docs: def load_genes(path):
      """ load a file listing gene and transcript IDs
      
      Args:
          path: path to file containing gene IDs and transcript IDs e.g.
              gene_1    transcript_1.1    length_1    denovo_count
              gene_2    transcript_2.1    length_3    denovo_count
      
      Returns:
          dict of transcripts eg {'CTC1': ["ENST00000315684", "ENST00000485511"]}
      """
      
      with open(path, 'rt') as f:
          lines = [ x.split('\t')[:2] for x in f if not x.startswith('hgnc') ]
      
      transcripts = {}
      for symbol, tx in lines:
          if symbol not in transcripts:
              transcripts[symbol] = []
          transcripts[symbol].append(tx)
      
      return transcripts