Unit 4.1 – Mendel’s Laws of Inheritance 🌱
1 Gregor Mendel’s Ground-breaking Work 🧑🌾
- Mendel performed systematic hybridisation experiments on garden pea (Pisum sativum) for seven seasons (1856 – 1863), recording thousands of plants and analysing the results with statistics 📊 :contentReference[oaicite:0]{index=0}.
- His large sample size and repeated crosses across generations gave universal rules rather than chance observations :contentReference[oaicite:1]{index=1}.
2 Why the Pea Plant Was Perfect 🌿
- Availability of many true-breeding lines (self-pollinated for generations so the same trait reappears) :contentReference[oaicite:2]{index=2}.
- Seven clear characters, each expressed as two contrasting traits (e.g., tall / dwarf, round / wrinkled seeds) made scoring unambiguous :contentReference[oaicite:3]{index=3}.
- Short life cycle, easy artificial pollination and abundant offspring meant quick, reliable data.
| Character | Trait 1 | Trait 2 |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
3 Key Genetic Terms 🔑
- True-breeding line – gives the same trait on self-pollination for many generations 🌾.
- Filial generations – F1 = first hybrid generation, F2 = offspring of self-pollinated F1.
- Dominant trait – appears in F1; recessive trait – masked in F1 but re-emerges in F2.
4 Mendel’s Classic One-Gene Cross 🔬
- Parental (P) cross: Tall × Dwarf plants.
- F1 observation: All offspring are tall – the dwarf form disappears 🌱.
- F1 self-pollination → F2 results: Tall : Dwarf = \(3:1\) (\(3/4\) tall, \(1/4\) dwarf) :contentReference[oaicite:4]{index=4}.
- No blending: every F2 plant is clearly tall or clearly dwarf.
Interpretation 😊
- Each trait is governed by a pair of factors (today called alleles).
- The two factors separate (segregate) during gamete formation, so each gamete gets only one factor.
- When dominant (T) and recessive (t) meet in F1 (Tt), the dominant hides the recessive. Segregation in F2 recreates all possible combinations.
5 Mendel’s Fundamental Laws 📜
- Law of Dominance – one factor in a pair can mask the other in the hybrid.
- Law of Segregation – the two factors of a trait segregate independently into gametes; a single gamete carries only one of the pair.
(These two laws explain the \(3:1\) monohybrid ratio seen above.)
6 Why Mendel’s Approach Still Matters 💡
- Introduced controlled crosses and quantitative analysis to biology.
- Provided a predictive framework still used in Punnett squares and modern genetics.
- Highlighted the importance of choosing clear-cut traits and large data sets.
7 High-Yield NEET Nuggets 🎯
- Monohybrid ratio \(3:1\) in F2 is a must-know.
- Definitions of dominant, recessive, allele and true-breeding line.
- Pea plant advantages – short generation time, seven contrasting traits.
- Law of Segregation – foundation for understanding Mendelian questions.
