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I haven't decided what to major in, but I'm quite interested in genetic engineering.

Hi,

Is it the same to say "what to study" instead of "what to major in" in the above? Thanks.
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The sentence is okay, but "major in" implies college; "study" does not. In your second example, you'd have to add "in college," if that's what you mean.

(Genetic engineering is getting a bad rap lately.)
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Hi,

Is it the same to say "what to study" instead of "what to major in" in

I see 'major in' as a feature of AmE but not of BrE, unless BrE has changed since I lived there.

Best wishes, Clive