Perks appear every three levels.
You must take a Perk at least one level before the next one is available or it is lost.
You will be able to take Sniper on Level 24 (or Level 21 in Fallout 1) and the Living Aatomy any time after Level 12 or at it. So you can take Sniper and then Living Anatomy at the next Perl Level (27, 30, 33....)
In Fallout 2, you can boost your Doctor skill by obtaining a PipBoy Medical Enhancer, which requires you to do a mcertain quest in New Reno and Broken Hills and then make specific steps in order to get this reward.
Since this is so cryptic, click on the spoiler tag to see an excerpt from Per's guide:
[spoiler:f74c2f74fe](Broken Hills Residential Area) If you don't kill the scorpion you might want to steal the Spectacles off it. You can even steal its tail! If you want to hear it talk, get caught stealing from it. (...) (New Reno Commercial Row) Here's also where you can obtain one of the most obscure items in the entire game: the Pip Boy Medical Enhancer! You can give the Spectacles (from Broken Hills) to him and ask for a favour or a discount, but don't ask for a discount - it doesn't have any effect, and this way you don't get the Enhancer. Instead choose any of the other two lines, and then when you talk to Renesco you'll have the option of asking him how the glasses are. Do this 20 times and he'll become so exasperated he gives you the Enhancer, which upon consumption yields 500 xp and a permanent Doctor +10%![/spoiler:f74c2f74fe]