Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen in aphids, daphnia, rotifers, and some other invertebrates, as well as in some plants. Among vertebrates, there are several genera of fish, amphibians, and reptiles that exhibit differing forms of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis (an incomplete form of parthenogenesis).
The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as systems where XX is female and XY is male), but male if the female sex is determined by unlike chromosomes (such as systems where WZ is female and ZZ is male), because the process involves the inheritance and subsequent duplication of only a single sex chromosome. The offspring may be capable of sexual reproduction, if this mode exists in the species. A parthenogenetic offspring is sometimes called a parthenogen. As with all types of asexual reproduction, there are both costs (reduced genetic diversity generated and susceptibility to adverse mutation) and benefits (reproduction without the need for a male) associated with parthenogenesis.

Parthenogenesis is distinct from artificial animal cloning, a process where the new organism is identical to the cell donor. Parthenogenesis is truly a reproductive process which creates a new individual or individuals from the naturally varied genetic material contained in the eggs of the mother. A litter of animals resulting from parthenogenesis may contain all genetically unique siblings without any twins or multiple numbers from the same genetic material. Parthenogenic offspring of a parthenogen are, however, all genetically identical to each other and to the mother, as a parthenogen is homozygous.
The alternation between parthenogenesis and sexual reproduction is called heterogamy. Forms of reproduction related to parthenogenesis but that require the presence of sperm are known as gynogenesis and hybridogenesis.

Reptiles

Most reptiles reproduce sexually, but parthenogenesis has been observed in certain species of rock lizards, geckos, whiptails, and Komodo Dragons.
Recently, the Komodo dragon which normally reproduces sexually was found to also be able to reproduce asexually by parthenogenesis. Because the genetics of sex determination in Komodo Dragons uses the WZ system (where WZ is female, ZZ is male, WW is inviable) the offspring of this process will be ZZ (male) or WW (inviable), with no WZ females being born. A case has been documented of a Komodo Dragon switching back to sexual reproduction after a parthenogenetic event. It has been postulated that this gives an advantage to colonisation of islands, where a single female could theoretically have male offspring asexually, then switch to sexual reproduction to maintain higher level of genetic diversity than asexual reproduction alone can generate.Parthenogenesis may also occur when males and females are both present, as the wild Komodo dragon population is approximately 75 per cent male.

Insects

An example of non-viable parthenogenesis is common among domesticated honey bees. The queen bee is the only fertile female in the hive; if she dies without the possibility for a viable replacement queen, it is not uncommon for the worker bees to lay eggs. Worker bees are unable to mate, and the unfertilized eggs produce only drones (males), which can only mate with a queen. Thus, in a relatively short period, all the worker bees die off, and the new drones follow. In one subspecies from South Africa (Apis mellifera capensis) workers are capable of producing diploid eggs parthenogenetically, and thus the queen can be replaced if she dies. It is believed that a few other bees may be truly parthenogenetic (for example, at least one species of small carpenter bee, in the genus Ceratina), and many parasitic wasps are known to be parthenogenetic, sometimes due to infections by Wolbachia.

Mammals

In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice: see Kaguya. In theory, the process could be used to reproduce humans after extensive testing and perfection.