Heredity & Evolution – Class 10 (CBSE)
(Study Notes – IIT‑JEE / Competitive Exam Focus)
1. Overview
| Term | Definition |
|---|---|
| Heredity | Transmission of traits from parents to offspring |
| Evolution | Change in the frequency of alleles in a population over successive generations |
| Genotype | Genetic constitution (alleles) of an organism |
| Phenotype | Observable characteristics (traits) of an organism |
Key Idea – Heredity explains how traits are passed on; evolution explains why and how populations change over time.
2. Mendelian Genetics – Foundations
2.1. Mendel’s Laws
| Law | Statement |
|---|---|
| Law of Segregation | Alleles segregate independently during gamete formation |
| Law of Independent Assortment | Genes for different traits assort independently (except for linked genes) |
| Law of Dominance | In a heterozygote, one allele masks the expression of the other |
2.2. Punnett Square – Monohybrid Cross
| A (dominant) | a (recessive) | |
|---|---|---|
| A | AA | Aa |
| a | aA | aa |
- Genotype ratio: 1 : 2 : 1
- Phenotype ratio: 3 : 1 (if A is dominant)
2.3. Dihybrid Cross (2 × 2)
| Aa | Aa | |
|---|---|---|
| Aa | AA Aa Aa aa | AA Aa Aa aa |
| Aa | AA Aa Aa aa | AA Aa Aa aa |
- Genotype ratio: 9 : 3 : 3 : 1
- Phenotype ratio: 9 : 3 : 3 : 1 (for two independent dominant traits)
2.4. Incomplete Dominance & Codominance
| Scenario | Example | Genotype | Phenotype |
|---|---|---|---|
| Incomplete Dominance | Red + white petal → pink | Rr | Pink |
| Codominance | AB blood type (A + B) | AB | Type AB |
2.5. Multiple Alleles & Sex‑Linked Traits
| Trait | Alleles | Genotypes | Phenotype |
|---|---|---|---|
| ABO blood group | I<sup>A</sup>, I<sup>B</sup>, i | IAIA, IAi, IBIB, IBi, ii | A, B, AB, O |
| Color‑blindness | X<sup>C</sup>, X<sup>c</sup> | X<sup>C</sup>Y, X<sup>c</sup>Y, X<sup>C</sup>X<sup>c</sup>, X<sup>c</sup>X<sup>c</sup> | Normal, color‑blind (male), normal (female), color‑blind (female) |
3. Hardy‑Weinberg Equilibrium (HWE)
3.1. Assumptions
- Infinite population size (no genetic drift)
- No mutation, migration, or selection
- Random mating
- No linkage between loci
3.2. Derivation
Let p = frequency of allele A, q = frequency of allele a (p + q = 1).
| Genotype | Frequency |
|---|---|
| AA | p² |
| Aa | 2pq |
| aa | q² |
Proof
p² + 2pq + q² = (p + q)² = 1² = 1.- The genotype frequencies remain constant across generations if the assumptions hold.
3.3. Deviations from HWE
| Cause | Effect on Genotype Frequencies |
|---|---|
| Selection | Skews frequencies toward advantageous alleles |
| Mutation | Introduces new alleles, changes p & q |
| Migration | Alters allele frequencies via gene flow |
| Genetic Drift | Random fluctuations, especially in small populations |
| Non‑random Mating | Increases homozygosity (inbreeding) |
3.4. Important Formulae
| Formula | Meaning |
|---|---|
p + q = 1 |
Sum of allele frequencies |
p² + 2pq + q² = 1 |
Sum of genotype frequencies |
q = √(q²) |
Estimate of allele frequency from phenotype frequency (e.g., recessive) |
p = 1 – q |
Complementary allele frequency |