If a boy exhibits a sex-linked trait that is carried on the X chromosome, what must be true? His mother exhibited that trait.
His father exhibited that trait.
His father's mother carried an allele for that trait.
His mother carried at least one allele for that trait.

C. His mother carried at least one allele for that trait

Explanation:

-Men have a single allele of each gene on the X chromosome, inherited from their mother, and a single allele of each gene on the Y chromosome, from their father.

-A sex-linked trait is a trait that is controlled by a gene or an allele located on the sex chromosome.

-X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele.

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