The inheritance pattern suggests a Sex-linked recessive trait (specifically X-linked).

1. Recessive: Since the parents are unaffected (phenotypically normal) but have affected offspring, the trait must be recessive (eliminating dominant options).
2. Sex-linked: The disease appears only in sons, while daughters remain unaffected. This is characteristic of X-linked recessive inheritance where the mother is a carrier (heterozygous). She passes the affected X chromosome to 50% of her sons, who express the disease because they have only one X chromosome. Daughters receive a normal dominant X chromosome from the unaffected father, preventing the expression of the disease .