A very rare disorder is shown to depend on the genotype at three unlinked autosomal loci, A/a, B/b and C/c and that to be affected one must be homozygous for the rare allele at all three loci, that is, aabbcc. The MZ twin disorder concordance is 100%, implying that the penetrance is 100%. A couple have a child with this disorder. What is the chance that their next child will be affected?