A 23-year-old woman with cystic fibrosis attends her general practitioner surgery for a routine postnatal appointment after giving birth to her second child (a boy). They already have a daughter who has cystic fibrosis. Her partner, the biological parent of both children, has neither cystic fibrosis nor a family history of cystic fibrosis. What is the chance of the couple’s son having cystic fibrosis?