Can a b neg father have an A pos son?

Yes, it is possible for a B negative father to have an A positive son.

Blood type is determined by the presence or absence of certain antigens on the surface of red blood cells. There are two main blood group systems: ABO and Rh (Rhesus).

The ABO blood group system is determined by the presence or absence of two antigens on the red blood cells, A and B.

People with type A blood have only A antigens, people with type B blood have only B antigens, people with type AB blood have both A and B antigens, and people with type O blood have neither A nor B antigens. Type O blood is also called the universal donor because it can be safely transfused to people with any blood type. Blood group A can only receive donations from blood groups A and O. Blood group B can only receive donations from blood groups B and O.

The Rh blood group system is determined by the presence or absence of one antigen on the red blood cells, the Rh factor. People who have the Rh factor are Rh positive, and those who don't have the Rh factor are Rh negative. Rh negative blood can only receive donations from other Rh negative donors.

The A antigen is dominant over the B antigen, so if a person has one A allele and one B allele, they will have type A blood. In order for a child to have A positive blood, they must inherit an A allele from one parent and a positive Rh factor from the other parent. A B negative father can have an A positive son if the mother of the child has A postive blood