Let's say that we have a human society somewhere away from Earth which is psychologically ready for genetic transhumanism... perhaps humans just aren't coping with the environment, perhaps the local religion supports it, perhaps the local wildlife are a bit much to handle, or perhaps people just want their kids to have better bodies than theirs.

So, the geneticists whip up the DNA to insert into an embryo so that the baby will be born with the desired alterations. It's tested and approved for general release.

Now, the thing is, the parents paying for their child to receive this upgrade want their grandchildren and their descendants to inherit the augmentation... regardless of whether their child is male or female, and regardless of whether their child's reproductive partner is enhanced... without their grandchild having to be modified at the embryonic stage.

Conversely, those commissioning certain enhancements may not want the enhancements propagated to the next generation, except under specific circumstances... perhaps only if the reproductive partner also has the enhancement, or perhaps only if a particular environmental factor is present or absent.

So, the geneticists add an extra chromosome pair to the zygote, containing all the necessary genes for the enhancement. The added chromosomes are replicated along with the others as usual during mitosis (normal cell division), and have the desired effect upon the individual carrying them.

Now, the difference that allows both backwards-compatibility and selective non-transmission is only noticeable during meiosis (the cell divisions that produce haploid gametes).

Normally, before mitosis or meiosis occurs, a cell's DNA is replicated once. To prevent the creation of too many copies during mitosis or meiosis, a protein that binds to a start sequence on the chromosome is produced, and replication starts from there. The copies do not have the starter protein bound to them, and so are not themselves copied.

The difference is that the backwards-compatible chromosomes have an extra start-sequence that is different to the natural one. During mitosis, this is of no consequence. However, during meiosis, during DNA replication, another, different starter protein is produced that only binds to the extra different start sequence. This results in the two cells that result from the first meiotic division having four copies of the extra chromosomes rather than two. Then, during the second meiotic division, the paired chromosomes are split between the resultant cells. This results in the gametes being haploid with respect to the original chromosomes, but being diploid with respect to the new chromosomes.

So, whether the enhanced person's gamete is an egg or a sperm, on fertilisation with the gamete of an unenhanced person, the resultant zygote is fully diploid and enhanced, gaining both copies of the enhancement chromosome from their enhanced parent.

Now the trick is to not end up with extra copies of the enhancement chromosome when both parents are enhanced. This may be achieved by signal proteins on the surface of the gametes. When fertilisation occurs, if both egg and sperm are carrying the enhancement chromosome, they each have a male or female specific marker protein on their surface. If the sperm detects the female marker or the egg detects the male marker, a process similar to X-inactivation occurs to one of the enhancement chromosomes within that gamete, rendering that chromosome inactive. However, unlike X-inactivation, the inactivated enhancement chromosomes are destroyed shortly after fertilisation.

In order to transmit an enhancement chromosome only when reproducing with a similarly enhanced partner, meiosis is left unchanged, so that the resultant gametes are fully haploid. If the other gamete does not have the requisite marker, the enhancement chromosome is inactivated and destroyed. When both gametes have the enhancement chromosomes, the zygote should be properly diploid for all chromosomes.

When the enhancement chromosome must be transmitted only in the presence or absence of a particular environmental marker, that chromosome has only an alternate start sequence, and during meiosis, the alternate starter protein is produced only in the presence or absence of the marker, and the enhancement chromosome is destroyed in the absence of the alternate starter protein, resulting in unenhanced gametes. The alternate starter protein is always produced during mitosis.

So... is this feasible or would it have problems? Could it be improved?

  • $\begingroup$ @DWKraus, The entire point of all this is to create a potentially very heavily enhanced subject who is still interfertile with unenhanced humans, rather than creating a new species $\endgroup$
    – Monty Wild
    Jun 5, 2020 at 5:00
  • $\begingroup$ How do you delete out the extra chromosomes? $\endgroup$
    – DWKraus
    Jun 5, 2020 at 5:10
  • $\begingroup$ @DWKraus There are a number of ways... such as marking them as unnecessary with a special protein, which facilitates absorption into a vacuole which contains deoxyribonuclease, which breaks down the DNA, or perhaps with the use of a DNA Depolymerase. It doesn't really matter how, just that the end result is that the excess copies are inactivated and destroyed. $\endgroup$
    – Monty Wild
    Jun 5, 2020 at 5:51
  • $\begingroup$ Healthy individual should have 23 pairs of chromosomes, too little or too much they develop turner syndrome or down syndrome respectively. The rest we call them mixed. $\endgroup$
    – user6760
    Jun 5, 2020 at 7:00
  • $\begingroup$ Sorry I mixed up, should be Edward syndrome for extra chromosome. $\endgroup$
    – user6760
    Jun 5, 2020 at 7:12

3 Answers 3


Why put the enhancements into the base human genome itself?  Why not put them in their own artificially engineered organelles? 

Like mitochondria, these "metachondria" would be genetically isolated.  The traits encoded in them don't participate in the chromosomal square-dance of meiosis.  Unlike mitochondria, metachondria would be inheritable from either or both of the parents.  Whatever mechanism destroys paternal mitochondria simply need not affect these artificial organelles. 

Having your enhancements locked inside metachondrial bodies gives you more flexibility in your engineering.  You can have traits that are only carried through paternal lines, if p-metachondria self-destruct during egg formation.  Likewise, m-metachondria that self-destruct during sperm formation only pass through maternal lines, just like the mitochondria that inspired them.  You can have u-metachondria that pass universally, regardless of which parental line. 

Do you want traits that pass paternally, but only express when the maternal line is also properly enhanced?  Then you make p-metachondria that lie dormant except when the matching m-metachondria are present.  That way, if Daddy marries the wrong girl, but Junior marries the right girl, the grand-kids still get to be part of the enhanced extended family.  Do you want more flexibility than that?  Ok, how about p-metachondria that lie dormant unless Mommie has been taking the right supplements since before conception? 

The best thing is the option of a fail-safe kill-switch.  Engineer things such that the right drug purges any metachondria from the reproductive system. You're back to baseline humanity in a single generation, if you need to be.  If there's an undesirable metachondrial mutation, it can be not merely treated but outright eradicated. 

Don't patch the human genome itself.  Leave all that code intact.  Instead, write an overlay.  Write a plug-in.  Write something that's easier to roll back, easier to upgrade, easier to debug in isolation. 

An entire and separate artificial organelle body earns you a wider range of solutions than just a few strands of customized DNA could every buy.  It's not just new code; it's a new sub-processor handling the new code. 

  • $\begingroup$ In a sense, the additional chromosomes are seperate to the basic human genome. By putting the additional chromosomes in seperate organelles, you're effectively requiring that they be bigger and more complicated, as they'll need to have their own protein synthesis mechanism rather than piggybacking on pre-existing mechanisms. Having an extra organelle with its own metabolic requirements is not as efficient as time-sharing use of existing mechanisms, and there may come a time when efficiency is critical. $\endgroup$
    – Monty Wild
    Jun 6, 2020 at 5:32
  • $\begingroup$ Additionally, human sperm do have mitochondria... quite a lot of them, actually, but they are left behind during fertilisation. It would be difficult to include another organelle without adding yet more unnecessary weight, and to do it in such a way that it would be carried through the egg's cell membrane without being destroyed by the acrosome. $\endgroup$
    – Monty Wild
    Jun 6, 2020 at 5:53
  • $\begingroup$ Yep. Encapsulating the metahuman genome in metachondria isn't something you get for free. We can expect costs and risks typical of other von Neumann nanobots. On the other hand, loose plasmid constructs and artificial chromosomes aren't free of risk, either. What does having more than one square-dance on the same dance floor entail? Are we looking at potential disastrous mutations from cross-over mismatches? Could we have alternate start sequences attaching to non-alternate chromosomes? How could we even get to "tested and approved for general release" without a clean fail-safe kill switch? $\endgroup$ Jun 6, 2020 at 15:27

Well, as long as the enhanced humans don't have any changes to the core structure of their genes (no extra chromosomes and such), they are automatically backwards compatible with normal humans since they are still human.

Then the inheritance issue could be solved by being able to decide if a gene is recessive or dominant. That way if they want their child to be able to spread, they can force the gene to be dominant or maybe even super dominant so that it also overrules normal dominant genes.

Now this would still create an issue when both partners have a dominant enhancement, since that is just a limitation of biology, but that could be an interesting part of the story. They might be forced to seek the assistance of a geneticist to make a custom combination of their genes.


It should work, but...

  • It would be simpler to rearrange the genes on the chromosomes so that the transhumans would be incapable of mating with non-transhumans since the genes wouldn't line up. Or if you add a 24th pair of chromosomes, then the resulting children with a normal will have mismatched chromosomes like a trisomy (resulting in an infertile mule?) Thus preventing uncontrolled gene transfer. The transhumans would be a new species.
  • But maybe you don't want to start a new species altogether. An insertable copy of the gene with built-in CRISPR could excise the gene in the absence of a suppressor, also added, which needs to be homozygous to function. All transhumans would be homozygous positive for the suppressor, all normals would be homozygous negative.Two transhuman parents have homozygous suppressor kids who become transhumans. A transhuman and a normal have heterozygous kids who delete the genes (including the suppressor) and are normals.
  • A fun alternative would be to have normals all have a built-in anti-CRISPR defense gene engineered in so they're protected from people misusing CRISPR. All inserted genes from transhuman people would be excised as pro-CRISPR, and would result in a normal, whereas all transhumans wouldn't have the anti-CRIPSR gene. The anti-CRISPR would (ironically) CRISPR itself into the chromosome copies from the transhuman parent so the anti-CRISPR gene would always be homozygous. Again, two transhumans will have transhuman kids, but a transhuman and a normal will have the anti-CRISPR and give birth to normal kids. Just a thought.
  • This should give you two additional alternatives that allow interbreeding. Your idea should still work, but messing with chromosome migration seems like dangerous territory. (I know, wild gene rearrangements like I'm suggesting sound PERFECTLY safe, right?) The last option (CRISPR defense) allows for a society that was abused by uncontrolled gene manipulation but doesn't want it any more, and doesn't rely on "good" actors who design their kids to only pass on genes to other transhumans. It does require everyone but transhumans to accept gene alteration (even if just for defense.)
  • $\begingroup$ This rather defeats the purpose... for a single enhanced person to be able to have all his or her children also be enhanced, or for an enhancement to be passed on according to conditions, without making the enhanced person a new species. $\endgroup$
    – Monty Wild
    Jun 5, 2020 at 5:36
  • $\begingroup$ In the suppressor scenario, the children will be homozygous suppressor with a transhuman (genes pass) but heterozygous with a normal (genes are excised). In the anti-CRISPR scenario, the genes are passed to children when mated with a Transhuman, but excised when mated with a normal with anti-CRISPR. Isn't that the outcome you wanted? Two transhumans have transhuman kids, but if anyone mates with a normal, the kids are normal? $\endgroup$
    – DWKraus
    Jun 5, 2020 at 5:46
  • $\begingroup$ This would make for an interesting model for "immortal" or "faerie" races, where they can mate with humans, but the resulting children are always essentially human. Take that, fantasy plots! $\endgroup$
    – DWKraus
    Jun 5, 2020 at 5:57
  • $\begingroup$ In some cases, yes... but the normal will not have any genetic modifications. For the non-transmission scenario, it's more a "Your kids can't have this if you don't reproduce with your own kind" thing rather than a "my parents don't want our kids getting your enhancements" thing. There's also the "If you leave the homeland/go to our enemies' homeland, your kids born there won't have your enhancements" thing. $\endgroup$
    – Monty Wild
    Jun 5, 2020 at 5:59
  • $\begingroup$ The middle scenario doesn't require any modification of the normal population. If it's a power thing, then make the transmission of the chromosome require a special co-factor medication the parents need to undergo meiosis while including the gene. If they leave home, they get no co-factor and they lose the special genes in the next generation. If they mate outside of government permission, no co-factor, no extra chromosome, normal kids. Cheating on your spouse? No co-factor, no extra chromosome, normal kids. $\endgroup$
    – DWKraus
    Jun 5, 2020 at 6:08

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