What is a sex-linked gene?

A sex-linked gene refers specifically to a gene that is located on one of the sex chromosomes, which in humans are the X and Y chromosomes. This type of gene can have unique inheritance patterns, especially since males (XY) and females (XX) have different combinations of these chromosomes. For example, traits linked to the X chromosome can manifest differently in males and females due to the presence of a single X in males compared to two X chromosomes in females.

The other options describe different types of genes or concepts that do not specifically pertain to the definition of a sex-linked gene. A gene present on any chromosome does not capture the distinction of being on a sex chromosome. A gene that causes sex determination refers to genes that influence whether an organism develops as male or female but does not encompass the broader definition of sex-linked genes. Lastly, a gene unaffected by environmental factors represents a type of gene stability that is not relevant to the location or categorization based on sex chromosomes. Ultimately, the correct choice highlights the specific placement of the gene on the sex chromosomes, which is central to its classification and inheritance patterns.