USA Biology Olympiad (USABO) Practice Exam

What is the relationship called when one gene masks the expression of another gene?

1. Dihybrid

2. Epistatic

3. Polygenic

4. Codominant

The correct answer is: Epistatic

The relationship where one gene masks the expression of another gene is known as epistasis. In this genetic interaction, when the genotype of one gene can influence or completely suppress the phenotype associated with another gene, it leads to a situation where the effect of one gene is not visible if the other gene has a specific genotype. This masking can result in a variety of phenotypic outcomes depending on the alleles present at both loci involved. For instance, in a classic example involving coat color in mice, one gene may determine whether the coat color is even visible by affecting pigmentation, while another gene determines the actual color. If the first gene is homozygous recessive, it will mask the expression of the second gene regardless of its alleles. This concept is key to understanding complex inheritance patterns and how traits are expressed. The other choices relate to different genetic concepts but do not involve the masking relationship described in the question. Dihybrid pertains to the inheritance of two traits simultaneously, polygenic refers to traits influenced by multiple genes, and codominant describes a situation where both alleles in a heterozygous genotype contribute equally to the phenotype without one masking the other.