Question

an allele whose trait only shows up when no dominant allele is present.
dominant allele
hidden allele
recessive allele
present allele

Explanation:

This is a definition matching question for genetic terminology. The described allele is masked by dominant alleles and only expresses its trait in homozygous (no dominant allele present) conditions, which is the definition of a recessive allele. "Hidden allele" is a non-standard term, while dominant alleles show their trait even when paired with a recessive one, and "present allele" is not a standard genetic term.

Answer:

recessive allele