📚 Class‑12 CBSE Biology – Genetics & Evolution
(Chapter 11 – Genetics & Evolution – 2024/25)
Aim: Understand the fundamentals of heredity, the mechanisms that generate genetic diversity, and the processes that drive evolution.
1. Simple Explanation
| Topic | What it means |
|---|---|
| Genetics | The study of how traits are passed from parents to offspring through genes (DNA segments). |
| Evolution | The change in the genetic composition of a population over successive generations, leading to the diversification of life. |
| Natural Selection | A key evolutionary force: traits that enhance survival/reproduction increase in frequency. |
| Molecular Basis | DNA → RNA → Protein is the central dogma; mutations are the raw material for evolution. |
2. Key Points
2.1 DNA & Gene Expression
- DNA Structure – Double helix, nucleotides (A, T, C, G).
- Replication – Semi‑conservative; DNA polymerase synthesises new strands.
- Transcription – DNA → mRNA (RNA polymerase).
- Translation – mRNA → Protein (ribosomes, tRNA).
- Gene Regulation – Promoters, enhancers, silencers, transcription factors.
2.2 Mendelian Genetics
| Concept | Symbol | Example |
|---|---|---|
| Alleles | A, a | Dominant (A), Recessive (a) |
| Genotype | Aa | Phenotype: Dominant trait |
| Phenotype | Tall | Observable trait |
| Punnett Square | Visual tool | Predict offspring genotypes |
| Hardy–Weinberg | (p^2 + 2pq + q^2 = 1) | Equilibrium conditions |
2.3 Non‑Mendelian Genetics
- Incomplete Dominance – Heterozygote shows intermediate phenotype.
- Codominance – Both alleles expressed (e.g., AB blood type).
- Multiple Alleles – More than two alleles for a gene (blood group).
- Polygenic Traits – Many genes contribute to a single trait (skin colour).
- Sex‑linked Inheritance – Genes on sex chromosomes (X‑linked recessive).
2.4 Gene Linkage & Recombination
- Genes close together on same chromosome tend to be inherited together.
- Recombination Frequency ( \frac{\text{cross‑overs}}{\text{total meioses}} \times 100% )
- Genetic Map Units (cM) – 1 cM ≈ 1% recombination.
2.5 Genetic Markers & Mapping
- DNA Markers – RFLPs, SSRs, SNPs.
- Mapping – Use recombination frequency to determine gene order and distance.
2.6 Evolutionary Mechanisms
| Mechanism | How it works | Example |
|---|---|---|
| Natural Selection | Differential reproduction | Peppered moth colour adaptation |
| Genetic Drift | Random fluctuations | Bottleneck effect in small populations |
| Gene Flow | Migration of alleles | Allele frequency change in island species |
| Mutation | Random DNA changes | Sickle‑cell allele in humans |
| Recombination | Creates new allele combinations | Haplotypes in humans |
2.7 Speciation
- Allopatric – Geographic isolation → divergent evolution.
- Sympatric – No geographical barrier (polyploidy in plants).
- Parapatric – Adjacent populations with some gene flow.
- Peripatric – Small isolated population; strong drift + selection.
2.8 Evidence for Evolution
- Fossil Record – Transitional fossils.
- Comparative Anatomy – Homologous structures.
- Embryology – Similar early stages in vertebrates.
- Molecular Biology – Similar DNA/protein sequences.
- Biogeography – Distribution patterns of species.
2.9 Molecular Evolution
- Molecular Clock – DNA mutations accumulate at a constant rate; used to estimate divergence time.
- Protein Evolution – Changes in amino acid sequences; can infer functional changes.
2.10 Biotechnology & Ethics
- Genetic Engineering – CRISPR‑Cas, GM crops.
- Ethical Issues – GMOs, cloning, gene therapy, consent.
3. Worked Example
3.1 Dihybrid Cross (Monohybrid × Monohybrid) – Incomplete Dominance
Parental genotypes:
- Plant A: ( A^a B^b ) (red petals, yellow flowers)
- Plant B: ( a^a b^b ) (white petals, pink flowers)
Goal: Predict phenotypic ratio of F₂.
Step 1: Understand alleles
| Gene | Alleles | Phenotype |
|---|---|---|
| Petal colour | (A^a) (red) | Red |
| Flower colour | (B^b) (yellow) | Yellow |
*Incomplete dominance