If ace is luffy's brother then why doesnt he have monkey.D at the beginning?

if ace is really luffy's brother then why doesnt he have monkey.D at the beginning of his name??? he is portgas.D. Ace, not monkey.D ace. all of luffy's relatives have monkey .D like Monkey.D.garp and Monkey.D. dragon and Monkey.D.luffy........ but no monkey.d.ace
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