Mendel’s Contributions to Heredity
Gregor Johann Mendel (1822-1884), an Austrian monk, is known as the **”Father of Genetics”** for his groundbreaking work on heredity. His experiments with pea plants laid the foundation for understanding how traits pass from parents to offspring.
## Why Mendel Chose Pea Plants
Mendel selected garden pea (*Pisum sativum*) for his experiments because:
– They had several contrasting traits that were easy to observe (tall vs dwarf, round vs wrinkled seeds)
– They had a short life span, allowing multiple generations to be studied quickly
– They produced many offspring
– They could self-pollinate as well as cross-pollinate
– Pure-breeding varieties were available
Mendel’s Experiments
Mendel conducted cross-pollination experiments with pea plants showing contrasting characteristics. He studied seven pairs of contrasting traits:
1. Plant height – Tall vs Dwarf
2. Seed shape – Round vs Wrinkled
3. Seed color – Yellow vs Green
4. Pod shape – Inflated vs Constricted
5. Pod color – Green vs Yellow
6. Flower position – Axial vs Terminal
7. Flower color – Violet vs White
Monohybrid Cross
Mendel crossed pure-breeding tall plants with pure-breeding dwarf plants. His observations were:
**F₁ Generation (First Filial):** All offspring were tall. No dwarf plants appeared, showing that tallness was dominant over dwarfness.
**F₂ Generation (Second Filial):** When F₁ plants self-pollinated, both tall and dwarf plants appeared in the ratio of approximately 3:1 (three tall plants for every one dwarf plant).
## Mendel’s Laws of Inheritance
From his experiments, Mendel proposed three fundamental laws:
### 1. Law of Dominance
When two contrasting traits are crossed, only one trait (the dominant trait) expresses itself in the F₁ generation, while the other trait (recessive trait) remains hidden. The recessive trait reappears in the F₂ generation.
### 2. Law of Segregation
The two alleles (forms of a gene) for a trait separate during gamete formation, so each gamete receives only one allele. During fertilization, offspring receive one allele from each parent, restoring the pair.
### 3. Law of Independent Assortment
When two or more traits are considered together, the inheritance of one trait is independent of the inheritance of another trait. This law applies to genes located on different chromosomes.
## Key Terminology Introduced
– **Trait:** A characteristic feature (like height or seed color)
– **Alleles:** Different forms of a gene (like tall and dwarf)
– **Dominant trait:** The trait that expresses itself (represented by capital letter, e.g., T)
– **Recessive trait:** The trait that remains hidden in presence of dominant allele (represented by small letter, e.g., t)
– **Genotype:** Genetic makeup (TT, Tt, or tt)
– **Phenotype:** Physical appearance (tall or dwarf)
– **Homozygous:** Both alleles are same (TT or tt)
– **Heterozygous:** Alleles are different (Tt)
## Significance of Mendel’s Work
Mendel’s work remained unrecognized during his lifetime but was rediscovered in 1900. His contributions helped scientists understand that hereditary information is passed in discrete units (now called genes), forming the basis of modern genetics and explaining how variation occurs in populations, which is essential for evolution.