🌱 Mendel’s Laws
🔬 What is a Monohybrid Cross?
A monohybrid cross is a genetic cross between two individuals focusing on one trait (e.g., plant height – tall or short).
Gregor Mendel studied inheritance patterns by crossing pure-breeding pea plants with contrasting traits, such as:
Tall (TT) × Short (tt)
🧬 Mendel’s First Law: Law of Dominance
🔹 Statement:
When two different alleles (e.g., T and t) are present in an organism, only the dominant trait (e.g., tallness) is expressed in the F₁ generation, while the recessive trait (shortness) is hidden.
🧪 Example:
TT (Tall) × tt (Short) → All offspring in F₁ = Tt (Tall)
✅ Observation: All plants are tall because T is dominant over t.
🧬 Mendel’s Second Law: Law of Segregation
🔹 Statement:
During gamete formation, the two alleles for a trait separate (segregate) from each other, so that each gamete receives only one allele.
🧪 Example in F₂ generation:
When F₁ plants (Tt) are self-crossed:
| Parent | Gametes | Cross | Result |
|---|---|---|---|
| Tt × Tt | T, t | Tt × Tt | TT, Tt, Tt, tt |
Genotypic Ratio: 1 TT: 2 Tt: 1 tt
Phenotypic Ratio: 3 Tall: 1 Short
✅ Observation: The recessive trait (shortness) reappears in 25% of F₂ plants.
📊 Summary Table:
| Generation | Genotype | Phenotype |
|---|---|---|
| P (Parent) | TT × tt | Tall × Short |
| F₁ | All Tt | All Tall |
| F₂ | TT, Tt, tt | 3 Tall: 1 Short |
🧠 Key Takeaways:
Dominant traits mask recessive ones in F₁.
Alleles segregate during gamete formation.
Traits reappear in predictable ratios in F₂.
🧬 📘 Diagram: Monohybrid Cross (Tall vs. Short Pea Plants)
Parental Generation (P):
TT (Tall) × tt (Short)
↓ ↓
Gametes: T Gametes: t
F₁ Generation:
T × t → All Tt (Tall)
All offspring are tall in F₁ because T (Tall) is dominant.
F₂ Generation (Selfing of F₁: Tt × Tt):
| T (from one parent) | t (from one parent) | |
|---|---|---|
| T (from other parent) | TT (Tall) | Tt (Tall) |
| t (from other parent) | Tt (Tall) | tt (Short) |
➡ Genotypic Ratio = 1:2:1 (TT: Tt: tt)
➡ Phenotypic Ratio = 3:1 (Tall: Short)