One of mom's chromosome 7's has a nonfunctional CTFR allele (cf−) next to a functional allele of another gene (A+). Her other homologue has a functional allele of the CTFR (CF+) gene next to a nonfunctional allele of the other gene (a−). Dad has functional alleles of both genes on both homologues (CF+, A+). A child of theirs has a maternal homologue that is CF+, A+; and a paternal homologue that is also CF+, A+. The best explanation for this is .