Male, Female, and Genderless
In TPPC, usually there are 3 kind of pokemon's genders. They are Male, written as M or (M), Female, written as F or (F), and Genderless, written as G or (G). Genderless stays for the pokemon that has no gender, such as Ditto, Voltorb, Porygon and many more.
Ungendered Pokemon
Ampharos (?)
Aside from male, female, and genderless gendered Pokemon, there are also ungendered pokemon; formally, they are known as 'U' or '(?)'. When version 7 was released, the symbol, (?), replaced the staple 'U' used to refer to them. The symbol indicates that the recognized Pokemon is capable of changing into it's own respective gender; for example, a Natu (?) can be changed into either a Female or a Male Natu, but Porygon (?) can only change into a genderless Pokemon. Most players have chosen to leave the Pokemon ungendered because they're considered rarer than common male/female Pokemon.
Gender and the Forums
Since the forums have no character that can represent the symbol of male and female, they usually replace the symbols with something else. Female is represented as ♀ ; or Š while Male is represented as ♂ ; or ‰ and Genderless pokemon will never show a symbol on the forum. Ungendered pokemon usually shown as (?)
Gender Glitches
Back to the older versions, there were some glitch occured to the gender. Because of this glitch, some pokemon have the gender they're not supposedly have. LoBo was known for his Blissey G, and other Pokemon including Pidgey G and Vulpix G have turned up occasionally. Recently, some trainers have been trying to trade these in the Trade Forum.