Concept:A gene present only on the X-chromosome is called a sex-linked gene because its transmission is connected with sex chromosomes.
Explanation:The X-chromosome is one of the sex chromosomes.
When a gene has no corresponding allele on the Y-chromosome, it is said to be sex-linked or X-linked.
Such a gene is not limited to expression in one sex; it can affect both males and females.
For example, a father passes his X-chromosome only to his daughters, while a mother can pass hers to both sons and daughters.
Therefore, the term that describes a gene located only on the X-chromosome is "sex-linked."
Answer:C. sex-linked