Topic 4

• Meiosis


  A type of cell division that produces daughter cells which are genetically different
• Meiosis


  1. Parent cell divides once

  2. Parent cell divides twice

  3. Results in 4 haploid daughter cells
• Haploid cells


  • Have one set of chromosomes
• Diploid cells


  • Have two sets of chromosomes
• Independent segregation


  Homologous chromosomes line up randomly at the equator during meiosis I, resulting in random distribution to daughter cells
• Crossing over


  Homologous chromosomes exchange genetic material during meiosis I, resulting in new combinations of alleles
• Meiosis vs Mitosis


  • Meiosis has two nuclear divisions, mitosis has one
  • Meiosis produces haploid cells, mitosis produces diploid cells
  • Meiosis introduces genetic variation, mitosis produces genetically identical cells
• Identifying meiosis in life cycles


  1. Look for transition from diploid to haploid cells

  2. Ignore labels about gamete production
• Fusion of haploid gametes during fertilization further increases genetic variation
• Humans have 2^23 (over 8 million) possible chromosome combinations in gametes
• Combining gametes results in 2^(2*23) (over 70 trillion) possible chromosome combinations in offspring
• Genetic variation from meiosis and fertilization is why humans are not genetically identical (except for identical twins)