def compute_min_distance_mendelian(proband_allele, parent_alleles):
    """Commute the smallest distance between the given proband STR expansion size, and parental STR expansion  size.

    Args:
        proband_allele (int): the proband's allele length.
        parent_alleles (list of allele sizes): list of parental allele lengths.

    Return:
          int: the smallest distance (in base-pairs) between one of the parent_alleles, and the proband_allele.
    """

    return min([abs(int(proband_allele) - int(pa)) for pa in parent_alleles])