Pokémon in the Generation 1 games don't have a gender, but they can be traded to Generation 2 games, where most Pokémon have genders. What determines the gender of a Generation 1 Pokémon when it is traded to a Generation 2 game?
The Attack IVs.
From the Bulbapedia:
In Generation II, a Pokémon's gender was determined through its Attack IV. Due to this, a female Pokémon could never have a maximized Attack stat (unless they were a member of an all-female species, such as Smoochum), because female Pokémon were given the lower portion of the IV range. This also prevented female Pokémon of a species with a gender ratio of seven males to one female, such as starter Pokémon or Eevee, from being Shiny, due to that also being based partly on the Attack IV.
87.5% male ratio: Attack IV needs to be 0 or 1 to be female all others result in a male
75% female ratio: Attack IV needs to be below 12 to be female
75% male ratio: Attack IV must be below 4 to be female
50-50 ratio: Attack IV must be below 8 to be female