Add the closest marker to the given QTL.

    :arg qtl: a row of the QTL list.
    :arg map_list: the genetic map containing the list of markers.

def add_marker_to_qtl(qtl, map_list):
    """Add the closest marker to the given QTL.

    :arg qtl: a row of the QTL list.
    :arg map_list: the genetic map containing the list of markers.

    """
    closest = ''
    diff = None
    for marker in map_list:
        if qtl[1] == marker[1]:
            tmp_diff = float(qtl[2]) - float(marker[2])
            if diff is None or abs(diff) > abs(tmp_diff):
                diff = tmp_diff
                closest = marker
    if closest != '':
        closest = closest[0]
    return closest