You’re right, Ronan looks like the spitting image of his maternal grandfather, John. His mouth, his eyes, even his hair. But shouldn’t we see even a small glimpse of Allen in his features if Ronan is his son?
Not necessarily. Also, people change over the years. A kid of 25 can look like his mother and by the time he is middle-aged, he looks like his father.
I looked like my father as a kid, now I favor my mother’s side. Of course, both Allen & Farrow could solve this tomorrow with a test. What a thing to do to a kid!