Genetic memory is a well-explored trope in science fiction. Maybe it doesn't actually work in humans (though there was speculation on RNA-encoded memories a couple of decades ago) but it might be possible to engineer it.

I'm fairly certain that the Forerunner store their memories in genetic code, which accounts for how Bornstellar became the IsoDidact and inherited the original's memories.

: How do multiple personalities share space in the one brain?

They don't. I'm pretty sure that multiple personalities is mostly a figment of Hollywood's imagination.
Well, multiple identities, anyway. Multiple personalities are exhibited by people with bipolar disorder.

: What is the
: context switch time? If it involved radical rewrites then one personality
: couldn't have a real time chat with another. And yet they do. (If I
: remember correctly)

: So we are back to assuming a generic, multi-personality capable brain with
: fast context switching.

If we go with my idea of personalities running within the Domain and communicating with human meatpuppets, the communication from the Geass to the meatpuppet could be done via a mechanism similar to auditory hallucinations.

: I feel dirty taking this as EVIDENCE that Bears ideas are possible. It's all
: a bit woo-ish and very new age and un hard science - its not even soft
: science - it's virtually homeopathy! :-) It is magic wearing science
: lipstick.

It's always homeopathy with you, isn't it? Never anything like astrology or ESP... or string theory. =D
But yeah, I guess we are trekking into the territory of "Sufficiently Advanced Science/Magic". The question is, will it be treated as Lovecraftian and beyond our understanding, or will it be given the same amount of thought that Star Trek devotes to the plot device of the week?

: I can't categorically state that it is impossible but I really don't like it.
: I don't read the fiction and feel a warm contented glow and it certainly
: doesn't feel like hard science fiction.

Same.

: Using the Flood's magic telephone, the same one that Cortana used to contact
: the Chief. And instead of having the personalities running in the brain -
: they run in a virtual brain hosted by the domain and the results of the
: simulation are fed into the brain.

Exactly!

: Why should native personalities be special?

Native personalities are are what remain when the Domain is shut down. Native personalities are what remain when the rest of the universe is destroyed.

: Maybe we could all be incorporeal
: and are just tied to a meat puppet for a while. OK, that's an old idea and
: probably is how many people think that we work. It's souls.

And honestly, that's the direction that Halo is going, as of Halo 4. Sure, 'Smart' AIs were always special, but Halo 4 reveals that even the Forerunner had trouble with organic-derived AIs. Something about the nature of souls prevents you from making them without destroying their bodies and prevents you from copying them. Oh, and since enslaving souls is evil, the souls are corrupted and driven insane.

Because it's fantasy, not science fiction.

: So Bear's ideas have a veneer of novelty and wear science lipstick but, to
: me, they are based on old ideas and old ways of thinking. But that could
: be a good thing for many people because then the ideas resonate with what
: they already think.

: I probably don't like them because they conflict with my world view. It could
: be simple prejudice.

I don't like them because they match up with my worldview. Science fiction is for exploring, not just for rehashing old concepts or dressing up real life in tinfoil and rubber foreheads.

: Makes more sense than the idea that we innately know Forerunner writing and
: understand Forerunner UIs.

Of course, Spartans have noticed that Covenant symbols are oddly familiar. But hey, are we really going to be treating flights of deja vu in the heat of combat as actual data?

: What plan? I have forgotten or I just missed it.

The 'plan' that gave rise to AIs, Spartan augmentation, and power armor.
You know... it's all... part of the plan.

*Joker laugh*

: I'm doing a reverse Sherlock
: - I'm more interested in remembering that the earth goes round the sun :-)

Odd, how that little tidbit of characterization fell away as more stories were published.

: Anyone with the ability to rewire our brains to become fast context switching
: multiple personality players could do anything. Given that level of
: competence how could she ever fail?

I'm thinking of writing an article on "Story-breaking plot devices". This will probably be included, as will teleportation... particularly trans-warp teleportation that can beam a person directly from Earth to Kronos. And, by extension, nuclear devices, grey goo, weaponized viruses, etc.