SSC CGL - Detailed Guide 2025
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Genetics and Evolution
Reference: Lucent GK, NCERT Class 6–12
Genetics is the study of heredity and variation. Evolution is the gradual change in species over time, leading to the development of new species.
1. Mendel's Laws of Inheritance
Gregor Johann Mendel, known as the Father of Genetics, conducted hybridization experiments on pea plants (Pisum sativum).
Key Terms:
- Gene: Unit of inheritance
- Alleles: Different forms of a gene (e.g., Tall (T) vs Dwarf (t))
- Genotype: Genetic makeup (TT, Tt, tt)
- Phenotype: Observable trait (Tall, Dwarf)
- Homozygous: Same alleles (TT or tt)
- Heterozygous: Different alleles (Tt)
- Dominant: Expressed even if one allele is present (T)
- Recessive: Expressed only when both alleles are recessive (t)
Mendel's Experiments:
- He crossed pure tall (TT) and pure dwarf (tt) plants
- F1 Generation: All tall (Tt) — dominant trait expressed
- F2 Generation: 3 tall : 1 dwarf — classic monohybrid ratio
Mendel's Laws:
| Law | Explanation |
|---|---|
| 1. Law of Dominance | In a heterozygote, dominant trait masks the recessive one |
| 2. Law of Segregation | Alleles segregate independently during gamete formation |
| 3. Law of Independent Assortment | Genes for different traits assort independently of one another |
Importance: Mendel's laws are the foundation of modern genetics.
2. DNA and Chromosomes
DNA (Deoxyribonucleic Acid):
| Feature | Description |
|---|---|
| Function | Carries genetic information |
| Location | Mostly in the nucleus (some in mitochondria) |
| Structure | Double helix (Watson and Crick model) |
| Components | Sugar (deoxyribose), Phosphate, Nitrogenous Bases |
| Bases | Adenine (A), Thymine (T), Cytosine (C), Guanine (G) |
| Base pairing rule | A–T and C–G |
Gene: Segment of DNA that codes for a specific protein
RNA (Ribonucleic Acid):
- Single-stranded
- Sugar: Ribose
- Bases: A, U (uracil), C, G
- Helps in protein synthesis
Proteins are synthesized in ribosomes using instructions from DNA (via RNA).
Chromosomes:
| Feature | Description |
|---|---|
| Made of | DNA + Proteins (histones) |
| Found in | Nucleus of every cell |
| Human cells contain | 46 chromosomes (23 pairs) |
| Sex chromosomes | XX (female), XY (male) |
3. Evolution Basics
Evolution is the gradual process by which living organisms develop and diversify from earlier forms.
Key Concepts:
| Term | Explanation |
|---|---|
| Variation | Differences among individuals of a species (due to mutation, recombination) |
| Natural Selection | Proposed by Charles Darwin — survival of the fittest |
| Speciation | Formation of new species from existing ones |
| Fossils | Remains of ancient organisms; evidence for evolution |
| Homologous Organs | Same structure, different function (e.g. forelimbs of humans and bats) |
| Analogous Organs | Different structure, same function (e.g. wings of birds and insects) |
| Vestigial Organs | Rudimentary organs (e.g. human appendix, wisdom teeth) |
Theories of Evolution:
- Lamarckism: Acquired characters are inherited (disproven)
- Darwinism: Natural selection — organisms better adapted to the environment survive and reproduce
Modern Theory of Evolution = Darwinism + genetics (Neo-Darwinism)
Evidences of Evolution:
- Fossils (archaeopteryx)
- Comparative anatomy (homologous/analogous organs)
- Embryology (similar early development)
- Molecular biology (DNA similarities)
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