docs: def get_geno_marker(self, marker, return_index=False):
          """Gets the genotypes for a given marker.
  
          Args:
              marker (str): The name of the marker.
              return_index (bool): Wether to return the marker's index or not.
  
          Returns:
              numpy.ndarray: The genotypes of the marker (additive format).
  
          """
          if self._mode != "r":
              raise UnsupportedOperation("not available in 'w' mode")
  
          # Check if the marker exists
          if marker not in self._bim.index:
              raise ValueError("{}: marker not in BIM".format(marker))
  
          # Seeking to the correct position
          seek_index = self._bim.loc[marker, "i"]
          self.seek(seek_index)
  
          if return_index:
              return self._read_current_marker(), seek_index
          return self._read_current_marker()