Why are X-linked recessive traits more frequently expressed in males?

• A. Males have two X chromosomes.
• B. A single dominant allele on the X is expressed.
• C. Males have only one X chromosome, so a single recessive allele on the X is expressed.
• D. The Y chromosome carries the recessive trait.

Answer: (C)

The main idea is that X-linked recessive traits appear in males because boys have only one X chromosome. With just one X, any recessive allele on that X has nothing on the other X to mask it, so the trait is expressed. In females, two X chromosomes mean a recessive allele must be present on both Xs to show the trait, or one to be masked by a dominant allele on the other X, so expression is less frequent. The Y chromosome doesn’t carry the same set of genes as the X, so it can’t provide a second X allele to reveal or mask an X-linked recessive trait.

So the statement that correctly explains the greater expression in males is that a single recessive allele on the X is expressed because males have only one X chromosome.