Which statement is true for X-linked recessive inheritance?

• A. Males are usually affected and females are carriers.
• B. Both sexes equally affected.
• C. Affected individuals must have two mutated copies.
• D. Carrier status is irrelevant.

Answer: (B)

In X-linked recessive inheritance, the gene is on the X chromosome and the disease manifests mainly in males because they have only one X chromosome. A single mutated X is enough for a male to be affected, since there isn’t a second X to provide a normal copy. Females have two X chromosomes, so a normal copy on the other X usually protects them from showing symptoms; they become carriers who can pass the mutated allele to offspring. Affected males will pass the mutated gene to all of their daughters (who become carriers) and none of their sons. When a carrier female is involved, each pregnancy has about a 50% chance that sons will be affected and daughters will be carriers. Females are rarely affected unless they have two mutated copies (homozygous) or there is skewed X-inactivation. Carrier status matters for risk to future children and relatives. Therefore, the typical pattern is that males are usually affected and females are carriers.