How can a mother with a positive blood group have a child with a negative blood group?

It has all to do with genetics and RH factor. It can happen. I know because I am having a negative blood group (O+) while my daughter has the blood group (A-). http://kidshealth.org/en/parents/rh.html http://www.rhnegativeregistry.com/Rh_Negative_Mother_Pregnancy_RhoGAM_Anti_D_HDN.html