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Inspired by this question: Sexual reproduction without biological sex

And, to a lesser extent, this question: Would a society of simultaneous hermaphrodites have gender roles?

I'm not particularly well-versed in biology at all, let alone the wide variance of sexual characteristics that exists in the world, so I apologize if I've misunderstood/misused any terminology here. (Or if my questions are somehow just beyond stupid in a way that I failed to realize.)

For my purposes, I'm imagining a (preferably as close to human as possible) species that has exactly one biological sex. Every individual of this biological sex has all the necessary anatomical requirements to be the father or mother in a reproductive exchange. In case it's somehow relevant, their anatomy is also arranged in such a way that one couple can be simultaneously participating in two exchanges. This means that both members of one couple could be simultaneously pregnant with a child fathered by the other member of the couple.

So the main question here: If you have two children of the same two parents, but with swapped parental roles in their conception (the father of one is the mother of the other and vice versa), is there any inherently meaningful difference between the two siblings beyond two siblings who have the same parents as each other, both of whom are in the same parental roles?

I realize that a society comprised entirely of such a species could easily create social constructs such as children taking the surname of their biological mother or labeling such cross-siblings as something indicative of a relationship closer than a half-sibling but further than "actual" siblings. I'm less concerned with purely social constructs that might arise than I am with actual, meaningful differences that would affect them in the future more than "actual siblings" would be expected to experience.

If a social construct were to arise from such differences (as I expect they would), then I'm definitely interested in those sorts of downstream effects. In fact, the main reason I'm asking the question is so that I can eventually come up with exactly those kinds of downstream effects. But I'd like to set the foundation of my social constructs on something more solid than just other social constructs.

For instance, as AlexP brought up in comments, the intra-uterine environment can be very important because of genomic imprinting and/or metabolic imprinting. Aside from potential disorders that might happen at the statistical fringe, what noticeably-common differences might we see between cross-siblings? Since both parents could nurse the child, would some differences be mitigated by having a child's father nurse them instead of the mother? Would differences in mitochondrial DNA cause significant differences? Are any noticeably-visible or significant characteristics only heritable from one parent? These are all sub-questions of which I'm unsure of their relevance to the larger discussion.

I wish I could be more specific in what I'm looking for, but I'm struggling to find the words to describe it at the moment. Hopefully, I can think of a better way to word all of this fairly soon and edit my question accordingly.

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    $\begingroup$ If this strange species uses a genetic framework similar to humans, then there will indeed be a little more difference between them than between two ordinary siblings. The small extra difference comes from their development in different environments; for example, different genomic imprinting or metabolic imprinting. In humans (and other mammals, of course), the intra-uterine environment is of great importance. $\endgroup$ – AlexP Jan 25 '18 at 2:03
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    $\begingroup$ @AlexP - imprinting is the right answer. Why don't you flesh it out? $\endgroup$ – Willk Jan 25 '18 at 2:21
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    $\begingroup$ The best use of Wikipedia articles is to provide an entry point. Now that you know what to look for (epigenetics, evo-devo, genomic and metabolic imprinting) you can start doing the research. Don't forget that siblings who are not identical twins can be quite different; the little extra difference due to different intra-uterine environments will most likely be much smaller. @Will, let's wait and see if somebody whose knowledge of biology is not decades out of date will answer. $\endgroup$ – AlexP Jan 25 '18 at 3:20
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    $\begingroup$ The biology I've learned is confused by the phrase 'simultaneous hermaphrodite". Hermaphrodites are just hermaphrodites. Two hermaphrodites copulating together can impregnate each other. Please don't try to impose a version of human reproductive gender roles on hermaphrodite species. They will have their own gender identities & be comfortable with them. Parents both Mum & Dad. Easy! Happens all the time. Please remember gender identities are social & cultural constructs. Don't worry they'll cope in their own way. Please drop the "simultaneous" nonsense, it's only confusing. $\endgroup$ – a4android Jan 25 '18 at 11:24
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    $\begingroup$ @a4android Dropping the "simultaneous" would drastically alter the scope of the question. Hermaphrodites are not "just hermaphrodites". Some species behave as one sex for part of their lifecycle, and transition to the other later--in some cases possibly even switching back and forth--but never have both reproductive systems active at once. Those are sequential hermaphrodites. Simultaneous hermaphrodites have both sets active simultaneously, and can impregnate each other. They are both types of hermaphrodites, but they have drastically different reproductive strategies. $\endgroup$ – Logan R. Kearsley Jan 25 '18 at 21:29
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That depends.

Do they use isogametes, or sperm and ova?

If they use isogametes, do they lay eggs, or use internal gestation / live birth?

If they use isogametes and lay eggs, the potential differences will be utterly negligible--limited to things like the effects of diseases or nutritional deficiencies in the mother that might compromise egg production.

If they use isogametes but have internal gestation, then there will still be no genetic differences, but the developmental environment has more opportunity to produce differences. I would not, however, expect them to be large enough or common enough to be of any social consequence. (Doesn't mean a society of such creatures couldn't still decide that the facts of parentage were important, though, purely as a social construct, maybe for purposes of inheritance or whatever.)

If they use dimorphic sperm and eggs, do their cells include any organelles with independent genetic material, like mitochondria or chloroplasts?

If their DNA (or equivalent) is strictly nuclear (or equivalent), then can we also assume "normal" reproduction of individuals with diploid genetic structure via haploid gametes that each carry half of the parents genetic information, despite their physical dimorphism? If so, there will be no genetic differences, and this is the same as the isogamete case--only the effects of the developmental environment may be relevant.

If they do have organelles with independent genetic material, then there will be potentially significant differences between children with different mothers in the same pair of parents, because organelles will come only from the mother. In humans, this is why certain metabolic diseases are passed strictly through the maternal line.

Additionally, if the assumption of "normal" haploid-diploid recombination of nuclear genetic material with equal contributions from each parent is wrong, and they have some other more complex reproductive system which introduces different proportions of genetic material from each parent independent of the organelles, then you could easily get very pronounced and obvious differences between children of different mothers.

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    $\begingroup$ This is exactly the kind of explainer I was hoping to get. Thank you! I'm holding off on marking this as the accepted answer for the moment because this is all so far out of my wheelhouse that I need to look up a bunch of terms to fully understand what you're saying and I may edit the question a little bit after doing a deeper dive on all the info here. I also need to decide whether I want to minimize or maximize differences between cross-siblings, but I'm still only at the very earliest stages of thinking this through at the moment. Any thoughts you might have would be greatly appreciated. $\endgroup$ – Aporia Jan 25 '18 at 22:44

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