Here's why:

* Location: The ap gene is located on the X chromosome, which is one of the sex chromosomes in fruit flies.

* Inheritance: Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). This means that females can be homozygous (ap ap) or heterozygous (ap +) for the ap gene, while males can only have one copy of the gene (ap Y or + Y).

Here's how this affects wing development:

* Females: A female fruit fly must inherit two copies of the ap allele (ap ap) to be wingless.

* Males: A male fruit fly needs only one copy of the ap allele (ap Y) to be wingless. This means that winglessness is more common in males, as they only need to inherit the ap gene from their mother.

Important Note: There are other genes involved in wing development in fruit flies, and some of these genes may also be sex-linked. However, ap is the most well-known and widely studied gene associated with winglessness in Drosophila.