What Defines My Child’s Blood Group?
Surprising as it may seem, blood group is not genetically determined at birth based on the contributions of both parents. To find out what it is, a blood sample must be taken from the child and subsequently analyzed.
Blood type is classified according to the presence or absence of antigens on the surface of red blood cells. The presence or absence of certain antigens is what determines that they are one type and not another.
There are three types of antigens (A, B and AB) that make up four different blood types or groups. Furthermore, type O is characterized by the absence of antigens on the surface of red blood cells.
Therefore, our genotype is composed of two possible expressions of this gene, which gives the following possible combinations: AA, AB, AB, AO, BO and OO. On the one hand, the A and B genes are dominant (they are expressed) and, on the other hand, the O gene is recessive (it is expressed only in the presence of a second O gene).
2 large groups: ABO and Rhesus
There are more than 20 different types of possible incompatibilities that form through two large groups: the ABO system and the Rh factor. Below is a description of each of them.
ABO system
Incompatibility, in this case, is usually milder and may appear in the first pregnancy. There are three different antigens: A, B and O, which will determine which groups can be A, B, AB or O.
People who have the A antigen on the surface of their red blood cells express antibodies — molecules that attack — against B. Those in group B do the opposite, they have antibodies against group A.
People in group O do not have A or B on their surface and therefore will be able to make antibodies against A and B. To conclude, those in group AB, having both antigens on their surface, will not produce antibodies or A and neither B.
For example, people whose wall is made up of type A antigens will be able to make antibodies against type B antigens. It would not make sense for them to make antibodies against type A, because they would be attacking themselves.
Rh factor
A person’s blood group is also determined with the indication of Rhesus factor, which indicates the presence or absence of a particular antigen on red blood cells. Rhesus factor can be positive (Rh+) or negative (Rh-).
The most frequent problems of maternal-fetal incompatibility (and therefore mother-father) depend mainly on the Rh factor.
When both parents have an Rh negative factor, regardless of their blood group, the child will necessarily have an Rh negative blood group. In other cases, the child may have a blood group with an Rh + or Rh -.
How is blood group inherited?
Blood groups are inherited from the parents: half from the mother and half from the father.
The group information of the ABO system is controlled by a single gene, in which A and B are dominant—like the brown gene dominates over the blonde or the dark-eyed gene over the light ones, in this case A or B dominates over O. Thus, both an AO and an AA person will be from group A.
Incompatibility between groups
When there is a mismatch, the baby’s cells are attacked by the mother’s antibodies; these destroy and can cause anemia and hyperbilirubinemia. It may be less severe in cases of incompatibility by the ABO system and more severe in the case of Rh.
It is rare that there is an incompatibility between groups, although it can happen in the following cases:
- Mother with group A, B or AB positive: no problem, it is not necessary to know the father’s group.
- Mother with positive O group: it is advisable that the father perform the exam because, if it is A, B or AB, the child could have jaundice at birth due to the ABO incompatibility. This is not a serious condition, but knowing the parent’s group speeds up the time of diagnosis.
- If the group is the same in both parents, there is no problem for the baby.
Definitely, it should be taken into account that the test to know the blood group is very important in pregnancy. It serves to find incompatibilities in the couple that can cause problems for the fetus. Therefore, prevention is of vital importance.
