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The female fetus's eggs form as she forms, just like all her other organs. They are each formed from her DNA, which is a mix of her mom's and dad's.
Remember Punnet Squares? Dad has brown eyes, no recessive blue, so he's BB, mom is blue-eyed, she's bb. (Eye color is actually more complex, but shut it.) So all of mom's eggs have b, and all of dad's sperm have B. Mom gets pregnant, a b egg with a B sperm. So her girl fetus's DNA is Bb, in every cell from the beginning. Now girl fetus develops ovaries, then eggs, which split one extra time like all gametes. Girl baby is born with brown eyes and full of eggs, half of which are B and half of which are b. Outwardly she has brown eyes like dad, but half her eggs contain recessive blue eyes from her mom, and HER fetus might one day be formed from a b egg, and have blue eyes and nothing but b eggs if she marries a blue-eyed dude.
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