The gene that is only located on X-choromosome is said to be

Step 1: Analyze the question The question asks what we call a gene that is found only on the X-chromosome (and not on the Y-chromosome).

Step 2: Explain the concept Humans have 23 pairs of chromosomes. One pair is the sex chromosomes:

• Females: XX
• Males: XY

The X-chromosome is bigger and carries many genes that have no matching gene on the small Y-chromosome. Genes located on a sex chromosome (usually the X) are called sex-linked genes. Their inheritance follows the pattern of the sex chromosomes.

Examples of X-linked traits: haemophilia, red-green colour blindness, and G6PD deficiency.

Step 3: Evaluate the options

• A. Expressed in females only: Wrong. Males also have an X-chromosome, and they express X-linked genes even more readily because they have only one X.
• B. Defective: Wrong. A gene on the X-chromosome is not necessarily faulty. Many are normal.
• C. Sex-linked: Correct. A gene carried on the sex chromosome is, by definition, sex-linked.
• D. Expressed in males only: Wrong. Females express these genes too, and they can be carriers or affected.

Step 4: Answer The gene is said to be sex-linked, which is option C.

Tip: Remember "Sex chromosome = Sex-linked". Sex-linked does not mean "only in one sex". It means the gene's location on the X (or Y) chromosome affects how it is inherited. Males (XY) show recessive X-linked conditions more often because they have no second X to mask the faulty allele.