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.
In terrestrial insects, mammals, reptiles, and birds, fertilization occurs within the female's body. In human reproduction, the male gametes (sperm cells) are introduced into the vagina of the female. A female gamete (ovum) is produced and released from the ovary into the oviduct. Fusion of the male and female gametes usually occurs in the oviduct (the Fallopian tube). Rarely, but more commonly when fertility drugs are used, two or more female gametes are released in humans at the same time. This results in twins or other multiple births. Just as rare is the division of a zygote into two cells by mitosis where each cell then develops to produce a new individual. These twins will be genetically identical.
In the majority of fish and amphibians, and most aquatic invertebrates, fertilization occurs externally, when both sexes release their gametes into the water. In most fungi, gametes are not released, but the hyphae of the two parents grow towards each other and fuse to achieve fertilization. In higher plants, pollination precedes fertilization.
External and Internal Fertilization
Human egg fertilization
How Strawberry Plants Reproduce
development of a human fetus
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