Epistasis is a genetic phenomenon where the expression of one gene influences the phenotype associated with another gene. This interaction can significantly affect traits, such as human blood types. In the context of blood type, the H gene produces a protein that is essential for attaching A and B antigens to the surface of red blood cells. The presence of a functional H protein, encoded by the dominant allele (H), is crucial for the expression of blood types A and B, which are determined by the alleles IA and IB, respectively.

When an individual possesses the recessive allele (h), which encodes an inactive form of the H protein, they cannot attach A or B antigens to their red blood cells. Consequently, even if they carry the alleles IA or IB, they will express blood type O if they are homozygous recessive for the H gene (hh). This illustrates how the H gene can mask the effects of the IA or IB alleles, demonstrating the concept of epistasis.

For example, if an individual has the genotype IAHh, they will have a functional H protein that allows the attachment of A antigens, resulting in blood type A. Conversely, an individual with the genotype IAhh will lack a functional H protein, leading to the absence of A antigens and resulting in blood type O, despite having the IA allele. This relationship highlights the importance of the H gene in determining blood type and exemplifies how one gene's product can affect the expression of another gene's phenotype.

In summary, epistasis is a critical concept in genetics that illustrates the complexity of inheritance patterns, where the interaction between different genes can lead to unexpected phenotypic outcomes.