Also keep in mind that the custom hair will typically be for only one gender. For example, if your mystery custom hair is a female hair style, then even if a male Sim inherited it, you wouldn't be able to tell (in-game) what the style was unless he passed it on to his female offspring and it became her dominant hair gene.
Your assumption about the SimDNA information is not quite correct, by the way. The first haircolor value is the dominant gene, the second is the recessive gene. Normally the dominant one would be the one the Sim is actually showing, but in the case of custom hair, if the dominant haircolor is the wrong gender for that Sim, the game will automatically display the Sim with the recessive haircolor instead. And if neither the recessive nor the dominant haircolor are the right gender for the Sim (I've seen this happen many times with inherited custom hair) then the game will assign the Sim a default Maxis hairstyle/color instead.
Thanks for the reply, in-depth info like what you just gave me is very hard to come by, it is much appreciated. Do you happen to know of any sites i might go to, so i can really dig into genetics? Other than the standard custom over maxis, black/red over brown/blond, etc.
I didn't know about the simDNA cheat until well into my current challenge, so i'm trying to back-track thru 2 deceased generations.