Since we're talking about chromosomes here, its pretty safe to assume that Bryan had a mutation that, unlike all the other muations that seem to define Marie's Children, was unacceptable to "Mommy" Marie. Though its not really mentioned in the story, I believe the Jebediah and the rest of the Savior-line either come from Marie's bloodline or they decended from someone else who was very much like Marie, only not so much into the killing and general chaotic creepiness. We know from the story that Jeb is very old but as spry as a kid, that he can move and fight with unnatural speed, strength, and accuracy, and that silver is as much a bane to him as to Marie's Children. Taking all of that into account, it seems likely that there is some kind of conneciton somewhere.
**UNdead / GirlCo: All we wanna do is eat your brains. We're not unreasonable, I mean, no one's going to eat your eyes.**
I had originally thought that it explained the existence of the saviours but you're quite right nothing mentioned about Jeb's being different. It could explain why Brian has a normal lifespan rather than the long livedness of say Jeb or Marie's kiddies, but I don't know if Brian's lifespan has been determined yet. It may be that MCss grow to adulthood then stop aging so Brian might have the extra lifespan yet... I also wondered if it was possibly the reason why Brian did not respond to the King's voice or commands the way the rest of MCs did or maybe this was because Brian could have been a King too.
as to what the properties of an XYX would be if Bri Bri and Robyn had made a baby
obviously the combos are :
XYZ - like the kings/Brian
XXZ - like Marie
XY - normal Male / ringer
XX - normal Female (although none of these seem to have come from a marie x human or at least not mentioned in the book)
XZ or YZ - abnormal (normal) Marie's Child mutant thing with nasty sharp pointy things, no need for bandaids and a dislike of silver
XYX - just realised as I was writing this it is Klinefelter syndrome which is mentioned in the book (I am such a tard)
which I guess leaves us with just XXX not being mentioned which everyone knows means you are bald and have a penchant for driving fast cars and extreme sports and ultimately end up being recruited as a government superspy and riding some kind of Bioweapon missile like a rodeo clown.