Not exactly... How people get Jenova genes does not seem to effect if they break down or not.
What is seems to come down to is if a particular Jenova "strain" can naturally pass it's own cells to other organisms outside itself. Which in turn seems to be linked to how well Jenova genes integrated themselves with whoever it's in. Sephiroth's cells are too stable for him to naturally make clones. So he doesn't break down. The Tsivets are the same way; no making clones but also no degradation. SOLDIERs are is this category too as are S Clones.
Isn't that just the same thing? If Genesis or whoever gets the mismatched cells to manifest Jenova powers, but due to the fact they are not
pure Jenova while manifesting said powers they suffer a downside, isn't that because their inadequate cellular structure was unable to handle it all?
That's what Hojo says to Zack in Chapter 4 when he talks to him about "Hollander's freak show." After Zack claims he doesn't feel that different from being modified by the experimental fusion chamber he stepped in, Hojo cooly says,
That's just the limit of your cellular structure. Or perhaps you would like to be altered to the point of degradation? Like some second-rate scientist I know has done?
And as explained in the game and the CC Complete Guide:
SOLDIER Degradation
A symptom of a particular flaw inherit within the G-Series SOLDIERs and their copies. It is due to an imperfect integration of Jenova cells into their bodies, and the symptoms worsen as their genetic information is spread to other beings. The first signs of degradation are changes in skin quality and hair color, and as the body’s natural immune system is damaged the subject’s skin and muscle tissue begin to degrade. Furthermore, the subject looses their ability to restrain themselves from giving in to brutal outbursts, which can temporarily raise their skills in combat, but they run the risk of eventually devolving into monsters.
This all stems from the flawed and imperfect integration of Jenova inherent to G-Types. Genesis never got pure Jenova cells, instead they were mutated cells from Gillian. Because of this, he will inevitably degrade when trying to manifest the powers and capabilities a "child" of Jenova could utilize without drawback.
The people who don't have stable Jenova integration is Genesis and Angeal, and all their clones. Genesis and Angeal don't need Hollander to make clones for them the same way Hojo makes clones of Sephiroth. They can do that fine on their own in the same way Jenova passed her virus on to the Cetra. The problem is, this somehow messes up their own DNA in the process.
Well, Angeal and Genesis do still require mako and whatever Copy Technology Shinra Scientists utilized. It's why Sephiroth and Zack found those tubes/capsules with monsters and people in them, slowly morphing into copies. They still gotta be bathed in Mako energy and given time to cook. Sephiroth Copies are made the same way SOLDIERs are made, too. You give them the cells and bathe them in Mako. Sephiroth Copies just don't physically mutate into a copy of their cellular donor. They just mentally snap and lose their ego.
Genesis and Angeal also probably did get more Jenova Cells and mako added to them when they did make SOLDIER. I don't see why they wouldn't as all the other SOLDIERs get those as well. So having more Jenova Cells added to them might be a contributing factor in making their issues worse. It's worth noting Gillian never develops any problems. As far as if Genesis and Angeal got mako earlier than when they went into SOLDIER... no where I looked for good info on the Gillian Proejct mentioned mako. So maybe Gillian had it, maybe she didn't.
No they actually didn't, because remember, they were being watched to analyzed for their capabilities as G-Type SOLDIERs. That's why Sephiroth was prevented from donating blood to treat Genesis's wound after his training room accident. Hollander specifically wanted to keep Genesis and presumably Angeal away from having pure Jenova cells administered to them. Either to hold it over their head, or to analyze their capabilities as Project G.
As a bit of a preview... here's a WIP I've been working of how Jenova' Cells map out....
View attachment 6462
...That is really fucking cool and accurate. That about sums it up
A minor quibble, but I would put Zack somewhere right in the middle of SOLDIER and the Sephiroth project, because while he didn't exhibit any of the qualities of a Sephiroth Copy, he was given S-Cells and made part of the Sephiroth Copy project. Zack has the unique distinction of having been administered Jenova cells AND S-cells.