Final answer:
In a testcross to determine recombination frequency in Drosophila, the female is always the heterozygous dominant (XWX") in the F1 generation.
Step-by-step explanation:
In a testcross to determine recombination frequency in Drosophila, the female is always the heterozygous dominant (XWX") in the F1 generation. This is because the male contributes the Y chromosome, which does not carry the gene for eye color. The female, on the other hand, provides one X chromosome with the dominant allele (XW) for red eyes and one X chromosome with the recessive allele (X") for white eyes.