Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

; each individual organism is thus distinctly female or male. The majority of animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 species (for example, all mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s and most reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s) are dioecious. The term is rarely used for animals, probably because it is the common state for animals. The term is most often used in plants. The majority of plant species are monoecious, and hence have either bisexual flowers or possesses both male and female flowers on the same plant; but still a significant number of plant species are dioecious. See plant sexuality
Plant sexuality
Plant sexuality covers the wide variety of sexual reproduction systems found across the plant kingdom. This article describes morphological aspects of sexual reproduction of plants....

, for a full description of more complicated situations in plants.

In plants

The term dioecious is typically used only in plants and plant species. Dioecious refers to a plant population having separate male and female plants. No individual plant can produce both microspore
In biology, a microspore is a small spore as contrasted to the larger megaspore.In botany, microspores develop into male gametophytes, whereas megaspores develop into female gametophytes. The combination of megaspores and microspores is found only in heterosporous organisms...

s and megaspore
Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These types of plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which is fertilized by sperm produced...

s; individuals of the species are either androecious (male, producing microspores) or gynoecious
Gynoecium is most commonly used as a collective term for all carpels in a flower. A carpel is the ovule and seed producing reproductive organ in flowering plants. Carpels are derived from ovule-bearing leaves which evolved to form a closed structure containing the ovules...

 (female, producing megaspores). Dioecious species cannot self-fertilize.

Individual plants are either male or female. From Greek for "two households". [Individual plants are not called dioecious; they are either gynoecious (female plants) or androecious (male plants).] In most dioecious species the female plant is of homogametic sex XX and the male plant is of heterogametic sex
Heterogametic sex
Heterogametic sex refers to the sex of a species in which the sex chromosomes are not the same. For example, in humans, males, with an X and a Y sex chromosome, would be referred to as the heterogametic sex, and females having two X sex chromosomes would be referred to as the homogametic...

 XY. Exceptions where the male plants are of homogametic sex are Potentilla fruticosa and species of Cotula
Cotula is a genus of flowering plant in the Asteraceae family. It includes about 80 species of plants known generally as water buttons or buttonweeds....

. Some plant genera are exclusively dioecious, such as willows
Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere...

 and poplars.
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