Ch 9 covers Mendel's experiments in heredity, the concepts of dominant and recessive traits, sex determination, and the theory of evolution — how species change over time through natural selection.
Mendel crossed pea plants and discovered: Law of Dominance — one trait dominates in F₁ (all tall from Tt × TT). Law of Segregation — alleles separate during gamete formation. Monohybrid cross (Tt × Tt): F₂ ratio 3 tall : 1 short (genotype TT:Tt:tt = 1:2:1). Dihybrid cross: 9:3:3:1 in F₂ (independent assortment).
Humans: 23 pairs of chromosomes (22 autosomes + 1 sex chromosome pair). Female: XX, Male: XY. Father determines sex: if sperm carries X → daughter (XX), if sperm carries Y → son (XY). Each has 50% probability.
Variations accumulate over generations. Natural selection: organisms with favourable traits survive and reproduce more (Darwin). Speciation: geographical isolation → genetic drift → new species. Evidence: fossils (preserved remains), homologous organs (same origin, different function — e.g., human arm, whale flipper), analogous organs (different origin, same function — bird wing, insect wing).
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No. Evolution does not mean organisms become "better" or more complex over time. Evolution simply means adaptation to a particular environment through natural selection. Bacteria, despite being simple, are well-adapted and successful. Evolution has no direction or goal — it just leads to better fitness for current conditions.
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