Another common cause of being intersex is the crossing over of the testis-determining factor (SRY) from the Y chromosome to the X chromosome during meiosis. The SRY is then activated in only certain areas, causing development of testes in some areas by beginning a series of events starting with the upregulation of the transcription factor (SOX9), and in other areas not being active (causing the growth of ovarian tissues). Thus, testicular and ovarian tissues will both be present in the same individual.Fetuses before sexual differentiation are sometimes described as female by doctors explaining the process. This is not technically true. Before this stage, humans are simply undifferentiated and possess a paramesonephric duct, a mesonephric duct, and a genital tubercle. Evolution
The evolution of anisogamy may have contributed to the evolution of simultaneous hermaphroditism and sequential hermaphroditism but, as of 2016 it remains unclear if the evolution of anisogamy first led to hermaphroditism or gonochorism. : 213  It is possible that hermaphroditism evolved from gonochorism, or vice versa. Most studies on its evolution focus on plants, and its evolution in animals is unclear as of December 2017.Simultaneous hermaphroditism that exclusively reproduces through self-fertilization has evolved many times in plants and animals, but it might not last long evolutionarily. : 14

In animals
Joan Roughgarden and Priya Iyer argued that the last common ancestor for animals was hermaphroditic and that transitions from hermaphroditism to gonochorism were more numerous than the reverse.
However, their argument was based on paraphyletic Spiralia, assignments of sexual modes for the phylum level than the species level, and methods exclusively based on maximum parsimony.Hermaphroditism is polyphyletic in invertebrates where it evolved from gonochorism: 97  and gonochorism is also ancestral to hermaphroditic fishes. According to Nelson Çabej simultaneous hermaphroditism in animals most likely evolved due to a limited number of mating partners. In plants
It is widely accepted that the first vascular plants were outcrossing hermaphrodites. In flowering plants, hermaphroditism is ancestral to dioecy. Flowers in angiosperms evolved to attract insects as vectors for pollination.Hermaphroditism in plants may promote self fertilization in pioneer populations. However, plants have evolved multiple different mechanisms to avoid self-fertilization in hermaphrodites, including sequential hermaphroditism, molecular recognition systems and mechanical or morphological mechanisms such as heterostyly. : 73, 74

See also
Asexual reproduction
Trioecy
Androgyny
Gonochorism
Gynandromorph
Self-pollination
Self-fertilization
Futanari
Non-binary gender

References
Further reading
External links

Britannica Online Encyclopedia: hermaphroditism (biology)
Current Biology – Gender trading in a hermaphrodite
The Evolution of Self-Fertile Hermaphroditism: The Fog Is Clearing
"Born True Hermaphrodite – Pictorial Profile", about Lynn Edward Harris