def calc_group_maf(group, vcf_record):
    """
    Calculate the minor allele frequency for a given group
    of samples.

    Arguments:
    - group: a Group containing the samples to examine
    - vcf_record: a vcf Record containing the calls
    """
    # we're only dealing with biallelic sites for now
    if len(vcf_record.ALT) > 1:
        return "."

    # the gt_type field is 0 for hom ref, 1 for het, 2 for
    # hom alt, and None for no call, so this is some super
    # convenient math
    calls = map(lambda s: vcf_record.genotype(s).gt_type, group.members)
    calls = list(filter(lambda c: c is not None, calls))

    if len(calls) == 0:
        return "."
    else:
        return sum(calls) / (len(calls) * 2)