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 player characters. 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.
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.
Some Pokémon evolutions can only be obtained if they are of a certain gender:
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.
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 share many differences, from appearance to moveset. However, since Generation II, a Nidoran♀ egg has a 50/50 chance of hatching into either a Nidoran♀ or a Nidoran♂, arguably making them related.
Volbeat and Illumise
Much like the Nidoran family, an Illumise egg may result in a Volbeat, making them related.
In the Fandom
A few Pokémon are considered "related" by fans of the franchise, due to perceived similarities. Among the most common examples is that of Tauros and Miltank. Due to Tauros and Miltank appearing to be based on the genders of a cattle, a bull and cow respectively, many fans consider them to be gender counterparts. However, Tauros and Miltank debuted in separate generations, generations I and II respectively, and a Miltank egg cannot hatch into a Tauros, nor can an egg resulting in a Tauros breeding with a Ditto become a Miltank. This contrasts with the relationship of the Nidoran family as explained above.
Another example is that of Latias and Latios, referred to by some fans as Lati@s. While there is arguably an obvious connection between these two Pokémon, they cannot breed, and thus cannot produce eggs of one another. Thus they are unrelated, in terms of reproduction anyway.
Starting 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.
In side games
Many side games in the Pokémon franchise also allow the player to choose between a male or female player character as well.
- 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.