The definition of Epistasis in gene biology is Choice D: when the allele of one gene masks the phenotype of another gene
Definition:
When the phenotypic effect of alleles at one gene are masked by alleles of another gene.
In essence, a gene is said to be epistatic when its presence suppresses the effect of a gene at another locus.
It is important to know that Epistatic genes are often called inhibiting genes because of their effect on other genes which are described as hypostatic.
Additionally, there are six common types of epistasis gene interactions:
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