Gender

From Bulbapedia, the community-driven Pokémon encyclopedia.

Gender is a feature released in Generation II, first for the Pokémon themselves, and then, in Pokémon Crystal, for the Pokémon Trainers. This feature allowed for Pokémon breeding, as well as introduced the concept of a Pokémon egg to the series. Gender makes no difference in the stats of a Pokémon after Generation II, unless the two Pokémon are a different species entirely, such as Nidoran.

1 Pokémon
2 Trainers

Pokémon

Gender also allowed for many Pokémon to have unofficial male and female counterparts, as demonstrated below.

Male	Female
Nidoran♂	Nidoran♀
Nidorino	Nidorina
Nidoking	Nidoqueen
Tauros	Miltank
Volbeat	Illumise
Latios	Latias
Gallade	Gardevoir*
Glalie*	Froslass
Mothim	Wormadam
*Can be either gender

Gender differences

Generation IV premiered minimal differences in sprite between two Pokémon of the same species. For example, a male Raichu will have the full tail seen in previous games, while the female is missing the very tip of it.

Gender-based evolution

Some Pokémon evolutions can only be obtained if they are of a certain gender:

Female Combee evolve into Vespiquen at level 21. Male Combee never evolve.
At level 20, female Burmy will evolve into Wormadam and male Burmy evolve into Mothim.
Only male Kirlia can evolve into Gallade.
Only female Snorunt can evolve into Froslass.

Single-gender Pokémon

Several Pokémon are also only of one gender, yet do not have an official or unofficial counterpart of the other. Most of the single-gender Pokémon with no counterpart are female, and only six evolution families total (five of which are female-only) are of this type.

Pokémon	Gender	Pokémon	Gender	Pokémon	Gender
Happiny	♀	Smoochum	♀	Tyrogue	♂
Chansey	♀	Jynx	♀	Hitmonlee	♂
Blissey	♀	Vespiquen	♀	Hitmonchan	♂
Kangaskhan	♀	Cresselia	♀	Hitmontop	♂

Trainers

In Pokémon Crystal, players were given the option of choosing their trainer's gender at the start of the game. Since then, every game in the main series has had that choice.