Most Pokémon are either male or female. However, depending on the species, players are more likely to encounter a Pokémon of a specific gender than the other. Gender plays a vital role in breeding, as offspring inherit the species of the mother and compatible moves from the father. However, Pokémon of either gender can be used if the partner is a Ditto.
From Generation III onward, due to how the core series Pokémon games incorrectly determine a Pokémon's gender from its personality value, the ratios stated below are idealized approximations. From Generation III to V, Pokémon are more likely to be male than the nominal ratio; from Generation VI to VII, Pokémon are more likely to be the more common gender than the nominal ratio (1:1 is unaffected). Pokémon which are always one particular gender are unaffected.
These Pokémon only appear as male. As a result, these Pokémon are incapable of producing offspring of the same species without Ditto or a female counterpart (in the case of Nidoran♂ family and Volbeat).
These Pokémon normally appear as male, rarely female. This specific gender ratio is used to make it more difficult in breeding more of them since players usually get them as gifts or Fossils rather than catch them in the wild. All starter and Fossil Pokémon are in this group, as well as some other Pokémon normally not found in the wild.
Generation VII added the most genderless Pokémon, with a current total of 27.
Prior to Generation VI, because Azurill's gender ratio does not match that of its evolved forms and its evolution is not dependent on gender, being female 75% of the time while Marill and Azumarill are female only 50% of the time, female Azurill has a 1/3 chance of evolving into a male Marill (depending on its personality value). Azurill is the only Pokémon to do this. From Generation VI on, Azurill retains its gender upon evolving, despite the gender ratios still differing.