Which allele type expresses a trait only when paired with an identical allele?

• A. Dominant allele
• B. Allele
• C. Genome
• D. Recessive allele

Answer: (D)

Recessive alleles express a trait only when two copies are present. If an individual has one recessive and one dominant allele, the dominant allele masks the recessive one and the recessive trait doesn’t appear. Only when the genotype is two copies of the recessive allele do you see that trait. For example, with a dominant allele A and a recessive allele a, the phenotype is dominant when the genotype is AA or Aa, but recessive when the genotype is aa.