  For a genetic locus carrying a strongly beneficial allele which has just
fixed in a large population, we study the ancestry at a linked neutral locus.
During this ``selective sweep'' the linkage between the two loci is broken up
by recombination and the ancestry at the neutral locus is modeled by a
structured coalescent in a random background. For large selection coefficients
$\alpha$ and under an appropriate scaling of the recombination rate, we derive
a sampling formula with an order of accuracy of $\mathcal{O}((\log
\alpha)^{-2})$ in probability. In particular we see that, with this order of
accuracy, in a sample of fixed size there are at most two nonsingleton families
of individuals which are identical by descent at the neutral locus from the
beginning of the sweep. This refines a formula going back to the work of
Maynard Smith and Haigh, and complements recent work of Schweinsberg and
Durrett on selective sweeps in the Moran model.
