Talk about Homozygous recessive epistasis

Bombay phenotype discussed earlier is an example of the homozygous recessive condition at one locus masking the expression of a second locus. There we established that the homozygous presence of the mutant form of the FUT1 gene masks the expression of the IA and IB alleles. Only individuals containing at least one wild-type FUT1 allele can form the A or B antigen. As a result, individuals whose genotypes include the IA or IB allele and who have no wild-type FUT1 allele are of the type O phenotype, regardless of their potential to make either antigen. An example of the outcome of matings between individuals heterozygous at both loci is illustrated. If many such individuals have children, the phenotypic ratio of 3 A: 6 AB: 3 B: 4 O is expected in their offspring.