Which term describes the allele that masks the expression of a recessive allele in a heterozygote?

• A. Allele
• B. Carrier
• C. Dominant
• D. Codon

Answer: (C)

In this situation the key idea is how traits are expressed when two different alleles meet in a heterozygote. The allele that determines the phenotype by suppressing the effect of the other allele is the dominant one. So in a genotype like Aa, the A (dominant) allele masks the recessive a allele, and the trait shown is the dominant one. The recessive allele only appears when both copies are recessive (aa).

The other terms don’t describe this masking role: an allele is just any variant of a gene; a carrier is someone who has a recessive allele but may not show the trait; a codon is a three-nucleotide unit that codes for an amino acid, not an allele.