Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is biparental reproduction. Dioecy has costs, since only about half the population directly produces offspring. It is one method for excluding self-fertilization and promoting allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Plants have several other methods of preventing self-fertilization including, for example, dichogamy, herkogamy, and self-incompatibility. Dioecy is a dimorphic sexual system, alongside gynodioecy and androdioecy. In zoology In zoology, dioecious species may be opposed to hermaphroditic species, meaning that an individual is either male or female, in which case the synonym gonochory is more often used. Most animal species are dioecious (gonochoric). Dioecy may also describe colonies within a species, such as the colonies of Siphonophorae (Portuguese man-of-war), which may be either dioecious or monoecious. In botany Land plants (embryophytes) differ from animals in that their life cycle involves alternation of generations. In animals, typically an individual produces haploid gametes of one kind, either sperms or egg cells. A sperm and an egg cell fuse to form a zygote that develops into a new individual. In land plants, by contrast, one generation – the sporophyte generation – consists of individuals that produce haploid spores rather than haploid gametes. Spores do not fuse, but germinate by dividing repeatedly by mitosis to give rise to haploid multicellular individuals which produce gametes – the gametophyte generation. A male gamete and a female gamete then fuse to produce a new diploid sporophyte. Sexual dimorphism is common in dioecious plants. In bryophytes (mosses, liverworts and hornworts), the gametophytes are fully independent plants, and do not distinguish more than one type of spore, producing isospores. Seed plants (spermatophytes), are heterosporic, producing spores of two different sizes (heterospores). Other vascular non-seed plants may be heterosporous or isosporous. Seed plant gametophytes are dependent on the sporophyte and develop within the spores, a condition known as endospory. In flowering plants, the male gametophytes develop within pollen grains produced by the sporophyte's stamens, and the female gametophytes develop within ovules produced by the sporophyte's carpels. The sporophyte generation of a seed plant is called "monoecious" when each sporophyte plant has both kinds of spore-producing organ, so ultimately produces both male and