What is epistasis?

• A. Epistasis increases the overall number of genes in a genome.
• B. Epistasis is a circumstance where the expression of one gene is modified by one or more other genes.
• C. Epistasis refers to crossing over during meiosis.
• D. Epistasis causes mutations to revert.

Answer: (B)

Epistasis is when the phenotype produced by one gene depends on the genotype at one or more other genes. Genes often work in pathways, so one gene’s product is needed for another’s effect. If the second gene isn’t functional, it can mask or modify the first gene’s influence, giving a phenotype that wouldn’t occur if the genes acted independently. A classic illustration is coat color in some animals, where one gene determines pigment production and another determines pigment deposition; interactions between these genes can produce unexpected colors, highlighting epistasis. This description matches the correct option because it captures how one gene’s expression is modified by other genes. The other choices aren’t about gene interactions: epistasis is not about increasing genome size, nor about crossing over during meiosis, nor about mutations reverting.