In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with one copy of each of the 23 chromosomes, and the zygote is a diploid cell with two copies of each of the 23 chromosomes.
Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).
Meiosis is followed by gametogenesis, the process during which the haploid daughter cells change into mature gametes. Males produce gametes called sperm in a process known as spermatogenesis, and females produce gametes called eggs in the process known as oogenesis.
Sexual reproduction produces genetically unique offspring. Crossing-over, independent assortment, and the random union of gametes work together to result in an amazing range of potential genetic variation.
