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Principles of Inheritance and Variation

NCERT Class 12 Biology Chapter 4

Principles of Inheritance and Variation for Class 12 Biology and NEET: Mendel’s laws, linkage, sex determination, mutation and genetic disorders.

8 topics Updated 8 Nov 2026

For: Class 12 and NEET

In the syllabus: NEET Class 12 syllabus

With your textbook

NCERT Biology (Class 12), Chapter 4 “Principles of Inheritance and Variation”, reprint 2026-27.

Open the official NCERT chapter (PDF, 19.2 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Mendel’s experiments on inheritance

    Mendel’s experiments for Class 12 Biology and NEET: why he chose the pea, how a cross is made, true-breeding lines and the seven contrasting traits.

    NCERT Section 4.1 8 min read 3 solved examples 6 practice questions

  2. Inheritance of one gene: the monohybrid cross

    Monohybrid cross for Class 12 Biology and NEET: 3 : 1 and 1 : 2 : 1 ratios, law of segregation, test cross, incomplete dominance, co-dominance, ABO.

    NCERT Section 4.2 19 min read 4 solved examples 10 practice questions

  3. Inheritance of two genes: the dihybrid cross

    Dihybrid cross for Class 12 Biology and NEET: the 9 : 3 : 3 : 1 ratio, the law of independent assortment, chromosomal theory, linkage and recombination.

    NCERT Section 4.3 16 min read 4 solved examples 9 practice questions

  4. Polygenic inheritance

    Polygenic inheritance for Class 12 Biology and NEET: polygenic traits with examples, human height, the additive skin-colour model and the environment.

    NCERT Section 4.4 9 min read 3 solved examples 7 practice questions

  5. Pleiotropy

    Pleiotropy for Class 12 Biology and NEET: pleiotropic genes with several effects, the metabolic-pathway mechanism, phenylketonuria and the pea starch gene.

    NCERT Section 4.5 8 min read 3 solved examples 7 practice questions

  6. Sex determination

    Sex determination for Class 12 Biology and NEET: XO, XY and ZW types, male and female heterogamety, humans, birds and the honey bee, with solved examples.

    NCERT Section 4.6 12 min read 4 solved examples 7 practice questions

  7. Mutation

    Mutation for Class 12 Biology and NEET: types of mutation, chromosomal aberrations, the sickle-cell point mutation, frame-shift mutations and mutagens.

    NCERT Section 4.7 10 min read 3 solved examples 8 practice questions

  8. Genetic disorders

    Genetic disorders for Class 12 Biology and NEET: pedigree analysis, colour blindness, haemophilia, sickle-cell anaemia, thalassemia and Down’s syndrome.

    NCERT Section 4.8 19 min read 4 solved examples 9 practice questions

Key formulas in this chapter

Mendel’s experiments

  • Mendel: garden pea, 1856–1863
  • true-breeding line: stable trait over several generations of selfing
  • 14 varieties = 7 pairs of contrasting traits
  • dominant: tall, violet, axial, inflated, green pod, round, yellow seed

Inheritance of one gene

  • monohybrid cross: TT × tt → F Tt, all tall
  • F: ¼ TT + ½ Tt + ¼ tt, genotypes 1: 2: 1
  • F phenotypes: 3 tall: 1 dwarf
  • (½ T + ½ t) gives ¼ TT + ½ Tt + ¼ tt
  • test cross: TT × tt → all dominant; Tt × tt → 1: 1
  • snapdragon F: 1 RR red: 2 Rr pink: 1 rr white
  • ABO: IAIA, IAi → A; IBIB, IBi → B; IAIB → AB; ii → O

Inheritance of two genes

  • dihybrid cross: RRYY × rryy → F RrYy (round yellow)
  • F gametes: RY, Ry, rY, ry, ¼ each
  • F phenotypes: (3: 1)(3: 1), that is 9: 3: 3: 1
  • F: 16 squares, 9 genotypes, 4 phenotypes
  • gamete types from n heterozygous unlinked loci = 2n
  • linked loci with r<50%: parental gametes exceed recombinant gametes
  • white–yellow 1.3%: tight; white–miniature wing 37.2%: loose

Polygenic inheritance

  • polygenic trait: 3 or more genes (NCERT); 2 or more (NHGRI); additive effects
  • 3 genes: 0–6 dominant alleles → 7 phenotype classes
  • AABBCC darkest; aabbcc lightest; 3 dominant alleles intermediate
  • AaBbCc × AaBbCc: 1: 6: 15: 20: 15: 6: 1 (of 64)
  • AaBbCc parents: aabbcc child (1/4)3 = 1/64

Pleiotropy

  • pleiotropy: 1 gene → several phenotypic effects
  • polygenic: 3 or more genes (NCERT) → 1 trait
  • PKU: enzyme for phenylalanine → tyrosine lacking
  • PKU: autosomal recessive; Pp × Pp → ¼ pp affected

Sex determination

  • XO: female XX, male XO; XY: female XX, male XY → male heterogamety
  • ZW: female ZW, male ZZ → female heterogamety
  • humans: 22 pairs of autosomes + XX (female) or XY (male)
  • typical human chromosome model: X:Y sperm = 1:1; XX:XY offspring ≈ 1:1
  • honey bee: female 2n = 32 (fertilised); male n = 16 (unfertilised)

Mutation

  • point mutation: change in a single base pair
  • sickle-cell: gene GAG → GTG; mRNA GAG → GUG; Glu → Val at position 6 of the beta chain
  • coding-region insertion/deletion not divisible by 3 → frame shift
  • coding-region insertion/deletion of 3n bases → downstream frame preserved
  • mutagens: chemical and physical factors, e.g. UV radiation

Genetic disorders

  • X-linked recessive: carrier mother × normal father → 1/2 of sons affected, daughters 1/2 carriers
  • autosomal recessive: carrier × carrier → 1/4 affected, 1/2 carriers
  • HbAHbS: carrier (sickle-cell trait); HbSHbS: diseased
  • classic full trisomy 21: 47 chromosomes; translocation/mosaic Down’s syndrome also occurs
  • Klinefelter’s syndrome: 47, XXY; Turner’s syndrome: 45, X0

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