Unaffected parents have affected sons → consistent with a carrier mother ( ) and normal father (XY).
No father-to-son transmission: the affected males' sons are unaffected (fathers pass the to sons in linked traits).
Affected daughters appear only when the father is affected ( ) and the mother is a carrier possible in both families and explains the filled circles in generation III.
X-linked dominant is unlikely because affected fathers would make all daughters affected (not seen).
It cannot be Y -linked (as females are also affected).
It cannot be autosomal dominant (as there should not be any skipping of generation).