What type of gene can mask the expression of a recessive gene?

A dominant gene can mask the expression of a recessive gene due to its ability to express its traits over those of a recessive counterpart. In genetics, traits are often determined by pairs of alleles—one inherited from each parent. When one allele is dominant, it will govern the organism's phenotype regardless of the presence of a recessive allele.

For instance, if we consider a simple trait like flower color in pea plants, one color may be due to a dominant gene (say purple) and the other due to a recessive gene (say white). If a plant has one allele for purple (dominant) and one for white (recessive), the plant will display the purple color because the dominant allele's effect is expressed. This illustrates the fundamental principle of dominance where the dominant allele can completely overshadow the recessive allele's effect in phenotypic expression.

Understanding this concept forms the basis of Mendelian genetics, where dominant traits will always be displayed in the phenotype when present, while recessive traits will only show if the organism carries two copies of the recessive allele.