An example of a sex-linked trait is the possession of facial hair in adult humans. Sex-linked traits are traits that are determined by genes located on the sex chromosomes (X and Y chromosomes). In humans, the gene for the ability to grow facial hair is located on the Y chromosome, which is present only in males. This means that the ability to grow facial hair is inherited from the father to the son and is not expressed in females, who have two X chromosomes. The other traits mentioned - ability to grow long hair in females, colour of the skin in humans, and ability to roll the tongue - are not sex-linked traits but rather are determined by other genes or a combination of genes and environmental factors.