docs: def read_source_alleles(file_name):
      """Reads an allele file."""
      alleles = {}
      open_function = open
      if file_name.endswith(".gz"):
          open_function = gzip.open
      with open_function(file_name, 'rb') as input_file:
          for line in input_file:
              row = line.rstrip("\r\n").split("\t")
              marker_name = row[0]
              allele1, allele2 = row[1].split(" ")
              alleles[marker_name] = {allele1, allele2}
  
      return alleles