The main way a recessive allele would be expressed, even when only one copy is present, would be sex-linked inheritance.

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In sex-linked inheritance, particularly with X-linked recessive traits, males (XY) express the trait when they inherit one copy of the recessive allele on the X chromosome, because they lack a second X chromosome that could carry a dominant allele to mask the recessive one.

The Main Way a Recessive Allele Would Be Expressed, Even