The third generation escaped the disease in this hypothetical family, but the son and both daughters in the second generation got the disease-causing mutation.
Myotonic muscular dystrophy (MMD) is an autosomal dominant disorder carried on chromosome 19 or chromosome 3. In this family, the father has the mutation on one of his chromosome 19s.
His square is colored in because he has the symptoms of MMD, as would be expected in a dominant disorder. Since he has one chromosome 19 with the MMD mutation and another that doesn’t have it, he can pass either one to his children. He married an unaffected woman.
Unfortunately, all three of their children — one son and two daughters — inherited from their father a chromosome 19 with an MMD mutation. Even one mutation was enough to cause the disease in all three. One of the affected daughters married an unaffected man.
By luck, neither their two sons nor their daughter inherited a chromosome 19 with an MMD mutation from their affected mother. The father’s chromosome 19s are both normal.