Adjust oven rack to lower-middle position and heat oven to 400 degrees. Line rimmed baking sheet with parchment paper. Whisk all-purpose flour, whole-wheat flour, wheat germ, sugar, salt, baking powder, and baking soda together in large bowl. Combine buttermilk and 2 tablespoons melted butter in 2-cup liquid measuring cup.
Add wet ingredients to dry ingredients and stir with rubber spatula until dough just comes together. Turn out dough onto lightly floured counter and knead until cohesive mass forms, about 8 turns. Pat dough into 7-inch round and transfer to prepared sheet. Using sharp serrated knife, make ¼-inch-deep cross about 5 inches long on top of loaf. Bake until skewer inserted in center comes out clean and loaf registers 195 degrees, 45 to 50 minutes, rotating sheet halfway through baking.
Remove bread from oven. Brush with remaining 1 tablespoon melted butter. Transfer loaf to wire rack and let cool for at least 1 hour. Serve.