Which generation does Pokemon Go base its move type effectiveness on?

Solution 1:

Through testing on the subreddit /r/TheSilphRoad, the answers to 1 and 2 are:

• It uses a modified 6th Gen chart as evidenced by the Fairy and Steel types.
• There are no 4x or 0x attacks, so Ground can hit Flying, and Normal can hit Ghost etc.
• Values have been changed:
  • 2x --> 1.25x
  • 1/2x --> 0.85x

As for 3, Same Type Attack Bonus (STAB) has been found in the game files to be 1.25x aswell.

Solution 2:

There is STAB in the game it gives a 25% bonus to an attack. This is especially relevant given the charge meter is filled by 0.5 for each 1 HP of damage dealt.

source

source 2