let G be an algebraic group. which subgroups of G are codimension one subgroups.

closed as not a real question by Mariano SuárezAlvarez♦, Igor Rivin, Simon Thomas, Martin Brandenburg, Qiaochu Yuan Jun 27 '11 at 22:49
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Perhaps it is better to phrase the question in terms of Lie algebras. For instance, if you want to know which are the possible codimension one Lie subalgebras of a given finite dimensional Lie algebra then there is a result of Tits which address exactly this.
This result has been explored by Hoffman to provide a classification of codimension one subalgebras of Lie algebras in this paper. 

