How does the Bombay phenotype illustrate the concept of epistasis?

The Bombay phenotype serves as a classic example of epistasis, where the expression of one gene is affected by the presence of one or more other genes. In the case of the Bombay phenotype, individuals with this phenotype have a mutation in the H gene, which encodes for a protein necessary for the formation of the H antigen, an essential precursor for the A and B antigens on red blood cells.

Because the H antigen is required for the expression of the ABO blood group, those who have the Bombay phenotype will appear as type O blood regardless of the ABO gene they possess. This illustrates epistasis because the effects of the H gene overshadow the contributions of the ABO genes, demonstrating that the second gene (the H gene) greatly determines the phenotype (the blood type) and masks the expected expression caused by the ABO alleles.

This example highlights the complex interactions between genes, where one gene can influence the phenotypic expression of another, solidifying our understanding of how epistasis occurs in genetic traits.