Peter and Sarah have a son with haemophilia. Peter is unaffected and Sarah is a carrier. What is the probability of them having a son with haemophilia, and what advice can be given to Peter about his

Peter (unaffected male): $X^H Y$
Sarah (carrier female): $X^H X^h$

Peter's gametes: $X^H$, $Y$
Sarah's gametes: $X^H$, $X^h$

The Punnett square shows that there is a 25% chance for each pregnancy to result in a boy with haemophilia ($X^h Y$). This occurs when the son inherits the $X^h$ allele from his carrier mother and the $Y$ chromosome from his father.
The genetic diagram shows how the haemophilia allele ($X^h$) from Sarah can be passed to her son, resulting in the condition.

b) Peter can manage erectile dysfunction and maintain reproductive health by addressing the underlying causes.
He should consult a doctor for a proper diagnosis and treatment plan, as stress (mentioned in the problem) or other medical conditions could be factors. Adopting a healthy lifestyle, including a balanced diet, regular exercise, avoiding smoking and excessive alcohol, can significantly improve overall and reproductive health. Open communication with his wife, Sarah, can also help manage stress and strengthen their relationship.