Quote -  gamma on

Based on that, it seems obvious what the problem is.

Go into the material room, and look at your trans map.  Odds are if you check what the gamma is, it will say "Use render settings".  Change it to "custom setting" and set to 1.

Any texture file that's used for anything other than colour (bump, displacement, mask, transparency) should all have their gamma set to 1, no exceptions, all the time.