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EDIT: I unexpectedly had a possible solution to this come to me, and posted it as answer here. Please look it up and give feedback on it.


I'm trying to come up with a plausible sex determination system that has the characteristics listed below. It's intended to be general-use, i.e. the subject species could be a modified version of Homo sapiens, humanoid aliens, a nonhuman animal, etc.; for convenience, though, I'll be assuming that the species is human(oid).

  1. There are five sexes: female, male, and three kinds of simultaneous hermaphrodites (berm, merm and ferm, collectively known as herms).
  2. Sex determination involves three sex chromosomes: X (tied to females and ferms), Y (tied to males and merms), and Z (tied to herms in general).
  3. All of the hermaphrodite sexes are fairly androgynous in facial features and fertile in both the male and female organs; berms have the most androgynous-looking, and their physical development follows a pattern that is intermediate between males and females (e.g. on average they're between both in height), while merms and ferms parallel males and females in their dimorphism (e.g. merms develop heavier jaws, greater height, a "V-shaped" torso, and find it much easier to achieve muscle hypertrophy).
  4. All possible pairings between the five sexes can produce a child of any sex.
  5. The sex system is stable and is in no danger of having any of the sexes being outcompeted within the foreseeable future on an evolutionary timescale.

I've looked into the various existing systems, and couldn't find anything that appeared to be sufficient for my needs. For example, an XYZ system didn't work, even if I employed a second pair of sex chromosomes (like the white-throated sparrow appears to be slowly evolving to), because every version resulted in either males/females or herms being unable to produce the other.

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    $\begingroup$ +1 for berm, merm and ferm $\endgroup$
    – Daron
    Commented May 28, 2023 at 8:29
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    $\begingroup$ I conjecture having 6 sexes instead of 5 would make this much easier. $\endgroup$
    – Daron
    Commented May 28, 2023 at 8:37
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    $\begingroup$ There are six combinations XX, XY, YY, ZX, ZY, ZZ of course. $\endgroup$
    – Daron
    Commented May 28, 2023 at 11:00
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    $\begingroup$ A berm, merm, and ferm walk into a bar... $\endgroup$
    – user458
    Commented May 28, 2023 at 23:44
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    $\begingroup$ BTW, science-based and science-fiction aren't meant to be used together (they define the scope of the answers). Could you delete one of them? Do you want science-based answers (answers must comply with known science) or science-fiction answers (answers should reflect scientific understanding, but are not limited to known science)? $\endgroup$
    – JBH
    Commented May 31, 2023 at 12:35

8 Answers 8

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Carrier Chromosomes

The species is hybrid diploid-triploid. Most alleles come in pairs, but there are some behind the scenes helper chromosomes that come in triples instead.

In particular everyone has XX,YY,ZZ pairs that do not manifest anything, but are used to store data. This is needed to get the same ratio of female, male, unviable, merm, ferm, and berm.

The child's ses is determined by the manifesting pair XX*,XY*,YY*,ZX*,ZY*, or ZZ*. Here * is a third helper chromosome that just says manifest this one guys. Otherwise the entire chromosome is recessive.

These manifested pairs correspond to female, male, unviable, merm, ferm, and berm respectively. The nonexistent YY* pair is unviable. The embryo fails to attach to the uterine wall and is flushed out with next month's menstruation.

This is how reproduction works. A male and merm with male and merm children.

enter image description here

Each child gets one chromosome from each parent's XX,YY,ZZ pairs. I have not drawn the arrows for YY and ZZ to avoid cluttering the image. They work the same way as the XX pairs. Your homework is to draw the missing arrows.

The above gives the child data storage pairs. Each parent also contributes one chromosome from a randomly chosen data storage pairs to the child's manifesting pair. This ensures an even ratio of sexes. The child also gets one of the parent's * chromosomes.

The XYZ in the manifesting chromosomes do not pass to the child.

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  • $\begingroup$ This is a very interesting approach. Is it just me, or does the "helper chromosome" seem to be functioning like an epigenetic marker that activates whichever chromosome it's placed on? $\endgroup$
    – MarqFJA87
    Commented May 28, 2023 at 21:13
  • $\begingroup$ @MarqFJA87 Something like that. I am not familiar enough with epigenetics to know whether they are inherited the same way. $\endgroup$
    – Daron
    Commented May 30, 2023 at 13:21
  • $\begingroup$ Some epigenetic markers have been demonstrated to be inheritable, at the very least. $\endgroup$
    – MarqFJA87
    Commented May 30, 2023 at 15:44
  • $\begingroup$ My understanding leaves me with questions. A species that is sex determined by polyploidy effects all chromosomes. In many insects, for example, the female is diploid and the male haploid. When the female mates, female progeny are given half the chromosomes from mom and all the chromosomes from dad, and males are given only the mom's half. This means males are more technically similar to neuter. In your answer you seem to be describing a triploid species, or maybe a polysomy, where only one chromosome set is multiplied. Which individuals are diploid and which are triploid? $\endgroup$
    – user458
    Commented Jun 5, 2023 at 14:19
  • $\begingroup$ @fredsbend They are hybrid diploid-triploid. For example the XX YY ZZ are pairs and things like ZX* are triples. $\endgroup$
    – Daron
    Commented Jun 5, 2023 at 20:22
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Sex determination by chromosome ordering

All humans have all three chromosomes, but they are encapsulated in a protein structure that preserves ordering. The ordering determines which phenotype is manifested.

When two humans mate the new egg draws random chromosomes from both parents, but the protein that collects the chromosomes always collect one of each type. It uses a special nucleation protein that has three slots angled at 120 degrees, one for each type. Once it has captured three chromosomes it is inserted into an encapsulation. It can be rotated or up-down inverted before it is inserted, but once encapsulated it is fixed. Creating 6 possible orderings of chromosomes.

The orderings are:

  1. X Y Z
  2. X Z Y
  3. Y X Z
  4. Y Z X
  5. Z X Y
  6. Z Y X

If Z is first the type is always berm. The order of the X and Y after it are not important.

If X is first more female characteristics will be manifested. Whether it is a female or a ferm is determined by the second chromosome. If the second is an Y it enforces the female characteristics making a female, if it is a Z it enforces herm characteristics making a ferm.

The same for the male Y chromosome. If the second is an X it enforces the male characteristics making a male, if it is a Z it enforces herm characteristics making a merm.

How the proteins that make this happen exactly work, I can't tell. I'm not a biologist but a software engineer.

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    $\begingroup$ I like this approach. It's quite inventive IMO. $\endgroup$
    – MarqFJA87
    Commented May 31, 2023 at 19:18
  • $\begingroup$ The use of "Chromosome" in this doesn't make sense, but these could be gene sequences within a chromosome. $\endgroup$ Commented Jun 5, 2023 at 9:42
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HandWavium

Assuming your humans aren't 100% like us,the mating system might introduce a certain randomness to it, you can even say that it isn't (fully? if necessary) understood. Or that Hermaphrodites can choose what Chromosome to pass on due to a complicated system of chemicals in their brain, pretend chromosomes are like features you pass when you have a child, you don't choose if they keep your stellar physic or your poor eye sight, let fate have a hand on it.In your "universe"/"world" it could be a normal things, just how in our the Chromosome dictates sex, in your world it could not mean anything.

The blob comes to mind when talking about this subject.

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  • $\begingroup$ Good old "The Blob". $\endgroup$
    – Daron
    Commented May 31, 2023 at 16:38
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You may be describing Trioecy

Trioecy is a sexual system that contains both males, females, and hermaphrodites, and is found in both plants and animals. Now, as a disclaimer; on Earth, it is more common in plants, and none the animals that it occurs in are chordates, ie. human or any kind of humanoid/tetrapod/sci-fi stand in, however, you have a bit of leeway here when it comes to alien planets.

One drawback of the system is it tends to be unstable on Earth, gradually shifting into dioecy (male and female) or gynodioecy (female and hermaprodite). But all you really need is a plausible reason for the system to stay in place, particularly in science-fiction, the sky's literally the limit.

One factor to keep in mind is that sex and mating complexity tends to increase the longer and more reliably a species lives. Species with very low life expectancy or in sparse or uncompetitive environments will want to produce asexually as frequently as possible. It was only when ecosystems were filled with life, and particularly with a threat of being eaten and need to evolve, that taking care of one's young and more elaborate sexual systems were created (there are definitely more environmental factors that can come into play; this is a very oversimplified rundown but for now we'll leave it there).

So one particularly reason the system could develop and stay in place is that a more complex mating system meant that children are more likely to reach maturity and need to be more adaptable with higher genetic variation. And the reason trioecy may dominate is because it gives a higher percentage of the population a chance to pass on their genes than more typical dominance hierarchies that we see on Earth.

An important thing to remember is, when it comes to Life in the Universe, we have a sample size of one. There's nothing to stop you from saying that Trioecy just became a dominant form of sexual system in your world.

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    $\begingroup$ This interesting and all, but it doesn't actually answer the question that I posed: What would be a plausible sex determination system for the criteria that I outlined? $\endgroup$
    – MarqFJA87
    Commented May 28, 2023 at 3:36
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    $\begingroup$ @MarqFJA87 if you're talking about chromosomal determination it can be basically anything you want when it comes to aliens, there's no reason why an alien would use our specific chromosomes in particular. Intersex variation already exists in humans, you could simply make it more likely. In addition, chromosomes aren't the only method of determining sex; environmental affects like temperature or amount of one particular sex also matter. You could have it that a sex forms at the lowest temperature in the womb and vice versa, and all the herms are produced with the median temperatures in between. $\endgroup$
    – Rexotec
    Commented May 28, 2023 at 5:33
  • $\begingroup$ Thanks for the suggestions, but I want it to be chromosomally determined anyway. And honestly, you could substitute the letters denoting the chromosomes with whatever you want, I'm using the ones I did for convenience. $\endgroup$
    – MarqFJA87
    Commented May 28, 2023 at 10:53
  • $\begingroup$ @Rexotec Intersex conditions are not sex variations. Of the 30 or so recognized categories, almost all of them are developmental deficiencies. $\endgroup$
    – user458
    Commented Jun 5, 2023 at 14:29
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XO + YO + ZO system

Some species follow an XO sex-determination system, where they inherit one X chromosome from their mother, and zero or one X chromosomes from their father. A double-X (XX) individual becomes female, and a single-X (XO) individual becomes male.

My proposal is a variation on that system. Instead of inheriting only one type of chromosome in this fashion, your species inherits three: X, Y, and Z. Having more X than Y makes you female or ferm, while more Y than X results in male or merm. Herm-ness requires a double Z.

To give the full set:

  • XO + YO + ZO: non-viable
  • XX + YO + ZO: female
  • XO + YY + ZO: male
  • XX + YY + ZO: non-viable
  • XO + YO + ZZ: berm
  • XX + YO + ZZ: ferm
  • XO + YY + ZZ: merm
  • XX + YY + ZZ: berm

(Note that berms are twice as common as any other sex. If that is undesirable, change "XO + YO + ZZ" to "non-viable".)

Because all individuals have at least one copy of all three chromosomes, their children can be any biological sex.

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    $\begingroup$ You're making this dependent on the ratio between the sex chromosomes, huh? That is actually rather similar to fruit flies, for which sex is predicated on not only the number of X chromosomes but the ratio between them and the autosomes. $\endgroup$
    – MarqFJA87
    Commented Jun 1, 2023 at 7:06
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Wasn't expecting myself to provide a (possible) answer, but here I am.

Someone elsewhere drew my attention to the fact that microorganisms and fungi have an analogue to sexes, called mating types. Reading about it, especially the protozoan Tetrahymena thermophila and how it can essentially "choose" its mating type, led me to formulate the following sex-determination system.

There are two sex chromosomes: X0 and X*. The X0 chromosome is "neutral" with respect to sex determination (i.e. has no influence) but does carry multiple genes that play important roles in the individual's reproductive biology, and is shared by all sexes. The X*chromosome, meanwhile, contains a supergene locus that determines the individual's sex via five different alleles: XF (female), XM (emale), XH1 (berm), XH2 (ferm), and XH3 (ferm).

Stem and germ cells exhibit nuclear dimorphism: They have a somatic macronucleus, which contains the individual’s genome and governs the cell’s behavior; and a germline micronucleus, which contains the DNA information for encoding the five alleles of the X*chromosome. During gametogenesis, the gamete’s macronucleus only retains one of the five possible alleles of the sex-determining locus on the X*chromosome by way of a stochastic process of DNA rearrangement and deletion, while an identical copy of the micronucleus is passed on.

During the embryonic stage of prenatal development, somatic cells typically possess micronuclei, but gradually begin to lose them after the gonads begin to form, such that by the end of last trimester, only the germ cells and the stem cells retain micronuclei.

How plausible is this model?

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    $\begingroup$ You can use the <sup> HTML tag to add superscripts. See meta.stackexchange.com/questions/1777/… $\endgroup$
    – MJ713
    Commented May 31, 2023 at 22:21
  • $\begingroup$ @MJ713 Oh man, thanks a lot! Unfortunately it doesn't work with an asterisk as the content unless I add a space, but that's a minor annoyance. What do you think of this solution, BTW? $\endgroup$
    – MarqFJA87
    Commented Jun 1, 2023 at 7:04
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    $\begingroup$ @MarqFJA87: You can probably avoid that issue with asterisks by adding a backslash before the asterisk (as an escape character, to avoid it being interpreted as an attempt at formatting). $\endgroup$
    – V2Blast
    Commented Jun 7, 2023 at 19:55
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    $\begingroup$ @V2Blast Thanks, that worked like a charm! $\endgroup$
    – MarqFJA87
    Commented Jun 8, 2023 at 0:39
  • $\begingroup$ It's all Greek to me I'm afraid. $\endgroup$
    – Daron
    Commented Jun 9, 2023 at 18:07
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If we are talking about sentient species: It is possible that the sentient genus was split in two and left in that state for a prolonged period of time, during which one of the species were left with XY and the other evolved into XZ system, with Z a different descendant of X chromosome acting as Y but also functional in absense of X. Now there can be XX XY XZ YZ ZZ (YY does not work but ZZ does) now when the ocean is crossed and they started cross-breeding again.

In one subspecies, only one of sex chromosomes were active in females, as it is in humans (of XX only first or second X gets to be active) - so XZ may appear as ferm but also as fully typical female, while potentially producing herm offspring due to having a Z.

Of course, evolution would prefer to straighten this system out, but this is where sentience with its reproductive medicine comes into play, ensuring that even a specific combination is at reproductive disadvantage in field conditions, it is overcome with medicine.

I guess that some pairings will still not produce all kinds of sexes, because obviously XX and ZZ (or XZ) cannot produce anyone who has Y.

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Sorry for the late answer, but...

All possible pairings between the five sexes can produce a child of any sex.

That means that there is no sex at al. "Sex" does not mean more for this species as eye color for us. That is a unisex or sexless species, with an extra irrelevant property.

They are mating snails. By the snails, all snail have both male and female reproductive organs, and both mate with the other on both way:

enter image description here

What you call here sex, is no more like the color and texture of their shell.

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  • $\begingroup$ The other answers that were provided are conclusive proof that you're wrong. $\endgroup$
    – MarqFJA87
    Commented Apr 7 at 3:52
  • $\begingroup$ @MarqFJA87 1. That you could write everywhere where there are contracting answers, don't you think? 2. How about the idea that maybe my answer is only different? 3. How about thinking a little, where is the answer wrong? $\endgroup$
    – Gray Sheep
    Commented Apr 8 at 17:43

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