There are about one billion red blood cells in two or three drops of blood.
Within the ABO Blood Group system, the A and B genes are co-dominant, i.e. these will be expressed whenever the gene is present. The O gene is silent and only expressed when neither A nor B is present.
The Rh(D) group is also dominant and will be expressed if inherited from either parent. If the Rh(D) gene is not inherited from either biological parent then the individual will be Rh(D) negative.
The following charts outline the possible ABO and Rh(D) group combinations of parents and the possible blood types of their biological children.