Which allele is expressed only when present in two copies and can be masked by a dominant allele?

• A. Recessive allele
• B. Dominant allele
• C. Co-dominant allele
• D. Incomplete dominant allele

Answer: (A)

The idea here is how alleles are expressed depending on dominance. A recessive allele shows up in the phenotype only when an organism has two copies of it. If a dominant allele is present in even one copy, it masks the recessive allele in the heterozygous individual, so the recessive trait isn’t seen. For example, in a simple tall versus short plant system, tall is dominant over short; only plants with two short alleles are short, while any plant with at least one tall allele is tall. Co-dominance means both alleles are fully expressed in the offspring’s phenotype, and incomplete dominance gives a blended phenotype in the heterozygote. So the pattern described—expressed only when two copies are present and masked by a dominant allele—fits the recessive allele.