Most multicellular eukaryotes form gametes by
Meiosis
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. Parthenogenesis is a type of asexual reproduction in which the offspring develops from unfertilized eggs. Oncogenesis or tumorigenesis is the formation of cancer, where normal cells are transformed into cancer cells. Binary fission ("division in half") is a kind of asexual reproduction.