Which allele is often masked by a dominant allele?

The allele that is often masked by a dominant allele is a recessive allele. In genetic terms, alleles can be classified as dominant or recessive based on their influence on the phenotype of an organism.

When an organism has one dominant allele and one recessive allele for a particular trait, the dominant allele's trait will be expressed, while the effect of the recessive allele will not be observable in the phenotype. This means that the presence of a dominant allele overshadows the expression of the recessive allele in a heterozygous genotype (where two different alleles are present).

For instance, in a gene where "A" is a dominant allele and "a" is a recessive allele, the possible genotypes are AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive). Only the genotype "aa" will express the characteristics associated with the recessive allele, as "A" will dictate the phenotype in both "AA" and "Aa" cases. Therefore, the recessive allele is effectively masked by the presence of the dominant allele in the heterozygous condition.