Blood typing is a method of determining a person's blood type by serological methods. These blood types depend on whether or not there are certain antigens on the red blood cells & if there are antibodies to these substances. By careful planning a couple can decide & project its child's blood type which is quite important because not all blood groups are compatible with each other & if child & mother are of incompatible blood types, mixing of their blood may lead to blood clumping or agglutination, which may be fatal for both mother & the baby.

Each biological parent donates one of the two A/B/O alleles to his/her child. Alleles are possible types of a particular gene, in this case the blood type gene. There are three basic blood type alleles: A, B, and O which codes for A, B or none of the RBC surface antigens that decide corresponding blood types respectively. Children have two alleles, one inherited from each parent. The possible combinations of the three alleles are OO, AO, BO, AB, AA, and BB. If the parents don't know their blood type, there are lots of easy ways to learn the blood type.

Consequently, a person with blood type B may have a B and an O allele, or they may have two B alleles. If both parents are blood type B and both have a B and a recessive O, then their children will either be BB (if each parent passed on the B allele), BO (if one parent passed on B and the other parent passed on O), or OO (if both parents passed on the O allele). If the child is BB or BO, they have blood type B. If the child is OO, they will have blood type O.

Rh Factor is also an antigen the alleles for which are also inherited from our parents, but independently of the ABO blood type alleles. Someone who is Rh positive or "Rh+" has at least one Rh+ allele, but could have two in homozygous state.

Similar to A, B, O & AB blood types, Rhesus factor (Rh +/-) genetics is determined from the possible allele combinations inherited from each parent. The DNA of each parent carries two Rh alleles either positive (+) or negative (-). The positive Rh factor allele is dominant over the negative Rh factor allele meaning that if a parent's pair of Rh factor alleles are one positive (+) and one negative (-), the positive allele will dominate the negative allele and result in a positive Rh factor (Rh+).