Pewarisan Sifat Kelas 9 SMP (Part-3) Persilangan Dihibrid

Introduction

This tutorial provides a step-by-step guide to understanding dihibrid crosses, a key concept in genetics covered in the 9th-grade science curriculum. Dihybrid crosses help illustrate how two traits are inherited simultaneously, making it essential for mastering the fundamentals of heredity.

Step 1: Understand Key Genetic Terms

Before diving into dihibrid crosses, familiarize yourself with essential genetic terminology:

• Alleles: Different forms of a gene (e.g., A and a).
• Genotype: The genetic makeup of an individual (e.g., AA, Aa, or aa).
• Phenotype: The observable traits resulting from the genotype (e.g., tall or short plant).
• Homozygous: Having two identical alleles for a trait (e.g., AA or aa).
• Heterozygous: Having two different alleles for a trait (e.g., Aa).

Step 2: Learn About the Dihybrid Cross

A dihybrid cross involves two traits, each represented by two alleles. For example, consider the traits of seed shape (round vs. wrinkled) and seed color (yellow vs. green):

• Round seed shape (R) is dominant over wrinkled (r).
• Yellow seed color (Y) is dominant over green (y).

Example Genotypes

• Round Yellow: RrYy
• Wrinkled Green: rryy

Step 3: Set Up a Punnett Square

To visualize the results of a dihybrid cross, use a Punnett square:

1. Determine Parent Genotypes: Use two parents, e.g., RrYy x RrYy.
2. List Gametes: Each parent can produce four types of gametes: RY, Ry, rY, ry.
3. Draw the Punnett Square:
   • Create a 4x4 grid.
   • Label the rows with one parent's gametes and the columns with the other parent's.

Example Punnett Square

       RY    Ry    rY    ry
   -----------------------
RY |  RRYY  RRYy  RrYY  RrYy
   -----------------------
Ry |  RRYy  RRyy  RrYy  Rryy
   -----------------------
rY |  RrYY  RrYy  rrYY  rrYy
   -----------------------
ry |  RrYy  Rryy  rrYy  rryy

Step 4: Analyze the Results

Count the different genotypes and phenotypes from the square:

• Genotypes:

  • 1 RRYY
  • 2 RRYy
  • 2 RrYY
  • 4 RrYy
  • 1 rrYY
  • 2 Rryy
  • 1 rryy

• Phenotypes:

  • Round Yellow: 9 (RRYY + RRYy + RrYY + RrYy)
  • Round Green: 3 (Rryy)
  • Wrinkled Yellow: 3 (rrYY + RrYy)
  • Wrinkled Green: 1 (rryy)

Step 5: Practice with Exercises

To reinforce your understanding, practice with additional problems. Work through different parent combinations and create new Punnett squares. This will help solidify your grasp of dihibrid crosses.

Conclusion

Dihybrid crosses are a foundational concept in genetics, illustrating how two traits are inherited together. By mastering the setup of Punnett squares and analyzing results, you'll be well-prepared for more complex genetic topics. Continue practicing with various examples to enhance your skills and understanding further.