Sequence meiosis I and meiosis II, and explain the three sources of genetic variation meiosis introduces.
Sexual reproduction requires special cells, gametes (sperm and egg), that carry only half of the parent's chromosome number, so that fertilization -- combining one gamete from each parent -- restores the full number in the offspring. Humans, for example, have 46 chromosomes (23 homologous pairs) in nearly every cell, but gametes carry only 23. Meiosis is the specialized division that produces these haploid gametes from a diploid starting cell, and it takes two full rounds of division to do it. Meiosis I is the reduction division: homologous chromosome pairs (one from each parent, matching in size and gene content) separate from each other, so each resulting cell gets one full set of chromosomes rather than a pair -- notably, at this point each chromosome still consists of two sister chromatids, exactly like a chromosome entering mitosis. Meiosis II then separates those sister chromatids, much like an ordinary mitotic division, producing four haploid cells total from the original one diploid cell -- and unlike mitosis, all four resulting cells are genetically different from one another and from the parent cell.
That genetic difference is not an accident -- it comes from three distinct sources built into the process. During prophase I, homologous chromosomes pair up closely and can exchange corresponding segments in a process called crossing over, physically shuffling maternal and paternal genes onto the same chromosome. During metaphase I, each homologous pair lines up with its orientation toward either pole decided randomly, called independent assortment, so which original parent each chromosome in a gamete came from is essentially a coin flip repeated for all 23 pairs -- more than eight million possible combinations from that step alone in humans. Finally, random fertilization -- which of many possible sperm happens to fertilize which egg -- multiplies that variation even further. Together, these three sources of variation are a central reason full siblings, despite sharing both parents, are never genetically identical (except identical twins, who split from a single already-fertilized cell after fertilization).
Mastery looks like: They can sequence meiosis I and meiosis II, state the resulting number and genetic makeup of the daughter cells, and name the three sources of genetic variation meiosis introduces.
Common stumbles: Confusing meiosis I (separating homologous pairs) with meiosis II (separating sister chromatids); forgetting that meiosis produces four genetically different cells, not two identical ones like mitosis.