Pewarisan Sifat Kelas 9 SMP (Part-2) Persilangan Monohibrid

Introduction

This tutorial focuses on the inheritance of traits, specifically monohybrid crosses, which is a key concept in biology for Grade 9 students. Understanding monohybrid crosses is essential for grasping how traits are passed from parents to offspring.

Step 1: Understanding Monohybrid Crosses

Monohybrid crosses involve a single trait that is being examined. Here’s how to approach it:

• Identify the Traits: Choose a trait to study, such as flower color or seed shape.

Determine Parental Genotypes

• Use capital letters for dominant traits (e.g., "R" for round seeds) and lowercase letters for recessive traits (e.g., "r" for wrinkled seeds).
• Example: If you are crossing round seeds (RR) with wrinkled seeds (rr), the parental genotypes are RR and rr.

Step 2: Creating a Punnett Square

A Punnett square is a helpful tool for predicting the genetic outcomes of a cross.

1. Draw the Square: Create a 2x2 grid.

2. Label the Rows and Columns:

   • Write one parent's alleles across the top (e.g., R and R).
   • Write the other parent's alleles down the side (e.g., r and r).

3. Fill in the Squares:

   • Combine the alleles from the top and side to fill in the squares.
   • The resulting combinations will show the possible genotypes of the offspring.

   Example:

       R   R
     ---------
   r | Rr | Rr |
     ---------
   r | Rr | Rr |
   

Step 3: Analyzing the Results

Once the Punnett square is filled, analyze the results:

• Determine Genotypic Ratio: Count the combinations of genotypes. For example, in the Punnett square above, all offspring are Rr (heterozygous).
• Determine Phenotypic Ratio: Identify the physical traits of the offspring. In this case, all offspring will display the dominant trait (round seeds).

Step 4: Exploring Intermediate Inheritance

Intermediate inheritance occurs when traits blend rather than showing dominant/recessive patterns.

• Example Trait: Flower color (e.g., red and white flowers producing pink flowers).
• Genotypes: Use R for red, W for white.
• Cross: RR (red) with WW (white).
• Punnett Square: All offspring will have RW (pink flowers).

Step 5: Practice with Example Problems

To solidify your understanding, practice with various problems:

1. Crossing Different Traits: Try different traits to see how the outcomes vary.
2. Use of Punnett Squares: Create Punnett squares for each new cross you attempt.

Conclusion

Understanding monohybrid crosses and intermediate inheritance is crucial for mastering genetics. By using Punnett squares, you can predict the inheritance patterns of traits. Practice with various traits will enhance your understanding and prepare you for more complex genetic concepts in the future. Start exploring different genetic crosses to deepen your knowledge!