CBSE Class 10 Science: Heredity & Evolution — Notes 2026

Heredity and Variation

• Heredity: transmission of traits from parent to offspring through genes
• Variation: differences among individuals of same species; raw material for evolution
• Sources of variation: mutations, sexual reproduction (meiosis + random fertilisation)

Mendel's Experiments

• Used garden pea (Pisum sativum); 7 pairs of contrasting traits; large number of offspring
• Monohybrid cross (Tt × Tt): 3:1 phenotypic ratio; 1:2:1 genotypic ratio in F2
• Dihybrid cross (RRYY × rryy): 9:3:3:1 ratio in F2; shows independent assortment

How Traits are Inherited

• Genes on chromosomes; humans 46 chromosomes (23 pairs)
• Sex determination: XX (female), XY (male); father's sperm determines sex
• Sex-linked inheritance: colour blindness gene on X chromosome; more common in males (XY)

Evolution — Natural Selection

• Darwin's theory: variation exists; resources limited; individuals with favourable traits survive (survival of the fittest)
• Traits passed to offspring; over generations, favourable traits become more common
• Evidence: fossil record, comparative anatomy, homologous/analogous organs, biogeography

Speciation

• Speciation: formation of new species from existing ones over many generations
• Geographic isolation: separated populations accumulate different mutations; eventually cannot interbreed
• Reproductive isolation: prevents gene flow; final step in speciation

Human Evolution

• Homo sapiens evolved in Africa ~200,000 years ago; migrated globally
• Evidence: common ancestry with other primates; mitochondrial DNA analysis; fossil hominids
• Evolution is ongoing: antibiotic resistance in bacteria, evolution of pesticide resistance in insects

CBSE Exam Tips — Heredity

• Solve dihybrid cross for CBSE; always show all 16 combinations in Punnet square
• Explain the role of DNA in heredity and protein synthesis (brief mention)
• Define: gene, allele, phenotype, genotype, dominant, recessive