Inheritance of One Gene 🧬
1. Mendel’s Classic Monohybrid Cross 🌿
Mendel crossed tall pea plants (TT) with dwarf plants (tt). Every F1 seedling grew tall, showing that the tall trait (T) dominates the dwarf trait (t). :contentReference[oaicite:0]{index=0}
- F1 genotype:
Tt(all tall) - F2 phenotypes after self-pollinating
Tt × Tt: 3 tall : 1 dwarf - F2 genotypes:
TT : Tt : tt = 1 : 2 : 1
The same 3 : 1 phenotypic pattern appeared for every contrasting pair of traits Mendel studied. :contentReference[oaicite:1]{index=1}
2. Genes, Alleles & Helpful Terms 📖
- Gene
- A heritable unit that carries information for a trait.
- Alleles
- Different forms of the same gene (e.g.,
Tandt). - Genotype
- The allele pair an organism has (
TT,Ttortt). - Phenotype
- The visible trait (tall or dwarf).
- Homozygous
- Both alleles identical (
TTortt). - Heterozygous
- Alleles different (
Tt).
3. Punnett Square Magic 📊
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Probability view: \( \Bigl(\tfrac{1}{2}T + \tfrac{1}{2}t\Bigr)^{2} = \tfrac{1}{4}TT + \tfrac{1}{2}Tt + \tfrac{1}{4}tt \) 🍀
4. Mendel’s Two Big Rules ⚖️
- Law of Dominance – In a heterozygote, one allele (dominant) masks the other (recessive). :contentReference[oaicite:2]{index=2}
- Law of Segregation – Allele pairs separate during gamete formation, so each gamete carries only one allele. :contentReference[oaicite:3]{index=3}
5. Test Cross 🔍
To uncover whether a tall plant is TT or Tt, cross it with a recessive (tt) plant:
- If all offspring are tall → parent was
TT. - If offspring split 1 tall : 1 dwarf → parent was
Tt.
Quick, definitive, and NEET-friendly! :contentReference[oaicite:4]{index=4}
6. Beyond Simple Dominance 🌈
6.1 Incomplete Dominance 🌸
Snapdragon cross: red (RR) × white (rr) → F1 pink (Rr). Self-pollination gives 1 red : 2 pink : 1 white. The heterozygote shows an intermediate color. :contentReference[oaicite:5]{index=5}
6.2 Co-dominance 🩸
Human ABO blood group gene I has three alleles: IA, IB, i.
| Genotype | Blood Type |
|---|---|
| IAIA, IAi | A |
| IBIB, IBi | B |
| IAIB | AB (both sugars expressed) |
| ii | O (no sugar) |
With IAIB, both A and B sugars appear together—no blending, true co-dominance. :contentReference[oaicite:6]{index=6}
6.3 Multiple Alleles & Pleiotropy 🌀
- ABO system shows multiple alleles—three versions of one gene exist in the population.
- One gene can affect more than one trait. Example: pea seed gene
B/bcontrols both starch-grain size and seed shape. :contentReference[oaicite:7]{index=7}
Important Concepts for NEET 🚀
- The monohybrid 3 : 1 phenotype and 1 : 2 : 1 genotype ratios.
- Law of Segregation explains why recessive traits re-appear in F2.
- A test cross (dominant × recessive) instantly reveals unknown genotypes.
- Recognize complete dominance vs. incomplete dominance vs. co-dominance.
- Multiple alleles (ABO) and pleiotropy (starch gene) widen inheritance patterns.
Happy studying and keep exploring genetics 😊!
