HEREDITY – REVISION NOTES
1. Accumulation of Variation during Reproduction
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New individuals are similar but not identical to parents.
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Variations arise due to:
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Small inaccuracies in DNA copying
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Sexual reproduction (mixing of genes)
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Asexual reproduction → very little variation
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Sexual reproduction → more variation
Importance of Variation
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Helps organisms survive environmental changes
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Beneficial variations are selected by nature
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Basis of evolution
2. Heredity
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Heredity: Transmission of traits from parents to offspring.
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Ensures:
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Basic body design remains stable
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Subtle changes (variations) appear
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3. Inherited Traits
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Traits are controlled by genes.
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Genes are segments of DNA.
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DNA provides information for protein synthesis.
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Proteins determine physical and functional traits.
Example
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Gene → enzyme efficiency → hormone amount → plant height
4. Mendel’s Contributions
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Gregor Johann Mendel studied inheritance using pea plants.
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Chose contrasting traits:
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Tall / Short
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Round / Wrinkled seeds
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Violet / White flowers
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5. Monohybrid Cross
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Cross between plants differing in one trait.
F₁ Generation
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Only dominant trait expressed.
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No intermediate character.
F₂ Generation
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Traits reappear in 3 : 1 ratio
(Dominant : Recessive)
6. Dominant and Recessive Traits
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Dominant trait: Expressed even with one copy (T)
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Recessive trait: Expressed only when both copies are recessive (t)
Example:
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TT or Tt → Tall
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tt → Short
7. Dihybrid Cross
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Cross involving two traits.
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Shows independent inheritance of traits.
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Produces new combinations in F₂ generation.
8. Expression of Traits
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Each organism has two copies of each gene.
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One copy from mother, one from father.
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Genes control traits by controlling proteins.
9. Chromosomes and Genes
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Genes are located on chromosomes.
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Humans have:
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23 pairs of chromosomes
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22 pairs autosomes
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1 pair sex chromosomes
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10. Sex Determination in Human Beings
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Females: XX
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Males: XY
Key Points
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Mother always contributes X chromosome
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Father contributes X or Y
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X → Girl (XX)
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Y → Boy (XY)
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Father determines the sex of the child
11. Important Differences
Asexual vs Sexual Reproduction
Asexual Sexual