CP Biology – Inheritance Practice Test

What does it mean that males are hemizygous for X-linked genes?

They have two X chromosomes They have only one X chromosome, so one allele determines the trait They have no X chromosome They have more than two X chromosomes

Correct Answer:

They have only one X chromosome, so one allele determines the trait

Explanation:

Males are hemizygous for X-linked genes because they have only one X chromosome, so for each gene located on X there is just one allele present. That single allele directly influences the trait, since there isn’t a second X chromosome to provide a second allele that could mask it. The Y chromosome doesn’t carry most of the X-linked genes, so it doesn’t offer a compensating allele for those loci. This is why X-linked recessive conditions tend to show up in males more often: a male needs only one recessive allele on his single X to express the trait, whereas females would need two copies. In females, having two X chromosomes means one good copy can often mask a problematic allele, or the expression can be more complex due to mechanisms like X-inactivation.

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