http://www.nature.com/ng/journal/v46/n7/fig_tab/ng.2991_F1.html Day 257 has 22 protein-coding genes (browser view) including KITLG (KIT ligand).
A mutation near the KITLG gene increases the chance of being blonde by four-fold. It does this by changing the expression of the gene, which is involved in organizing melanocytes.
Click here to see all 8329707 letters of Day 257 with the rs12821256 blonde-hair mutation flashing.