A white-eyed mutant male fly was crossed with a normal red-eyed female.

The F₁ generation flies all exhibited red eyes, as expected if the white-eyed allele is recessive.

In the F₂ generation, all of the white-eyed flies were male, indicating that the eye-color gene is sex-linked: located on the X chromosome.

Note: "gene" should read "allele" in the diagram.