Belonging to, or proceeding from, the original stock; native; hence, not counterfeit, spurious, false, or adulterated; authentic; real; natural; true; pure; as, a genuine text; a genuine production; genuine materials.
If you are referring to synthetic meaning "man-made", then the opposite would be "natural" or "genuine". There is another usage of synthetic, in which the opposite is "analytic".
Real, genuine.
genuine, real
The opposite of "fake" is "genuine" or "real."
The opposite could be true, or real, genuine.
real, genuine
The opposite of bastard is genuine and/or born within wedlock, of identifiable roots.
what is opposite meaning of lucid
Genuine, natural, real, unaffected, unpretentious
Haram (Muslim word for "not clean/genuine")
mindedness...........
the opposite meaning is to be rememberd or known not to be forgotten