Heredity (and, in the older syllabus, Evolution) gives 3–5 marks in Class 10 with one Punnett square question almost every year. These notes give the crosses step by step.
Key terms
- Gene: unit of inheritance. Allele: alternative form (T, t). Genotype (TT, Tt, tt) vs phenotype (tall, dwarf).
- Dominant trait shows even with one allele; recessive needs two.
Monohybrid cross
TT (tall) × tt (dwarf) → F₁ all Tt (tall). F₁ × F₁: Tt × Tt → TT, Tt, Tt, tt → phenotype 3 : 1, genotype 1 : 2 : 1. Conclusion: traits are inherited as separate units; the recessive is hidden, not lost.
Dihybrid cross
RRYY (round yellow) × rryy (wrinkled green) → F₁ RrYy. F₁ × F₁ → 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Conclusion: traits are inherited independently.
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How traits are expressed
A gene codes for a protein (enzyme); the amount/kind of protein decides the trait, e.g. a plant hormone made by an enzyme controls height. Each trait has one copy from each parent (via gametes with half the chromosomes).
Sex determination
- Humans: 22 pairs + XX (female) or XY (male). The father’s sperm (X or Y) decides the child’s sex — 50 : 50.
- Some animals: temperature (turtles), or environment.
Evolution basics (if in your syllabus)
- Acquired traits (learned/injury) are not inherited; inherited traits change through variation + natural selection + genetic drift.
- Speciation by geographical isolation, genetic drift, natural selection.
- Evidence: homologous organs (forelimbs), analogous organs (wings of bat and bird), fossils, embryology.
- Humans are not ‘more evolved’ than bacteria — all present species are equally evolved.
Common mistakes
- Writing 3 : 1 as the genotype ratio
- Saying the mother decides the child’s sex
- Calling wings of a bird and bat homologous
Related: Life Processes Class 10.
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Frequently asked questions
Are these Heredity and Evolution Class 10 Notes notes enough for the exam?
They cover every formula, definition and question type that appears in school and board papers for this chapter. Use them with the NCERT examples, exercise questions and the last five years' papers to be fully prepared.
How should I revise Heredity and Evolution Class 10 Notes?
Revise the notes after 1 day, 7 days and 30 days. Solve five questions each time instead of only reading, and keep an error list of the steps you get wrong.
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