The fusion of a female gamete (egg) with a male gamete (sperm) to form a single cell, the zygote. Gametes are haploid because their nuclei contain only one set of chromosomes. In contrast, most animal cells are diploid since their nuclei contain two sets of chromosomes, one set derived from the egg and the other from the sperm. The reduction from the diploid to the haploid number occurs in a specialized set of two cell divisions, known as meiosis, during which gametes are formed in the gonads of an adult. Fertilization restores the diploid chromosome number. In the human, normal eggs and sperm contain 23 chromosomes each, whereas the zygote and most of its daughter cells contain 23 pairs, or 46, chromosomes per cell nucleus.