  We give a closed formula for the number of partitions $\lambda$ of $n$ such
that the corresponding irreducible representation $V_\lambda$ of $S_n$ has
non-trivial determinant. We determine how many of these partitions are
self-conjugate and how many are hooks. This is achieved by characterizing the
$2$-core towers of such partitions. We also obtain a formula for the number of
partitions of $n$ such that the associated permutation representation of $S_n$
has non-trivial determinant.
