This section covers reproduction (how organisms produce offspring), inheritance (how characteristics are passed on through DNA), variation, and natural selection — the mechanism driving evolution.
Asexual: one parent, genetically identical offspring (clones), mitosis. Examples: binary fission (bacteria), runners (strawberry). Sexual: two parents, gametes (sex cells) fuse at fertilisation, offspring genetically different, meiosis produces gametes. Human: male (testes produce sperm) and female (ovaries produce eggs). Fertilisation in oviduct → zygote → embryo implants in uterus wall. Placenta: exchanges O₂, nutrients, CO₂, and waste between mother and fetus.
DNA: double helix in chromosomes (23 pairs in humans). Gene: section of DNA coding for a protein/characteristic. Alleles: different forms of a gene. Dominant (expressed with one copy, capital letter) vs recessive (needs two copies, lowercase). Genotype: allele combination (AA, Aa, aa). Phenotype: observable characteristic. Monohybrid cross: predict offspring ratios using Punnett squares. 3:1 ratio from Aa × Aa cross. Codominance: both alleles expressed equally (e.g., blood groups). Sex-linked: gene on X chromosome (e.g., colour blindness).
Variation: differences between individuals. Continuous (range of values, e.g., height) vs discontinuous (distinct categories, e.g., blood group). Caused by: genes, environment, or both. Mutation: change in DNA sequence — can be harmful, neutral, or beneficial. Natural selection: individuals best adapted to environment survive and reproduce, passing on advantageous alleles. Over generations, the population evolves. Selective breeding: humans select organisms with desired traits for breeding.
Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent (diploid). Used for: growth, repair, and asexual reproduction. Meiosis produces four genetically different daughter cells with half the chromosome number (haploid — gametes). Occurs only in reproductive organs. Meiosis involves two divisions and creates genetic variation through: independent assortment (random orientation of chromosomes) and crossing over (exchange of genetic material between homologous chromosomes). This variation is essential for natural selection and evolution.
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