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    Inheritance, Variation & Evolution

    Revision notes · Biology

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    Lesson step 1 of 4DNA, Genes & Chromosomes

    DNA, Genes & Chromosomes

    DNA (deoxyribonucleic acid) is a polymer made of two strands twisted into a double helix. It is found in the nucleus of cells, packaged into structures called chromosomes.

    Human cells have 23 pairs of chromosomes (46 total). Gametes (sex cells) have 23 chromosomes (haploid). The 23rd pair of chromosomes determines biological sex: XX for female and XY for male.

    A gene is a small section of DNA on a chromosome that codes for a particular protein. Different versions of the same gene are called alleles.

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    Key Terms

    Dominant
    An allele that is always expressed when present.
    Recessive
    An allele only expressed when two copies are present.
    Natural selection
    The process by which organisms with favourable traits survive and reproduce more successfully.
    Meiosis
    Cell division producing four genetically different haploid gametes.