Aman with type ab blood and a woman with type b blood have a baby has type o blood. should there be any reason for concern? why?

Yes, there should be a concern because it was theoretically impossible for type AB blood parent to have a child with type O blood.
Blood type is coded by two alleles, which have three possible type: A, B and O. The O alleles is recessive, so you need two O allele(from both parent) to have type O blood child.
A person with type AB blood type allele should be one A and one B, so it is impossible to get O allele him.

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