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When performing major genetic engineering, it is usual to begin with a single cell from which the genetically-engineered organism develops, the altered genome resulting in a different course of development that ends with the resultant organism having the desired traits.

It therefore stands to reason that if you started with an adult subject, and somehow introduced the same altered genes into that subject's cells, that subject wouldn't likely spontaneously develop the new traits, unless those traits were very simple, as in producing a new protein or not producing an old one (as in gene therapy).

In order for gene therapy to give an unaltered adult subject traits that would develop naturally in an altered zygote as it develops, it would be necessary to devise a process by which that adult subject could develop the altered traits, and have a modified virus deliver the genetic material (the Transform Genes) created to direct that task to the subject's cells. The subject's altered cells would then perform the tasks necessary to express the desired traits, tasks that might differ significantly to the process of expressing those traits during natural development.

After the desired traits are expressed, it would be necessary for the transform genes to edit themselves out of the subject's genome, and for the heritable genes to either be introduced, or to be left behind (depending upon the approach chosen by the genetic engineers: an all-in-one treatment, or an initial and a follow-up treatment), the process of giving an unaltered adult the desired traits being significantly different to the process of maintaining traits inherited from conception in an adult subject.

Have I missed anything significant in this process of transforming an unaltered adult subject into an equivalent genetically altered adult subject?

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    $\begingroup$ This sounds a little like butterfly metamorphosis. The issue being that, while the cells were in between one behavior and another, they would be capable of neither. You might need external life support (or enzyme/catalyst support) while the translation was in process. You'd also probably need a specialized method of removing shed material afterwards, like hyperactive lymph. $\endgroup$
    – Robert Rapplean
    Commented Sep 18 at 2:06
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    $\begingroup$ First of all, I LOVE the idea. Having said that, I don't think it's possible for someone who's lost an arm to grow a new arm even though genetic knowledge to do so exists in the genome. The child (and its genetics) is "wired for change" while the adult is not. So... your transform genes must of necessity return the body's engine to the child's state to allow the trait to develop, right? But having done so, wouldn't unintended trait changes also occur? Simplistically, growing taller, for example. $\endgroup$
    – JBH
    Commented Sep 18 at 2:37
  • $\begingroup$ @JBH Exactly one of the points that should be addressed in an answer... (though some species can regenerate missing limbs without growing bigger overall). It is going to be a matter of controlling all the side-effects. $\endgroup$
    – Monty Wild ♦
    Commented Sep 18 at 2:55
  • $\begingroup$ @JBH as long as we're world building fantasy science we might claim the regenerative properties of newts and axolotl (they don't get bigger) as something we can gene therapy into an organism, you then just need to cut or skim a portion of the body off at the appropriate site for the development of the new limb or organ to encourage it's growth, there may be techniques to trigger that kind of growth without those regenerative genes as well, localised hormone treatments maybe? to retrigger early development responses in cells coupled with regular stem cell injections to the region perhaps? $\endgroup$
    – Pelinore
    Commented Sep 18 at 9:21
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    $\begingroup$ @RobertRapplean axolotl ☝️ $\endgroup$
    – Pelinore
    Commented Sep 18 at 9:21

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Yes, organisms can and do change shape and function during lifetime. Yes, developmental processes do exist. A woman does not look like a girl, a man does not speak like a boy, a frog does not look like a tadpole and so on. There are many species in which striking changes occur during lifetime.

The only problem I see is actually designing the process, somehow inserting all the required genes in the correct places in the genome, complete with their regulatory sequences, and finally triggering the process.

In particular, the question is asking about a process of metamorphosis. Most people know about the massive changes in body shape and function which change a caterpillar into a butterfly, or a tadpole into a frog; but even we mammals go through a process of abrupt change in anatomy and function immediately after birth, when for example the cardio-vascular system is dramatically rearranged: the foramen ovale (which in fetal circulation shunts blood from the right atrium of the heart into the left atrium) closes, the ductus arteriosus (which in fetal circulation connects the pulmonary artery with the aorta, thus avoiding the entire pulmonary circulation) closes and is resorbed, the umbilical arteries and veins close and are resorbed, fetal hemoglobin (hemoglobin F) is replaced with adult hemoglobin (hemoglobin A).

The difficulty lies in engineering such a process. While we understand how some of these processes work macroscopically, and while we understand how some of these processes are triggered, we are nowhere near being able to design them. They are complicated. They are full of details. Their genetic base is poorly understood. For a simple example, we humans have both the genes to make hemoglobin A, and hemoglobin F, and various forms of embryonic hemoglobin. Which kind of hemoglobin we make at each stage in life is regulated by a genetic machinery which responds to environmental factors.

All in all, it is really really complicated. We must first design the macroscopic process: which cells need to proliferate, which cells need to die, what are their distinguishing features, how does this correlate with other biological systems. Then we must design the signalling: how is this proliferation and apoptosis directly controlled by hormonal or other signals. Then we must design the genetic base, complete with regulator genes. Then we must pack the entire genetic program in some sort of giant virus (because ordinary viruses are obviously too small), and we must add the mechanism to insert the whole thing in the right place on a chromosome.

Yes, there is nothing metaphysical in developmental processes. In the end we can imagine that at some point in the future we will be able to design complex developmental processes and somehow insert them into the genome of willing people.

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    $\begingroup$ Maybe even an extra dominant chromosome.... I'm already on the waitlist for extra thumbs. $\endgroup$
    – Escaped dental patient.
    Commented Sep 18 at 7:49
  • $\begingroup$ "pack the entire genetic program in some sort of giant virus (because ordinary viruses are obviously too small)" A virus is mostly just the DNA code, just snip and insert it into a small one, you might have a 'big' one after you do that of course, but you don't need to 'start' with a big one 🤗 $\endgroup$
    – Pelinore
    Commented Sep 18 at 9:34
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    $\begingroup$ “into the genome of willing people” — I do hope we can train future viruses to ask consent before infection. :-) $\endgroup$
    – SRM
    Commented Sep 18 at 12:09
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    $\begingroup$ @AlexP There’s been some significant work in biological fitness function definition in the last 5 years, enough that I’d consider it plausible sci fi to assume we will close that gap. $\endgroup$
    – SRM
    Commented Sep 18 at 12:11
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    $\begingroup$ @SRM: Yes I fully agree that for science-fiction purposes it is perfectly fine. I would go even further and say that at least some tinkering would be even fit for hard science-fiction. We have plenty of natural examples, we just need to accumulate sufficient knowledge. It is much less far out there than fusion bottles and reactionless spaceship engines. $\endgroup$
    – AlexP
    Commented Sep 18 at 12:14

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