Here's why:

* Sex chromosomes determine an individual's sex. Females typically have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY).

* Genes located on these chromosomes are directly linked to an individual's sex and are therefore called sex-linked genes.

There are two main types of sex-linked genes:

* X-linked genes: Located on the X chromosome. These genes can affect both males and females, but they are expressed differently due to the difference in the number of X chromosomes.

* Y-linked genes: Located on the Y chromosome. These genes are only found in males and are passed directly from father to son.

Examples of sex-linked traits include:

* Red-green color blindness: An X-linked recessive trait.

* Hemophilia: An X-linked recessive trait.

* Testicular feminization: An X-linked recessive trait.

* Hairy ears: A Y-linked trait.