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