fertilization

In sexual reproduction, the union of two gametes (sex cells, often called egg or ovum, and sperm) to produce a zygote, which combines the genetic material contributed by each parent. In self-fertilization the male and female gametes come from the same plant; in cross-fertilization they come from different plants. Self-fertilization rarely occurs in animals; usually even hermaphrodite animals cross-fertilize each other.

The fusion of gametes combines the genetic material contributed by each parent. To avoid doubling the amount of inherited information every generation, each gamete contains only half the amount of inherited information – it is haploid. This is achieved by halving the number of chromosomes when gametes are being produced. When the gametes fuse the full amount of information is restored (diploid state). Gametes are therefore produced by a specialized form of cell division, known as meiosis, which is only used for this purpose. This type of cell division has ways of mixing genes before the gametes are produced. This promotes variation (see natural selection) by producing gametes with different combinations of genes.