Can a o positive mother with A positive father have A negative son?

The short answer is YES. The long answer is that everybody carries two copies of any particular gene. Some genes are 'dominant' so will overrule a 'recessive' gene. In blood grouping, groups A and B are both dominant over O, and RhD 'positive' is dominant over 'negative'. Therefore the O positive mother must have two group O genes and one positive and one negative RhD gene. The father could be A/A or A/O and again must have one positive and one negative RhD gene. The son will have inherited an O from the mother and an A from the father making him group A. He will also have inherited both parents RhD negative genes making him 'negative'. JM. Consultant Haematologist