Meiosis and Genetic Variation

• In humans:


  • Meiosis produces gametes (sperm and eggs) with 23 chromosomes each, allowing for genetic diversity in offspring.
• Meiosis:


  • Halves the chromosome number.
  • Generates genetic diversity through crossing over and independent assortment.
  • Produces gametes for sexual reproduction.
• Chromosome condensation in Meiosis:

  • Pairing of homologous chromosomes
  • Crossing over during prophase I
• Meiosis Overview:


  • Starts with a diploid cell
  • Involves two rounds of division: Meiosis I and Meiosis II
  • Results in four haploid cells
• Meiosis as a Reduction Division

  Meiosis is a specialized type of cell division that produces gametes (sex cells) with half the number of chromosomes as the parent cell. This reduction in chromosome number is crucial for sexual reproduction.
• Key Terms of Meiosis:

  • Diploid (2n): Cells with two sets of chromosomes (one from each parent)
  • Haploid (n): Cells with one set of chromosomes
• Two Rounds of Segregation in Meiosis:


  1. Meiosis I: Reduction Division

  • Homologous chromosomes separate
  • Reduces chromosome number from 2n to n

  1. Meiosis II: Equational Division

  • Sister chromatids separate
  • Chromosome number remains n
• Need for Meiosis in Sexual Life Cycles:

  • Maintains a constant chromosome number across generations
  • Introduces genetic variation through:

  1. Crossing over (exchange of genetic material between homologous chromosomes)
  2. Independent assortment of chromosomes
• Down Syndrome and Non-disjunction


  Down syndrome is a genetic disorder that serves as an excellent example of what can go wrong during meiosis, specifically due to a process called non-disjunction.
• Down Syndrome:

  • Caused by the presence of an extra copy of chromosome 21 (trisomy 21)
  • Most commonly results from non-disjunction during maternal meiosis I
• Non-disjunction:

  • Failure of chromosomes or chromatids to separate properly during meiosis or mitosis
  • Can occur in Meiosis I or Meiosis II
• How non-disjunction leads to Down syndrome:

  1. During normal meiosis:

  • A parent cell with 46 chromosomes produces gametes with 23 chromosomes

  1. With non-disjunction of chromosome 21:

  • Some gametes receive 24 chromosomes (two copies of chromosome 21)
  • Others receive 22 chromosomes (no copy of chromosome 21)

  1. If a gamete with 24 chromosomes fertilizes a normal gamete:

  • The resulting zygote has 47 chromosomes, including 3 copies of chromosome 21
  • This leads to Down syndrome
• Meiosis as a Source of Variation

  Meiosis is not just about reducing chromosome numbers; it's also a crucial mechanism for generating genetic diversity. This diversity is essential for evolution and adaptation in sexually reproducing organisms. There are two main processes: random orientation of bivalents and crossing over.
• Random Orientation of Bivalents

  During metaphase I of meiosis, homologous chromosome pairs (bivalents) align at the cell's equator. The orientation of these pairs is random, leading to different combinations of maternal and paternal chromosomes in the gametes.
• Key points of Random Orientation of Bivalents:

  • Each homologous pair can align in two possible orientations
  • The number of possible combinations = 2n2n, where n is the number of chromosome pairs
• Crossing Over


  Crossing over occurs during prophase I of meiosis. It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
• Key points of crossing over:

  • Creates new combinations of alleles on each chromosome
  • Occurs at points called chiasmata
  • Frequency of crossing over varies along the chromosome
• Combined Effect on Genetic Variation

  The combination of random orientation and crossing over leads to an enormous potential for genetic diversity:

  1. Random orientation shuffles whole chromosomes
  2. Crossing over creates new combinations within each chromosome

  This two-level mixing results in gametes that are genetically unique, even among siblings.
• Importance of Genetic Variation

  The genetic diversity generated by meiosis is crucial for:

  • Adaptation to changing environments
  • Evolution of species over time
  • Reducing the impact of harmful recessive alleles in populations
• Recombinant chromatids 


  chromatids that contain a mixture of maternal and paternal genetic material due to crossing over.