code: def iter_geno_marker(self, markers, return_index=False):
          
          if self._mode != "r":
              raise UnsupportedOperation("not available in 'w' mode")
  
          # If string, we change to list
          if isinstance(markers, str):
              markers = [markers]
  
          # Iterating over all markers
          if return_index:
              for marker in markers:
                  geno, seek = self.get_geno_marker(marker, return_index=True)
                  yield marker, geno, seek
          else:
              for marker in markers:
                  yield marker, self.get_geno_marker(marker)
docs: Iterates over genotypes for a list of markers.
  
          Args:
              markers (list): The list of markers to iterate onto.
              return_index (bool): Wether to return the marker's index or not.
  
          Returns:
              tuple: The name of the marker as a string, and its genotypes as a
              :py:class:`numpy.ndarray` (additive format).