Elements of Heredity and Variations: Historical Background and Mendel’s Laws of Inheritance – Part 2

 

Introduction

In Part 1, we explored the historical background of heredity and the groundbreaking contributions of Gregor Mendel. In this part, we study about the core of Mendel’s work – his famous Laws of Inheritance which laid the foundation of classical genetics.

Mendel’s Experiments: An Overview

Mendel studied seven contrasting traits in pea plants such as:

1. Seed shape (Round/Wrinkled)

2. Plant height (Tall/Dwarf)

3. Flower color (Purple/White)

He conducted monohybrid and dihybrid crosses, carefully recording the appearance of traits in successive generations.

Mendel’s Three Laws of Inheritance

1. Law of Dominance

Definition: When two different alleles (forms of a gene) are present in a pair, one is dominant and its trait is expressed, while the other is recessive and masked.

Example: In a cross between tall (TT) and dwarf (tt) pea plants, all offspring (Tt) are tall.

Dominant allele = T (tall)   and Recessive allele = t (dwarf)

2. Law of Segregation

Definition: During gamete formation, the two alleles for a trait separate, so each gamete carries only one allele.

This explains why recessive traits can reappear in the F2 generation.

Example: Crossing Tt × Tt gives a 3:1 ratio in the next generation (Tall: Dwarf).

3. Law of Independent Assortment

Definition: Alleles of different traits are distributed independently of each other during gamete formation.

This law was derived from dihybrid crosses, such as:

• Round Yellow (RRYY) × Wrinkled Green (rryy)

The F2 generation displayed a 9:3:3:1 ratio:

9 Round Yellow
3 Round Green
3 Wrinkled Yellow
1 Wrinkled Green

Importance of Mendel’s Laws

1. Foundation of modern genetics

2. Explained inheritance patterns

3. Led to the discovery of genes and alleles

4. Inspired later scientists like Bateson, Punnett, and Morgan

Limitations of Mendel’s Laws

Although revolutionary, Mendel’s laws don’t explain:

1. Incomplete dominance (e.g., pink flowers from red × white)

2. Codominance (e.g., AB blood group)

3. Polygenic inheritance (e.g., height, skin color)

4. Linked genes (genes located close together on the same chromosome)

These were addressed by Neo-Mendelian genetics and molecular biology.

Conclusion (Part 2)

Mendel’s principles revolutionized the understanding of heredity and remain central to genetics today. They explain how traits are passed on and predict genetic outcomes, laying the foundation for fields like genetic engineering, medicine, and evolutionary biology.

🔗 Explore More:

1. Human Inheritance Patterns

2. Chromosomes and DNA

3. Modern Modifications to Mendel’s Laws