Something tells me we will all save ourselves a lot of headache if we just assume Mohinder's type O negative and therefore can safely inject his blood into anyone and anyone without them reacting to some normal antigen =p (A, B, Rh factor, etc)
However, I think the antibodies he carries aren't connected to the "three simple genes" (as he calls them) that determine whether or not someone has a power. They're just like the antibodies to any other invasive particle that get in the blood... some people have them, some don't, but they're biologically produced by his B-cells, coded in their genome.
Now, WHY they're coded in his genome and whether or not that was done intentionally - and if there are others who carry the ability to make them - is another question entirely. If I remember my biology correctly (heh), what antibodies a person can and can't produce are determined when their B cells first develop and divide, before they "fix" their genomes to react to various antigens... before they develop specificity. (Basically every type of antibody we can produce has a corresponding specific set of B cells that produce it. Those cells start off as general cells, that become specific for producing a certain antibody as they divide. By the time you're an adult, they're done 'choosing' their specificity). So it seems likely that Mohinder was exposed to the virus, or something similar to it, in his infancy or during his development. Considering his sister had it, and therefore his parents could easily have carried it (harmlessly, themselves being powerless), I don't see why not.