Heredity and Evolution

Chapter Overview

This chapter explains heredity, variation and the inheritance of traits. It also introduces Mendel's experiments, dominant and recessive traits, sex determination, natural selection, speciation, fossils and evidence for evolution.

Topics Covered

Heredity

Variation

Genes

Alleles

Traits

Dominant and Recessive Traits

Genotype and Phenotype

Mendel's Experiments

Monohybrid Cross

Dihybrid Cross

Inheritance of Traits

Sex Determination

Inherited and Acquired Traits

Variation and Evolution

Natural Selection

Genetic Drift

Geographical Isolation

Speciation

Fossils

Evolution and Classification

Homologous Structures

Analogous Structures

Molecular Evidence

Human Evolution

Quick Revision

Heredity is the transmission of inherited characteristics from parents to offspring.
Genes carry hereditary information and can have alternative forms called alleles.
Genotype refers to genetic constitution, while phenotype refers to observable characteristics.
A classical monohybrid cross under complete dominance gives a typical F₂ phenotype ratio of 3 : 1.
The corresponding classical F₂ genotype ratio is 1 : 2 : 1.
Typical human females have XX sex chromosomes and typical human males have XY sex chromosomes.
Heritable variations can contribute to evolutionary change.
Natural selection can alter the frequency of heritable characteristics in a population.
Geographical isolation can reduce gene flow and contribute to speciation.
Fossils, comparative anatomy and molecular evidence help scientists study evolutionary history.

Key Concepts

Concept	Meaning
Gene	Unit of heredity carrying genetic information.
Allele	Alternative form of a gene.
Genotype	Genetic constitution associated with a trait.
Phenotype	Observable expression of a trait.
Variation	Differences among individuals of a population.
Natural Selection	Differential survival and reproduction associated with heritable characteristics.
Speciation	Formation of new species.
Fossil	Preserved remain, impression or trace of ancient life.

Important Mendelian Ratios

Rr × Rr → RR : Rr : rr = 1 : 2 : 1

Classical F₂ genotypic ratio of a monohybrid cross.

Dominant : Recessive Phenotype = 3 : 1

Under complete dominance.

9 : 3 : 3 : 1

Classical F₂ phenotypic ratio of a standard dihybrid cross under Mendelian assumptions.

Human Sex Determination

X Ovum + X Sperm → XX

X Ovum + Y Sperm → XY

An ovum normally carries an X chromosome, while sperm may carry either an X or a Y chromosome.

Evolution – Basic Flow

Heritable Variation → Differential Survival and Reproduction → Change in Population → Evolutionary Change

Evolution is a change in inherited characteristics of populations over generations.

Official Study Sources

Students should refer to official NCERT and CBSE resources for curriculum-aligned preparation.

NCERT Official Textbooks → CBSE Academic Curriculum 2026-27 → CBSE Assessment Resources →