Crossing Over: Occurring in Prophase I, non-sister chromatids exchange segments at points called chiasmata. This results in recombinant chromosomes that carry unique combinations of maternal and paternal alleles.
Independent Assortment: During Metaphase I, the random orientation of homologous pairs leads to possible combinations of chromosomes in gametes, where is the haploid number. For humans, this results in over 8 million possible combinations.
Random Fertilization: The fusion of any two unique gametes (sperm and egg) further multiplies the potential genetic combinations. This ensures that every offspring is genetically distinct from its parents and siblings.
Understanding the differences between mitosis and meiosis is critical for grasping how somatic growth differs from gamete production.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth and repair | Gamete production |
| Divisions | One | Two |
| Daughter Cells | 2 (Diploid, ) | 4 (Haploid, ) |
| Variation | Genetically identical | Genetically unique |
| Synapsis | Does not occur | Occurs in Prophase I |
While Meiosis II resembles mitosis in that sister chromatids are separated, the starting cells in Meiosis II are already haploid, whereas mitosis typically starts with a diploid cell.
Chromosome Counting: Always count centromeres to determine the number of chromosomes. In Meiosis I, the number of chromosomes is halved, but each chromosome still consists of two chromatids until Meiosis II.
Nondisjunction: Be prepared to identify errors where chromosomes fail to separate. If it occurs in Meiosis I, all four gametes will be abnormal; if in Meiosis II, only two gametes are typically affected.
Timing of Variation: Remember that genetic variation is introduced early in the process (Prophase I and Metaphase I). By the time Meiosis II begins, the genetic blueprint of the daughter cells is already determined.
