CBSE Class 12 Biology: Principles of Inheritance — Notes 2026

Mendel's Laws

• Law of Dominance: one allele (dominant) masks the expression of another (recessive); T allele for tallness dominant over t for shortness
• Law of Segregation: allele pairs separate during gamete formation; each gamete receives one allele
• Law of Independent Assortment: genes on different chromosomes assort independently; basis for dihybrid ratio 9:3:3:1

Monohybrid Cross

• Tt × Tt → 1TT:2Tt:1tt (genotype ratio); 3 tall:1 short (phenotype ratio = 3:1)
• Test cross: Tt × tt → 1Tt:1tt; 50% tall:50% short; used to determine unknown genotype
• Incomplete dominance: RR (red) × WW (white) → RW (pink); F₂ = 1:2:1 phenotype ratio (no 3:1)

Dihybrid Cross

• RRYY × rryy → RrYy (F₁); F₁ × F₁ → 9 R_Y_:3 R_yy:3 rrY_:1 rryy (9:3:3:1)
• Proves Law of Independent Assortment when genes on different chromosomes
• Linked genes: do not assort independently; linkage reduces recombination frequency; Morgan's fruit fly work

Chromosomal Theory of Inheritance

• Sutton and Boveri (1902): genes located on chromosomes; chromosome behaviour during meiosis explains Mendel's laws
• Morgan: Drosophila (fruit fly) genetics; confirmed chromosomal theory; discovered sex-linked inheritance
• Crossing over: exchange of chromosome segments between homologs during meiosis I; creates recombinant chromosomes

Sex Determination

• XY (human): XX = female, XY = male; Y carries SRY gene (testis-determining factor)
• ZW (birds): ZZ = male, ZW = female; opposite of mammals
• Environmental sex determination: Turtle — temperature-dependent; above certain temperature → more females

Mutations

• Point mutation: change in single nucleotide; sickle cell anaemia (GAG → GTG in beta-globin gene)
• Chromosomal mutation: deletion, duplication, inversion, translocation of chromosome segments
• Aneuploidy: abnormal chromosome number; Down's syndrome (trisomy 21, 47 chromosomes); Turner's (45, X); Klinefelter's (47, XXY)

CBSE Board Focus

• Inheritance: 7–8 marks; Mendel's cross diagrams (Punnett square), chromosomal theory, sex determination
• Draw Punnett square for F₁ × F₁ dihybrid cross; label all 16 boxes; group phenotypes
• Compare: complete dominance, incomplete dominance, codominance with examples (ABO blood group for codominance)