Final answer:
Meiosis occurs during the formation of sex cells in humans to create cells with either the HbS or Hbs allele for sickle cell disease.
Step-by-step explanation:
Meiosis occurs during the formation of sex cells (gametes) in humans. In males, meiosis occurs during spermatogenesis to produce sperm cells, while in females, meiosis occurs during oogenesis to produce egg cells. During meiosis, homologous chromosomes separate, resulting in the formation of cells with either the HbS or Hbs allele for sickle cell disease.