What kind of mutations would be necessary to cause humans to start producing fertile hermaphrodites?
There are a few ways a human can be born a hermaphrodite. According to Wikipedia:
- It can be caused by the division of one ovum, followed by fertilization of each haploid ovum and fusion of the two zygotes early in development.
- Alternately, an ovum can be fertilized by two sperm followed by trisomic rescue in one or more daughter cells.
- Two ova fertilized by two sperm will occasionally fuse to form a tetragametic chimera. If one male zygote and one female zygote fuse, a hermaphroditic individual may result.
- It can be associated with mutation in the SRY gene.
The same wiki has this to say about fertility:
There are no documented cases in which both types of gonadal tissue function.
Therefore a hermaphrodite may have both female and male sexual characteristics, but will only be able to produce either female or male gametes (ova or sperm).
The wiki also says:
Although fertility is possible in true hermaphrodites, there has yet to be a documented case where both gonadal tissues function, contrary to the misconception that hermaphrodites can impregnate themselves. As of 2010, there have been at least 11 reported cases of fertility in true hermaphrodite humans in the scientific literature, with one case of a person with XY-predominant (96%) mosaic giving birth.
As to how hermaphroditism might be the norm rather than a rarity... I cannot think of a way in which hermaphroditism may be, or might have been, a characteristic reinforced by natural selection in our species. As an author of a fictional world you might come up with one - or it might be due to artificial selection, or genetic engineering.
There is a research paper about this subject although if you want more than the abstract, you'll have to pay for it.
Although I am not a doctor, from my understanding, you
Chimeras are the result of fusion of two zygotes to form a single embryo, producing an individual with genetically different kinds of tissue. If the fused zygotes are of different sex, the individual develops both ovarian and testicular tissues. The majority of these people are best reared as females and many pregnancies with living offspring have been reported in persons reared as females, and several cases has fathered a child. During ovulation, a negative pressure occurs in the lumen of the oviduct and it produces a vacuum effect which has made several pregnancies possible in subjects lacking an ipsilateral ovary by allowing the transperitoneal migration of oocyte from the contralateral gonad. Self-fertilization was reported in many flowering plants, in a kind of fish and in a case of rabbit. They have both eggs and sperms in their body and at fertilization, one sperm cell fuses with oocyte to form an embryo. Self-fertilization may also occur in human. A scenario is presented here for a woman to have a son without a father: she is a chimera of 46,XX/46,XY type resulting from the fusion of two zygotes of different sex types and she develops both ovary and testis in her body. Since XX cells tend to gather on the left side while XY cells on the right, she develops an ovary on the left side with a oviduct and a testis on the right side located in an ovarian position with no duct. Müllerian duct regression on the right side is mediated by the antimüllerian hormone derived from the ipsilateral testis and testosterone secreted from Leydig cells does not prevent the regression of the Wolffian duct. Therefore, neither an oviduct nor an epididymis and vas deferens is present next to the testis on the right side, and lumens of a well-developed rete testis have an open access to the abdominal cavity allowing the sperms to be picked-up by the contralateral oviduct. Both gonads are functional and produce spermatozoa and oocyte respectively after puberty. At the time of ovulation, estrogens increase the motility of the oviduct on the left side which results in a negative pressure in the tube and oocyte and sperms are picked-up into the tube with the help of this vacuum effect, taking both gametes to the fertilization site in the oviduct. Since the sperm contains a Y chromosome, this fertilization gives rise to a XY male embryo.
Apparently there is also a person who has given birth to children (Although I cannot find a super official site to corroborate)
The term ‘hermaphrodite’ is generally considered to be offensive when applied to humans. The correct term is intersex, and is something that is already far more common than you might think.
The belief that Homo sapiens is absolutely dimorphic with the respect to sex chromosome composition, gonadal structure, hormone levels, and the structure of the internal genital duct systems and external genitalia, derives from the platonic ideal that for each sex there is a single, universally correct developmental pathway and outcome. We surveyed the medical literature from 1955 to the present for studies of the frequency of deviation from the ideal male or female. We conclude that this frequency may be as high as 2% of live births. The frequency of individuals receiving “corrective” genital surgery, however, probably runs between 1 and 2 per 1,000 live births (0.1–0.2%).” Am. J. Hum. Biol. 12:151–166, 2000. © 2000 Wiley-Liss, Inc.
I would suggest you do some more research about intersex people if you intend to write a story about this topic as intersex individuals tend to be marginalized quite a bit. Here are some resources that may be a good place to start.
note: I apologize for any formatting issues on this post. I am thumping this on my phone and I haven’t posted too many answers.