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Class 10 Science Notes — Chapter 8: Heredity

Mendel's experiments, laws of inheritance, and sex determination.

Detailed NCERT notes

  • Heredity = transmission of traits from parents to offspring via genes on chromosomes.
  • Mendel's work with garden pea (Pisum sativum) — 7 contrasting traits.
  • Monohybrid cross (Tt × Tt): F1 all tall (Tt); F2 ratio 3 tall : 1 short — Law of Segregation (alleles separate during gamete formation).
  • Dihybrid cross (RRYY × rryy): F1 all round-yellow (RrYy); F2 ratio 9:3:3:1 — Law of Independent Assortment (alleles of different genes segregate independently).
  • Terminology: gene, allele (variants of a gene), dominant (masks the other), recessive (only expressed when both alleles same), homozygous (TT or tt), heterozygous (Tt), phenotype (visible trait), genotype (genetic makeup).
  • Punnett square is used to predict offspring genotype/phenotype.
  • Only traits controlled by genes are inherited; acquired traits (learnt skills, physical injuries) are NOT inherited (Lamarck's idea rejected).
  • Sex chromosomes in humans: female XX, male XY. Mother gives X; father gives X or Y → 50% chance of each sex; father's contribution decides sex.
  • Basis of evolution: variations arise → those beneficial in environment get selected → over generations, species change.
  • Note: 'Evolution' portion beyond the basics has been rationalised in NCERT; focus on inheritance mechanism.

Formulas & key results

  • Monohybrid F2: 3:1 (phenotype), 1:2:1 (genotype)
  • Dihybrid F2: 9:3:3:1 (phenotype)
  • Sex determination: XX (female), XY (male)

Mind map

  • Genes, alleles, dominant/recessive, genotype/phenotype
  • Monohybrid → segregation; Dihybrid → independent assortment
  • Punnett squares
  • Sex determination in humans

Tricks & shortcuts

  • Dominant capital, recessive small (T, t).
  • Sex chromosome: mother always X; father decides X (girl) or Y (boy).

Common mistakes to avoid

  • Believing acquired characters can be inherited.
  • Assuming mother determines sex.

Competency-based questions & answers

  1. Q. Why is the sex of the child determined by the father?
    A. Mother contributes only X; father contributes X or Y. XX → girl, XY → boy — decided by father's gamete.
  2. Q. Two tall pea plants (Tt) are crossed. What is the probability of a short offspring?
    A. Tt × Tt → TT, Tt, Tt, tt ⇒ P(short = tt) = 1/4 = 25%.
  3. Q. Explain: 'A trait acquired during a lifetime is not inherited.'
    A. Only changes in DNA (in reproductive cells) pass on. A muscle built by exercise is not coded in DNA, so it cannot be inherited.