The Bombay phenotype is an example of epistasis in which the homozygous recessive condition at one locus masks the expression of a second. The hh individual is phenotypically type O regardless of the I genotype.

The Mendelian dominance of the h locus and the multiple alleles inheritance pf ABO blood types yield offspring ratios that also differ from classic 9:3:3:1 dihybrid ratio.