Human ABO blood type is determined by three alleles, two of which (I^A and I^B) produce gene products that modify the H antigen produced by protein activity of an independently assorting H gene. A rare abnormality known as the 'Bombay phenotype' is the result of epistatic interaction between the gene for the ABO blood group and the H gene. Individuals with the Bombay phenotype appear to have blood type O based on the inability of both anti-A antibody and anti-B antibody to detect an antigen. The apparent blood type O in Bombay phenotype is due to the absence of H antigen as a result of homozygous recessive mutations of the H gene. Individuals with the Bombay phenotype have the hh genotype. Use the information above to make predictions about the outcome of the cross shown below.

IᴬIᴮHh×IᴬIᴮHh

Key Concepts

ABO Blood Group System

The ABO blood group system is determined by the presence of specific antigens on the surface of red blood cells, which are encoded by three alleles: I^A, I^B, and i. The I^A and I^B alleles are co-dominant, meaning that both can be expressed simultaneously, resulting in blood types A (I^A I^A or I^A i), B (I^B I^B or I^B i), AB (I^A I^B), or O (ii). Understanding this system is crucial for predicting blood type inheritance in offspring.

Epistasis

Epistasis refers to the interaction between genes where the expression of one gene is affected by one or more other genes. In the case of the Bombay phenotype, the H gene's product is necessary for the expression of A and B antigens. If an individual has two recessive alleles (hh) for the H gene, they will not produce the H antigen, leading to a phenotype that appears as blood type O, regardless of their ABO genotype.

Genotype and Phenotype

Genotype refers to the genetic constitution of an individual, while phenotype is the observable physical or biochemical characteristics resulting from the genotype and environmental influences. In the context of the Bombay phenotype, individuals with the hh genotype exhibit a phenotype that masks their true ABO blood type, demonstrating how genotype can influence phenotype through epistatic interactions.