Genetics and Evolution
1. Introduction
Genetics is the study of heredity and variation. Evolution explains how species change over time. This chapter integrates Mendelian genetics with molecular mechanisms and evolutionary theory.
2. Mendel's Laws
2.1 Law of Segregation
Alleles separate during gamete formation, so each gamete carries only one allele for each trait.
2.2 Law of Independent Assortment
Alleles of different genes assort independently during gamete formation (applies only to genes on different chromosomes).
2.3 Monohybrid Cross
Cross between parents differing in one trait. F₂ ratio: 3:1 (dominant:recessive).
2.4 Dihybrid Cross
Cross between parents differing in two traits. F₂ ratio: 9:3:3:1.
3. Chromosomal Theory of Inheritance
Sutton and Boveri proposed that genes are located on chromosomes. The behaviour of chromosomes during meiosis parallels the segregation and independent assortment of alleles.
4. Linkage and Crossing Over
Linkage: Genes on the same chromosome tend to be inherited together. Crossing over: Exchange of segments between homologous chromosomes during prophase I of meiosis. Leads to recombination.
Recombination frequency is proportional to the distance between genes. Used to construct genetic maps (1% recombination = 1 map unit = 1 centimorgan).
5. Sex Determination
In humans: XX (female), XY (male). Males produce X and Y sperm; females produce X ova. Sex is determined by the sperm.
In birds: ZZ (male), ZW (female). In insects (grasshopper): XX (female), XO (male).
6. Mutations
Sudden heritable changes in DNA. Types:
- Gene mutations: Point mutations (base substitution, insertion, deletion).
- Chromosomal mutations: Deletion, duplication, inversion, translocation.
7. Pedigree Analysis
Study of inheritance patterns in families. Symbols: square (male), circle (female), shaded (affected).
8. DNA as Genetic Material
8.1 Griffith's Transformation Experiment
R strain (non-virulent) + heat-killed S strain (virulent) → mouse dies. Proved that some 'transforming principle' transfers genetic information.
8.2 Avery, MacLeod and McCarty
Identified the transforming principle as DNA (not protein or RNA).
8.3 Hershey-Chase Experiment
Used radioactive ³²P (DNA) and ³⁵S (protein) to infect E. coli with bacteriophage. Only ³²P entered the bacterial cell, confirming DNA as genetic material.
9. Theories of Evolution
Lamarckism: Inheritance of acquired characters (use and disuse). Example: Giraffe stretching neck.
Darwin's Theory: Natural selection — organisms with favourable variations survive and reproduce more.
Modern Synthetic Theory: Combines Darwinian selection with genetics. Sources of variation: mutation, recombination, gene flow, genetic drift.
10. Hardy-Weinberg Principle
Allele frequencies in a population remain constant from generation to generation in the absence of evolutionary influences.
p² + 2pq + q² = 1 (where p = dominant allele frequency, q = recessive allele frequency).
Factors disrupting equilibrium: Mutation, gene flow, genetic drift, natural selection, non-random mating.
11. Human Evolution
Humans evolved in Africa. Key ancestors: Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens.
12. Worked Problems
Problem 1: In a population, 16% show a recessive trait. Find allele frequencies. Solution: q² = 0.16 ⇒ q = 0.4. p = 1 - 0.4 = 0.6. Carriers: 2pq = 2×0.6×0.4 = 0.48.
13. Common Mistakes
'Students often forget that Mendel's law of independent assortment applies only to genes on different chromosomes. Linked genes on the same chromosome do not assort independently.'
14. ISC Exam Focus
| Topic | Theory Marks | Practical Marks |
|---|---|---|
| Mendel's laws | 4 | 2 |
| Linkage and mapping | 3 | 2 |
| DNA as genetic material | 3 | 1 |
| Evolution theories | 3 | 1 |
15. Self-Test Questions
- Explain Mendel's law of segregation with a monohybrid cross.
- What is the chromosomal theory of inheritance? How did Sutton and Boveri contribute?
- Describe the Hershey-Chase experiment and its significance.
- State the Hardy-Weinberg principle. What factors disturb its equilibrium?
- Distinguish between homologous and analogous organs with examples.
16. Quick Revision Chart
| Genetic Term | Definition |
|---|---|
| Allele | Alternative form of a gene |
| Homozygous | Same alleles at a locus (AA or aa) |
| Heterozygous | Different alleles at a locus (Aa) |
| Genotype | Genetic constitution |
| Phenotype | Observable characteristic |
| Dominant | Allele expressed in heterozygote |
| Recessive | Allele masked in heterozygote |
| Test cross | Cross with recessive homozygote |
| Back cross | Cross with parent genotype |
| Linkage | Genes on same chromosome inherited together |
| Recombination | New combination due to crossing over |
