For the longest time, blood typing seemed to be a simple area of genetic inheritance from parents. The individual got one alllele from mom and one allele from dad and their phenotype was formed from there. Blood type is a codominate trait which is composed of three different alleles O, A and B. Both A and B are codominant to eachother while O is recessive to both A and B. Therfore, if an individual has the alleles O and B, they would have a type B blood. If an individual had the A and B allele, they would have type AB blood. These alleles determine the type of sugar which is attached to the red blood cells. Antigens recognize only these types of sugars in the blood as beeing good. If an individual with type B blood had A sugars in their blood, their body would attack them as foreign. This is a key concept in blood transfusions between individuals. If an individual was given a blood type that did not match theirs, they would have a major immunological response. The Bombay pheonotype puts a wrench in this simple pattern of inheritance. This is an example of recessive epistasis where if an individual possesses two of the recessive alleles for the bombay gene, they will expressive the bombay phenotype and have type O blood, despite the alleles inherited from their parents. The Bombay phenotype codes for a substrate which the sugars from the A and B allele bind two. Without the substrate, the A and B sugars will not bind and they will exhibit type O blood. With this understanding, no more parents will get blamed for stepping out when they did not.
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