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In my modern-day setting, people who use magic are called Fjǫlkunnigr, who gain four different powers based on core aspects of their personality called Megin. There are only three ways for someone to become Fjǫlkunnigr: binding their souls to one of twelve ancient artefacts called Divine Tools, touching magic tainted-objects or inheritance. Inheritance occurs because Fjǫlkynngi is a dominant genetic mutation transmitted from parent to offspring, and second-generation Fjǫlkunnigr will not have their Megin active until their bodies reach full physical and sexual maturity.

There also exists a faction called the Degenerates consisting of humans parasitised by a species of mind-controlling and gene-altering pentastomids hailing from the Late Cambrian. These pentastomids’ modus operandi is after turning adult humans into genderless humanoids that lay pentastomid eggs; these pentastomids cherry pick desirable genetic traits from said hosts, which they then pass on to their offspring, allowing them to modify future hosts via a retrovirus. The problem is that the Degenerates’ four ruling kings are first-gen Fjǫlkunnigr, and their pentastomid “offspring” can’t convert parasitised humans into Fjǫlkunnigr.

There would probably be a lot of evolutionary pressure for the Degenerates to evolve toward a species-wide acceptance of the Fjǫlkynngi gene. So why haven’t they?

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    $\begingroup$ I'm having a little problem understanding what it is that's exactly going on, and what the problem is. Do the degenerates that stole traits have magic, but their offspring don't? $\endgroup$ – DWKraus Sep 11 '20 at 2:30
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    $\begingroup$ @DWKraus That's the whole plot synopsis. $\endgroup$ – Arbiter Elegantiae Sep 11 '20 at 2:34
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    $\begingroup$ For reference, pantastomids are parasitic arthropods, sometimes known as tongue worms, and considered by some to be crustaceans en.wikipedia.org/wiki/Pentastomida $\endgroup$ – Tom J Nowell Sep 11 '20 at 14:25
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    $\begingroup$ xkcd.com/483 $\endgroup$ – user253751 Sep 11 '20 at 14:28
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    $\begingroup$ So a parasite effectively sterilize its host, than produce another generation of parasites which carry a "genetic payload" selected from their (parasite) parent's host. And those offspring parasites infect new hosts and modify their genes with that payload? $\endgroup$ – G0BLiN Sep 11 '20 at 19:08
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Puzzling genetics:

If your parasites that have stolen genes from magic hosts can use magic, but their offspring can't, here are a few of the routes to how this could work.

  • Sexual maturity as a developmental step is required for the magic to work. Since the subsequent offspring render their hosts into asexual beings to complete their life cycle, the gene transfer takes place AFTER the gender alteration. Since the subsequent hosts are essentially sexless, this critical developmental step is missed and the parasites can't use magic because their hosts didn't have the gene when they underwent sexual maturity.
  • The magic gene involves some kind of unstable genetic structure that the parasites involuntarily correct as part of the assimilation process. Since the parasite involuntarily eliminates (for example) trinucleotide repeats over a certain number in the genes they edit out, they are accidentally eliminating a critical function of the magic.
  • The magic abilities are incompatible with some gene the parasites possess. The parasites might have a related gene that competitively inhibits expression of the magic (the parasites ARE magic, so they can't POSSESS magic). The initial parasites are able to do magic because a significant number of the host cells remain uninfected by parasite DNA, and in the second generation of parasites, all the cells genetically transformed by the parasite are ALSO transformed with the incompatible parasite gene.
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  • $\begingroup$ I like all those. I'd propose another possibility: It is not a single gene which decides magic ability. It is a mix of different genes on different chromosomes. Mixing and matching genes and traits is all well and good, but you mix up the wrong ingredients and the recipe doesn't work any more. Like making a cake. Mix less sugar than you should and the yeast doesn't have enough fuel for the cake to rise. The only way for offspring to be magical is to be exact clone of a parent, which is not possible for the parasites. $\endgroup$ – jo1storm Sep 11 '20 at 15:05
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Have you ever had a text document open, used "replace all", and regretted it? It matches something it's not supposed to and blats it.

For example "corp"->"corporation" changes "Microsoft corp" to "Microsoft corporation", and then you realised it changed "Incorporate" to "Incorporationorate".

Consider the human DNA, and parasite DNA, as big text documents. Consider these retrovirusese as find-and-replace operations in a text document.

Genetics is rarely 1-gene 1-result. Often it takes many sequences of DNA working together to accomplish a result. Theres junk DNA serving no useful purpose, genes repeated, and some genes control multiple things.

The same string of DNA may do two different things in two different organisms. Humans and bananas share something like 50% of their genes, a banana retrovirus to "make the fruit sweeter" may match a shared sequence and wreck havoc on a human.

Running a retrovirus meant for humans on the parasite nearly never matches, and when it does, usually nothing important changes.

But the magic mutation occurs in a sequence of genes in humans, which also occur in the parasite.

The retrovirus to make humans magical also targets the same sequence of DNA in the parasite, except in the parasite, the thing its matching controls something important, which the retrovirus changes.

After a few failures, the parasite has learnt not to make a retrovirus of that particular trait.

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Since the magic gene only activates for second generation inheritors, you could reason that for some reason or another they make these humans gender-less first, then do their gene editing.

Because of this de-sexing, you could reason that the gene is dormant and unable to be activated due to a lack of the proper hormones. Though i'm not sure on the science parts, considering you also have magic I don't believe it will be too unbelievable.

You can also make it a cultural thing: Perhaps they have these abilities, but do not use them as only the original rulers have that right? This works better if these parasites are more hive minded and extremely traditional, perhaps even having a caste system not unlike bees?

These are just some possible solutions to your problem. Hope I could be of assistance!

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Lets approach this from real world biology, assuming the existence of the parasite and desired effects.

1: How do The Parasites Acquire Genes?

Horizontal gene transfer! Happens all the time in bacteria. However acquiring a gene doesn't mean it's useful or helpful. That's why your degenerates stop expressing gendered features.

2: The Various Effects on Degenerates?

The infection replaces key parts which are essential for the degenerates to survive and reproduce on their own. But that's not important, it's the parasites reproduction that has the evolutionary pressure. Things that impact the parasites fitness are what matters here. Degenerates don't have babies, and parasites infect normal humans, so why keep gendered traits?

As long as the parasite can continue to reproduce, they don't need to worry about preserving those parts of the genome during infection.

3: Laying eggs

The parasites lay them, using the resources of the host, this is how parasites work in the real world. Otherwise, it isn't really a parasite, more like a virus.

4: The Fjǫlkynngi gene?

Just because your genes code to create a Giraffe doesn't mean you turn into a Giraffe. Perhaps if your genome was replaced during the zygote stage you might follow that development process, but in adult humans this process has already happened. So you're much more likely to die within a day or two than turn into a giraffe.

Now lets say the parasite acquired this gene, and transmitted it to a degenerate, that doesn't mean the genes effects somehow apply

This does mean however that if somebody who already has this gene is infected, then the infected magic user can use magic. That would explain your kings.

So all you need is for the gene to result in an organ or developmental change.

This also means that the gene transferral might be successful if a parasite transforms a pregnant woman during the early stages of development.

5: Why Don't They Just Evolve to Bypass This?

There would probably be a lot of evolutionary pressure for the Degenerates to evolve toward a species-wide acceptance of the Fjǫlkynngi gene. So why haven’t they?

That's not how evolutionary pressure or selection works. Given a population, traits that help them survive are more likely to be passed on, and traits that make them less likely to survive aren't.

There is no guidance towards particular features, your genome doesn't look at other creatures and decide it wants one of their traits. For them to evolve acceptance of that genes benefit, they would need to have some accidental mutation that conveyed a slight survival advantage in that direction, combined with successful reproduction.

If another mutation conveyed a stronger version of that manifestation then it would result in a greater chance of reproduction, and so on until you've arrived at the goal. Not all mutations will lead in that direction though. Genetic mutations happen all the time and they have no effect or cause disease.

And that's why your degenerates didn't just evolve to use magic.

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