Males are more often affected by X-linked traits than are females because:

• A. they have only one X chromosome, so a single recessive allele will cause the trait to be expressed
• B. they have two X chromosomes, increasing the chance of inheriting the trait
• C. the Y chromosome can mask the effects of X-linked alleles
• D. they inherit X-linked traits only from their fathers

Answer: (A)

Verified step by step guidance

1. Understand that X-linked traits are associated with genes located on the X chromosome, which is one of the two sex chromosomes in humans.
2. Recall that males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).
3. Recognize that because males have only one X chromosome, any recessive allele on that X chromosome will be expressed, as there is no second X chromosome to potentially carry a dominant allele that could mask it.
4. Contrast this with females, who have two X chromosomes, so a recessive allele on one X can be masked by a dominant allele on the other X, making them less likely to express the trait unless they inherit two copies of the recessive allele.
5. Conclude that males are more often affected by X-linked recessive traits because a single recessive allele on their single X chromosome is sufficient to express the trait.