Gender

Gender is a concept introduced in Generation II, though touched upon in Generation I. In Gold and Silver Versions, all Pokémon of most species were assigned a gender (male or female), and then, in Pokémon Crystal, a choice of male or female player characters became available for the first time. This feature allowed for Pokémon breeding, as well as introducing 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.

Since Pokémon Crystal, Trainers can also have gender. Players get to choose play as a boy or a girl when beginning a new game.

Pokémon

Related species

Nidoran

The Nidoran family is a special case in terms of gender. Introduced in Generation I, before gender was known for all Pokémon, Nidoran♀ and Nidoran♂ are considered separate species of Pokémon and indeed have many differences, from appearance to moveset. However, a Nidoran♀ egg always has a 50/50 chance of hatching into either a Nidoran♀ or a Nidoran♂, confirming that they are related.

Male	Female
Nidoran♂	Nidoran♀
Nidorino	Nidorina
Nidoking	Nidoqueen

Other

In a manner similar to Nidoran, eggs produced by Illumise may hatch into Volbeat. While Latias and Latios are in the No eggs Group in the games, likely due to being Legendary Pokémon, Latias have produced eggs containing Latios in the anime, confirming that they are related. Mothim and Wormadam evolve from male- and female-gendered members of the same species.

Male	Female
Volbeat	Illumise
Latios	Latias
Mothim	Wormadam

Unofficial counterparts

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

Male	Female
Tauros	Miltank
Gallade	Gardevoir*
Glalie*	Froslass
Rufflet	Vullaby
Braviary	Mandibuzz
*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, though Cresselia has a genderless counterpart.

Pokémon	Gender	Pokémon	Gender	Pokémon	Gender	Pokémon	Gender
Happiny	♀	Smoochum	♀	Tyrogue	♂	Sawk	♂
Chansey	♀	Jynx	♀	Hitmonlee	♂	Tornadus	♂
Blissey	♀	Vespiquen	♀	Hitmonchan	♂	Thundurus	♂
Petilil	♀	Kangaskhan	♀	Hitmontop	♂	Landorus	♂
Lilligant	♀	Cresselia	♀	Throh	♂

Trainers

Choosing to play as male or female in Pokémon Black and White

Starting in Pokémon Crystal, players were given the option of choosing to play as a boy or girl Trainer at the start of the game. Since then, every game in the main series has included that choice.

Game	Male	Female
C	File:Gold crystal.png Ethan	File:Kris.gif Kris
R	File:Brendan.png Brendan	May
S	File:Brendan.png Brendan	May
FR	Red	Leaf
LG	Red	Leaf
E	Brendan	May
D	File:Koki.png Lucas	Dawn
P	File:Koki.png Lucas	Dawn
Pt	File:Lucasplatinum.png Lucas	File:Dawnplatinum.png Dawn
HG	File:HGSS Ethan.png Ethan	File:HGSS Lyra.png Lyra
SS	File:HGSS Ethan.png Ethan	File:HGSS Lyra.png Lyra
B	File:BW Hilbert.png Hilbert	File:BW Hilda.png Hilda
W	File:BW Hilbert.png Hilbert	File:BW Hilda.png Hilda

In side games

Many side games in the Pokémon franchise allow the player to choose between a male or female player character as well.

Game	Male	Female
Pokémon Card GB2	Mark	File:Mint 2.jpg Mint
Pokémon Ranger	Lunick	Solana
Pokémon Ranger: Shadows of Almia	Kellyn	Kate
Pokémon Ranger: Guardian Signs	Ben	Summer

Trivia

In Generation II, there was a feature called the Time Capsule allowing compatibility between this generation and Generation I. In Generation I, all Pokémon were genderless. But in Generation II, genders were introduced. And upon trading with the Time Capsule, the gender of the Pokémon would be redetermined, (if traded from Gen I to II), mostly through IVs.
Due to the programming in the Generation II games, female Pokémon can never have a maximized Attack stat unless they are a female-only species such as Miltank; this is because female Pokémon cannot have Attack IVs greater than or equal to a value based on their gender ratio. Female-only species have a 100%, or sixteen-out-of-sixteen, chance to be female, so their Attack IV can be anywhere in the 0-15 range, which is sixteen values. Pokémon who have a 12.5% chance to be female, such as starter Pokémon, only have a two-out-of-sixteen chance to be female, which means that they can only have a zero or one Attack IV. Because the majority of Pokémon species have a 50/50 gender ratio, a typical female Pokémon cannot have an Attack IV greater than seven (corresponding to the 0-7 IV range). In later generations, female Pokémon do not have these restrictions.
Although its English and French names suggest that it is exclusively male, Mr. Mime may be of either gender.
Since Azurill is 75% female, 1 in 3 of every female Azurill will change into a male upon evolving into Marill (based on Azurill's personality value) because unlike Azurill, Marill is 50% male and 50% female.
In the international versions of Generation II, due to how the battle field is programmed, Nidoran♂ and Nidoran♀ have their gender icon shown twice, in their name since it's part of it like in Generation I and another after the level number. This does not occur with the Japanese or Korean versions, due to the icon appearing after the Pokémon's name, thus avoiding redundancies.