Problem: Your friend from medical school is treating a young New York City boy with the same genetic disorder as your patient in LA. She takes a cheek swab from the boy and his parents, extracts DNA from their cheek cells, and sequences their genomes. As expected, she finds that the boy carries two mutant alleles in XF5. The mother is heterozygous, and the father has two wild-type alleles. She asks you for help understanding this. How can you explain that the father gave the child a mutant allele when he only has wild-type alleles?