Which gene type can mask the expression of another gene?

A dominant gene can mask the expression of another gene because it exerts its effect even when only one copy is present. In genetic terms, if an individual has one dominant allele and one recessive allele for a specific trait, the dominant allele will determine the phenotype; thus, the characteristics associated with the recessive allele will not be expressed.

For example, if we consider a trait governed by two alleles, A (dominant) and a (recessive), an individual with genotypes AA or Aa will express the trait associated with the dominant allele (A) while the recessive trait does not manifest unless the individual has two copies of the recessive allele (aa). This characteristic of dominance is crucial in understanding inheritance patterns in organisms.

In contrast, recessive genes require two copies to be expressed and wouldn't mask other gene expressions. Incomplete dominance refers to a situation where neither allele is completely dominant, resulting in a blending of traits, and co-dominance involves both alleles being fully expressed simultaneously rather than one masking the other.