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



what are the subgroups of an algebraic group with codimension one
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](http://www.ams.org/mathscinet-getitem?mr=120308) which address exactly this. 



> 
> Let $\mathfrak g$ be a finite
>  dimensional Lie algebra over a field of characteristic zero. If 
>  $\mathfrak h$ is a codimension one
>  subalgebra then there exists a 
>  morphism $\phi : \mathfrak g \to
> > \mathfrak{sl}(2)$ with kernel
>  contained in $\mathfrak h$.
> 
> 
> 


This result has been explored by Hoffman to provide a classification of codimension one 
subalgebras of Lie algebras in this [paper](http://www.ams.org/mathscinet-getitem?mr=1076859).


