Somewhere I remember reading in a biology text in high shcool about recessive sub grouping in blood typing. If what I am remembering is correct, then my ex could have given our son the A . Others have said this is possible. Thank you for time and consideration from an agrivated ex wife. A man who is B positive would not be expected to father a type A son with a type O mother.
Blood typing is a very poor method to resolve important questions about paternity.
If this is an issue that needs a clear cut resolution, you should consider DNA testing. One well-known example of epistasis in human blood type is the Bombay Phenotype . I have read your whole site and used the calculator and still do not have an answer. How is that possible. My doctor says it is not possible which led me to believe I was possibly adopted.