Sex linkage

In the fruit fly Drosophilia, eye color is sex-linked.

The allele for red eyes is dominant ($\text{X}^{W}$start text, X, end text, start superscript, W, end superscript) and the allele for white eyes is recessive ($\text{X}^{w}$start text, X, end text, start superscript, w, end superscript).

A scientist crosses two red-eyed flies and notice that half of their male offspring have white eyes.