Why does my son have a different blood group?

Dear Dr Ombeva

My husband and I are blood group A, yet our son is blood group O. How is this possible? Could there be a problem?

Peony

Dear Peony,

Thank you for writing in. Regarding your question, there is no cause for alarm. It is indeed perfectly normal and possible for parents who both are blood group A, like in your case, to give birth to a child who is blood group O.

The ABO blood grouping system has four main blood groups: O, A, B and AB. The combine together with the Rhesus system to produce the following blood groups: O-, O+, A-, A+, B-, B+, AB- and AB+. The four ABO blood groups arise from a child inheriting one or more of the alternative forms of this gene (or the alleles) namely A, B or O.

The table below shows the likely blood group outcomes for a child, based on the father’s and mother’s blood group. As you can see, when both parents are blood group O, all the children can ONLY be blood group O.

However when both parents are either blood group A each or blood group B each, then it is possible for the child to be group O. However, if any of the parents is blood group AB, then there is NO possibility of the child having a child with blood group O, even if the other parent is O.

- Dr Ombeva Malande is a paediatrics and child health expert