Heredity and Evolution

Heredity and Evolution is the most intellectually rich chapter in Class 10 Science — and one of the most policy-relevant for UPSC. Genetics underpins biotechnology (GM crops, CRISPR gene editing, gene therapy), forensic science, prenatal diagnosis, and the debate over sex-selective practices. Evolution is the unifying theory of modern biology, underpinning biodiversity conservation, understanding of antibiotic resistance, and vaccine development. This chapter is a direct bridge between NCERT biology and GS3 science & technology.

PART 1 — Quick Reference Tables

Mendel's Laws

Monohybrid Cross — Classic Example (Pea Flower Colour)

Dominance Variations

Human Chromosomes and Sex Determination

Evidence for Evolution

PART 2 — Detailed Notes

1. Heredity — Transmission of Traits

Gregor Mendel (1822–1884) — An Augustinian monk who experimented with pea plants (Pisum sativum) for 8 years (1856–1863). He chose peas because:

• Short generation time
• Large number of offspring
• Well-defined, contrasting traits (tall/dwarf, round/wrinkled, yellow/green seeds, etc.)
• Ability to control pollination

Key concepts:

• Alleles: Alternative forms of a gene (e.g., T for tall, t for dwarf)
• Dominant allele: Expressed when present (even in one copy) — T
• Recessive allele: Expressed only when present in two copies (homozygous) — tt
• Genotype: Genetic makeup (TT, Tt, tt)
• Phenotype: Observable characteristic (tall or dwarf)
• Homozygous: Two identical alleles (TT or tt)
• Heterozygous: Two different alleles (Tt)

Dihybrid cross (two traits simultaneously): Mendel crossed plants differing in two traits: seed colour (Yellow Y, dominant; green y, recessive) and seed shape (Round R, dominant; wrinkled r, recessive).

YYRR × yyrr → F1: all YyRr (yellow, round) F1 × F1: YyRr × YyRr → F2: 9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled

The 9:3:3:1 ratio in the F2 demonstrates independent assortment.

2. Chromosomes and DNA

Chromosomes are thread-like structures in the nucleus, made of DNA coiled around histone proteins (chromatin). Each chromosome carries thousands of genes. Humans have 46 chromosomes (23 pairs) in somatic (body) cells.

DNA (deoxyribonucleic acid) — the molecule of heredity. Double helix structure (Watson and Crick, 1953 — building on Rosalind Franklin's X-ray crystallography work). Two antiparallel strands of nucleotides connected by hydrogen bonds between complementary base pairs (A-T; G-C).

Gene: A specific sequence of DNA base pairs that encodes for a protein. One gene → one protein (one enzyme). The human genome contains approximately 20,000–25,000 protein-coding genes.

3. Mutations and Variation

Mutations are changes in DNA sequence. Can occur:

• Spontaneously during DNA replication (copying errors)
• Induced by mutagens — radiation (UV, X-rays, gamma rays), certain chemicals (benzene, tobacco smoke constituents)

Mutations may be:

• Beneficial: Provide a survival advantage (e.g., sickle cell trait — heterozygotes resistant to malaria)
• Neutral: No effect on fitness (most mutations)
• Harmful: Cause genetic diseases (e.g., sickle cell anaemia, cystic fibrosis, Huntington's disease)

Variation arises through:

1. Mutations
2. Genetic recombination during meiosis (crossing over)
3. Independent assortment of chromosomes
4. Random fertilisation

4. Lamarck vs Darwin — Evolution Theories

Jean-Baptiste Lamarck (1744–1829) — Theory of Inheritance of Acquired Characteristics:

• Organisms develop new organs or modify existing ones in response to environmental demands.
• These acquired characteristics are passed on to offspring.
• Classic (incorrect) example: Giraffes stretched their necks to reach leaves; offspring inherited longer necks.
• Why wrong: Body changes during an organism's lifetime (muscles built by exercise) do not change the DNA in reproductive cells and cannot be inherited.

Charles Darwin (1809–1882) — Theory of Natural Selection (On the Origin of Species, 1859):

1. Overproduction: All species produce more offspring than can survive.
2. Variation: Individuals in a population vary in their traits.
3. Competition: Limited resources cause competition.
4. Natural selection (survival of the fittest): Individuals with advantageous variations survive and reproduce more — "fittest" means best suited to environment, not physically strongest.
5. Inheritance: Successful variants pass their traits to offspring.
6. Gradual change over generations: Populations change over time → new species.

Alfred Russel Wallace independently arrived at the same theory as Darwin in 1858. Both Darwin and Wallace jointly presented the theory to the Linnean Society of London on 1 July 1858.

Neo-Darwinism (Modern Synthetic Theory): Combines Darwin's natural selection with Mendelian genetics and the discovery of DNA. Added concepts: mutation as the source of new variation, genetic drift (random changes in allele frequencies in small populations), gene flow (migration of individuals carrying different alleles).

5. Evidence for Evolution

Homologous structures — shared blueprint from a common ancestor: The forelimb of a human (grasping), a whale (swimming), a bat (flying), and a horse (running) have the same basic bone structure (humerus-radius/ulna-carpals-metacarpals-phalanges) but are adapted for different functions. This divergent evolution from a common ancestor is compelling evidence.

Analogous structures — similar function, different structure (convergent evolution): Wings of a bird (modified forelimb with feathers) and wings of an insect (modified exoskeleton outgrowth) perform the same function but are built entirely differently. No common ancestor for the wing itself — the same selective pressure (flight) produced similar solutions independently.

Archaeopteryx — the famous transitional fossil found in Germany (1861, Bavarian limestone, 150 million years old). It had features of both reptiles (teeth, clawed forelimbs, long bony tail) and birds (feathers, wishbone). A classic example of a transitional fossil linking two groups.

Human evolution timeline:

• Homo habilis ("handy man") — ~2.4–1.5 million years ago; first tool use; East Africa
• Homo erectus ("upright man") — ~2 million–100,000 years ago; fire use; left Africa to colonise Asia and Europe
• Neanderthals (Homo neanderthalensis) — ~400,000–40,000 years ago; Europe/Asia
• Homo sapiens ("wise man") — ~300,000 years ago to present; Out of Africa ~70,000 years ago

💡 Explainer: CRISPR-Cas9 — Evolution of Biotechnology

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a gene-editing tool derived from a bacterial immune system. Bacteria use CRISPR to recognise and cut viral DNA that has attacked them before — a natural acquired immunity with genetic memory.

Scientists (Jennifer Doudna and Emmanuelle Charpentier — Nobel Prize in Chemistry 2020) adapted this system into a molecular scissors tool that can cut any DNA sequence with precision. The cut DNA can then be repaired with deletion (gene knockout) or insertion (gene knock-in) of desired sequences.

Applications relevant to UPSC:

• GM crops: Precise gene editing without foreign DNA (cisgenic editing) — potentially easier regulatory approval
• Sickle cell disease cure: CRISPR-based gene therapy has shown promise in clinical trials; India has a significant sickle cell burden (12.4 million carriers; high prevalence in tribal populations)
• Cancer immunotherapy: Edit T-cells to better target cancer
• Designer babies controversy: In 2018, He Jiankui (China) edited human embryo genomes → international outcry; ethical red line crossed

🎯 UPSC Connect: Prenatal Diagnosis and PCPNDT Act

Genetic knowledge enables prenatal tests:

• Amniocentesis: Sample of amniotic fluid at 15–20 weeks; fetal chromosomes analysed
• Chorionic Villus Sampling (CVS): Sample of placental tissue at 10–12 weeks; earlier than amniocentesis
• NIPT (Non-Invasive Prenatal Test): Blood test at 10+ weeks; analyses fetal DNA in maternal blood

These tests can detect chromosomal abnormalities (Down syndrome — trisomy 21), genetic disorders (thalassemia, sickle cell), and fetal sex.

The PCPNDT Act 1994 (amended 2003) prohibits using these tests for sex determination and sex-selective abortion. All ultrasound machines must be registered; operators must maintain records of pregnant women tested. Despite the law, India's sex ratio at birth remains below 950 girls per 1,000 boys in several states.

PART 3 — Frameworks & Analysis

Framework: Genetics → Biotechnology → Policy

Framework: India's Genetic Disease Burden

Exam Strategy

Prelims traps:

• Sex of offspring is determined by the father (X or Y sperm), not the mother (all eggs carry X).
• Archaeopteryx is a reptile-bird transitional fossil — NOT a dinosaur itself (though closely related to small theropod dinosaurs).
• Analogous organs = same function, different origin (convergent evolution). Homologous organs = same origin/structure, different function (divergent evolution).
• Incomplete dominance produces a blended intermediate phenotype; codominance produces both parental phenotypes together (ABO blood group).
• CRISPR Nobel Prize in Chemistry was awarded in 2020 to Jennifer Doudna and Emmanuelle Charpentier.

Mains frameworks:

• GM crops/gene editing: Mendel → DNA → recombinant DNA → CRISPR → India's regulatory approach (GEAC) → ethical concerns
• PCPNDT: chromosomal sex determination → prenatal diagnosis technology → sex selection → PCPNDT Act → Beti Bachao → NFHS data
• AMR connection: bacterial evolution by natural selection explains how antibiotic resistance spreads through bacterial populations

Previous Year Questions

Q1 (Prelims 2021): With reference to CRISPR-Cas9 technology, which of the following statements is/are correct? (Tests: nature of the technology, applications, Nobel Prize)

Q2 (Prelims 2018): DNA Fingerprinting technology is based on the fact that… (Tests: principle of genetic variation underlying DNA fingerprinting)

Q3 (Mains GS3 2019): What is gene editing? Explain the ethical, legal and social implications of human genome editing. Evolution of genetics → CRISPR → He Jiankui controversy → policy framework

Q4 (Prelims 2017): Consider the following statements about "Sickle Cell Anaemia": [statements about inheritance pattern, affected populations, treatment] (Tests: autosomal recessive inheritance, affected groups in India)