Warning: Loads of pseudo-scientific geekiness ahead!
As I've mentioned before, COI assumes that while some Cetra bloodlines remained relatively pure, a wide variety of Cetra genes have migrated into the human population via interbreeding over thousands of years. While many (if not most) humans have a few random Cetra genes, those genes are usually dormant, recessive, and/or have negligible or minor effects.
However, certain individuals have synergistic combinations (i.e., complexes) of Cetra genes which gives them extraordinary abilities which are often reflected in psychic-type powers and the person's reactions to Mako and other things (e.g., Jenova Cells, etc.). Those Cetra gene complexes fall into three broad categories or functions -- combat, support, and special. Some people have only one complex, but others have two or more (typically with one primary and one or or more secondary).
EDIT: High level genes provide a broad array of talents, abilities, and powers, while lower level genes provide narrower, more specific abilities, etc. High level genes include the same abilities as lower level genes, but stronger versions and more of them.
Examples of high-level Cetra gene complexes are:
(1) combat -- Attack, Protect, etc.
(2) support -- Heal, Bind, Veil, Create, etc.
(3) special -- Command, Morph, etc.
EDIT: Examples of mid-level or low-level Cetra genes are: Persuade, etc.
As in real life, genetics alone do not dictate an individual's behavior but they can strongly influence it. ^_^
Here are my tentative ideas on the Cetra genes various COI characters would have....
Sephiroth - Command / Attack - This genetic combo is pretty damn scary, and I think it suits Sephiroth perfectly. Command means 'to lead' and/or 'to control'. Attack implies a focus on the aggressive and offensive application of force or power. Throw in Jenova-CORE (i.e., hyper-predator) cells and you've got a VERY nasty combination that shouts 'uber living weapon'. Which he basically is.... And yes, I deliberately decided to make Attack secondary to Command, because his ability (and desire) to dominate and control is more dangerous than his fighting skills, formidable as those fighting skills are.
Zack - Bind / Protect - Zack's Protect aspect is pretty obvious. Protect implies the application of force and power in a more defensive manner than Attack. However, Protect does not mean that the individual is any less dangerous than a person with the Attack genes. Also, Protect does not automatically imply heroism in the classic RPG sense. The urge to protect can be directed to protecting only yourself, a single person, a small group of people, many people, etc. Bind may seem an odd choice for Zack's primary gene complex, but if one thinks of all the various meanings of the word, it makes a strange sort of sense. Bind can mean 'to confine or restrict', but it can also mean 'to unite' or 'to connect' or 'to anchor', etc.. Because of his personality, Zack is a people-person -- he 'connects' with people. He can be extremely likable and people are instinctively drawn to him. For Sephiroth, I can see Zack serving as anchor (connecting Sephiroth to normal humans and humanity), translator, facilitator, etc.
Aerith - Heal / Bind - Aerith positively screams Heal genes, doesn't she? Heal implies 'to restore' or 'to purify' and it suits her perfectly. And like Zack, she has an aura that draws people to her.
Rufus - (combat-type) / ??? - Like Cloud, Rufus definitely will have a combat-type gene complex as primary. And like Cloud, I haven't quite decided what his secondary complex will be. It's not necessarily the same as Cloud's, either, because while they're very closely related, they aren't genetic clones. However, I do have a few ideas.
Vincent - (combat-type) / ??? - Perhaps some sort of shapeshifting gene as secondary? Or it could be primary. Help!
Hojo - if he has Command genes(a definite possibility), that raises interesting speculation as to exactly WHO is Sephiroth's biological father. ^_- Correction: I can really see Hojo having the Persuade gene.
EDIT: Reeve - probably the Create gene, just because of his intelligence and technological abilities.
SOLDIERs, especially the First Classes, definitely have some varying levels of combat-type Cetra genes.
Some Turks may also have some low level combat-type Cetra genes, considering that in COI, they're somewhat mako-enhanced.
As for the other characters, I'm completely open to suggestions on whether they should have any Cetra genes (and what type) or not.
EDIT: Just remember that Cetra genes are not only about behaviorial tendencies or desires, but the possession of special talents, abilities, or powers to accomplish those desires.
::looks around:: ::cricket chirping::
Okay, did I totally lose you guys? Or did I scare or bore everyone away? ^_^;;;;