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.