In Drosophila, the X-linked recessive mutations prune (pn) and garnet (g) recombine with a frequency of 0.4. Both of these mutations cause the eyes to be brown instead of dark red. Females homozygous for the pn mutation were crossed to males hemizygous for the g mutation, and the F1 daughters, all with dark red eyes, were crossed with their brown-eyed brothers. Predict the frequency of sons from this last cross that will have dark red eyes.