Which type of allele masks the effect of another allele?

A dominant allele is one that can mask the effect of another allele, which is typically recessive. In a situation where two different alleles for a trait are present, the dominant allele will influence the phenotype—the observable characteristics—while the recessive allele will not express its effect in the presence of the dominant one.

For example, if we're considering a trait such as flower color, if one allele codes for purple flowers (dominant) and another codes for white flowers (recessive), the presence of the purple allele will lead to purple flowers, therefore hiding the effect of the white allele. This distinctive characteristic of dominant alleles is crucial in genetic inheritance and understanding how traits are passed from one generation to the next.

The other types of alleles mentioned—recessive, co-dominant, and homozygous—do not fulfill this specific role of masking. Recessive alleles require two copies to express their trait, co-dominant alleles are expressed together rather than one masking the other, and homozygous pertains to having two identical alleles for a given gene rather than describing the functional interaction between different alleles.