Use homozygous plants for simplicity:
   • Parent 1: RRYY (round yellow)
   • Parent 2: rryy (wrinkled green)

Step 4: Create a Punnett Square

Use a Punnett square to visualize the crosses:

1. Set Up the Square: Create a 4x4 grid.
2. Fill in Gametes: Write the possible gametes from each parent on the top and side.
   • Parent 1 gametes: RY, RY, RY, RY
   • Parent 2 gametes: ry, ry, ry, ry
3. Complete the Square: Fill in the grid by combining the gametes.

Step 5: Analyze the Results

Count the phenotypic ratios:

1. Phenotypes Expected:

   • Round Yellow: 9
   • Round Green: 3
   • Wrinkled Yellow: 3
   • Wrinkled Green: 1

2. Phenotypic Ratio: The expected ratio is 9:3:3:1.

Step 6: Solve Practice Problems

To solidify your understanding, practice with different traits and parental combinations. Here’s a simple problem:

• Cross a heterozygous round yellow (RrYy) plant with a homozygous wrinkled green (rryy) plant.

1. Set up the Punnett square.
2. Determine the phenotypic ratio.

Conclusion

In this tutorial, you learned about dihybrid crosses, including key genetic terms, Mendel’s laws, how to set up and analyze a dihybrid cross, and how to solve related practice problems. Understanding these concepts is crucial for further studies in genetics. For more in-depth learning, consider reviewing the additional videos in the series linked in the description.