Recessive genes came together in all three children in the second generation of this hypothetical family, making it inevitable that the third generation would be carriers.
Spinal muscular atrophy (SMA)  is an autosomal recessive disorder carried on chromosome 5. In this family, the father and mother are both carriers of a mutation on chromosome 5. They have no symptoms of the disease, since it takes two such mutations in a recessive disorder to cause symptoms.
They had two daughters and one son, all of whom inherited a chromosome 5 with an SMA-causing mutation from each parent. They all have the disorder. One affected son married an unaffected woman. Their children are all unaffected carriers of SMA, having inherited one chromosome 5 with a mutation from their father and one without this mutation from their mother.
All of the children of this man had to inherit a chromosome 5 SMA mutation, because they had to get one chromosome 5 from their father and both of his chromosome 5s have this mutation. They couldn’t have inherited this kind of mutation from their mother because she’s not a carrier.