Why do you think Tally is suprised that David and his father look alike?

Tally is unaware of the way in which the "pretties" average looks..... the traits that are genetically passed down from their parents can make them so similar.

She blinked, her breath catching, for a moment unable to speak. David and his father somehow looked . . . alike.

“Sure. I know all about genes. I knew two sisters, uglies, who looked almost the same. But parents and children? That’s just weird.”

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“But families pass on nonaverage looks. Like our big noses.” The man tweaked his son’s nose, and David rolled his eyes. Tally realized that David’s nose was much bigger than any pretty’s. Why hadn’t she noticed that before now?

“That’s one of the things you give up, when you become pretty. The family nose,” his mother said. “Az? Why don’t you turn up the heat.”