Blood type is an important genetic trait that is passed down from parents to their children. The two most well known blood group systems are the ABO blood group and the Rh blood group. A person’s blood type is determined by the types of antigens found on their red blood cells. The ABO blood group has four major blood types: A, B, AB, and O. The Rh blood group is either positive or negative. So is a child’s blood type determined solely by the mother, or do both parents play a role?
Overview of Blood Types
As mentioned, the two main blood grouping systems are ABO and Rh. Here is a brief overview:
ABO Blood Groups
The ABO blood group consists of 4 main types:
- Type A – has A antigens on red cells
- Type B – has B antigens on red cells
- Type AB – has both A and B antigens on red cells
- Type O – has neither A nor B antigens on red cells
Rh Blood Groups
The Rh blood group consists of either Rh positive (+) or Rh negative (-). If your blood has the Rh antigen, you are Rh positive. If your blood lacks the Rh antigen, you are Rh negative.
Blood Type Compatibility
It’s important to have compatible blood types for blood transfusions. Here is the compatibility chart:
| Blood Type | Can Donate Blood To | Can Receive Blood From |
|---|---|---|
| A+ | A+, AB+ | A+, O+ |
| O+ | O+, A+, B+, AB+ | O+, O- |
| B+ | B+, AB+ | B+, O+ |
| AB+ | AB+ | Everyone |
| A- | A+, A-, AB+, AB- | A-, O- |
| O- | Everyone | O- |
| B- | B+, B-, AB+, AB- | B-, O- |
| AB- | AB+, AB- | AB-, A-, B-, O- |
This table shows which blood types are compatible for donating and receiving blood from others. For example, a person with type A+ blood can donate blood to people with types A+ and AB+, but can only receive blood from donors with types A+ or O+.
Now that we’ve covered the basics of blood types, let’s look at how they are inherited.
Genetics of Blood Type
Blood type is passed down from parents to their children through inheritance of the ABO and Rh genes. Since you receive one set of genes from each parent, both influence your blood type.
Inheriting the ABO Blood Group
The ABO blood group is controlled by a single gene located on chromosome 9. This gene codes for enzymes that add A or B antigens to your red blood cells.
There are three possible genotypes for the ABO gene:
- AA or AO – Produces type A antigens
- BB or BO – Produces type B antigens
- AB – Produces both A and B antigens
- OO – Produces no antigens (type O)
You inherit one ABO allele from each parent. The allele you inherit determines which antigens are produced:
| Genotype | Blood Type |
|---|---|
| AA or AO | A |
| BB or BO | B |
| AB | AB |
| OO | O |
For example, if you inherit an A allele from one parent and a B allele from the other, you will have the AB genotype and type AB blood. Since both parents contribute to the ABO genotype, blood type is clearly dependent on both the mother and father.
Inheriting the Rh Blood Group
The Rh blood group is determined by a single gene on chromosome 1. The allele for the Rh+ antigen is dominant over the allele for no antigen (Rh-).
- Rh+ – Has Rh antigen on red cells
- Rh- – Lacks Rh antigen on red cells
You inherit one allele from each parent. If you inherit at least one Rh+ allele, you will be Rh+. You need to inherit two Rh- alleles to be Rh-.
| Genotype | Blood Type |
|---|---|
| Rh+Rh+ or Rh+Rh- | Rh+ |
| Rh-Rh- | Rh- |
Again, both parents contribute to your Rh blood type. Having an Rh- mother does not guarantee an Rh- child if the father is Rh+.
Predicting a Child’s Blood Type
Now that we understand how blood types are inherited, let’s look at some examples of how to predict a child’s blood type based on the parents’ blood types:
Example 1
- Mother: Type A+
- Father: Type B+
Possible genotypes:
Mother – AO or AA
Father – BO or BB
If the child inherits an A from the mother and a B from the father, the child will have genotype AB and blood type AB+.
Example 2
- Mother: Type O-
- Father: Type AB+
Possible genotypes:
Mother – OO
Father – AB
Since the mother can only pass on an O allele, the child must inherit an O from her. If the child inherits an A from the father, the genotype will be AO and blood type A+. If the child inherits a B from the father, the genotype is BO and blood type is B+.
Example 3
- Mother: Type B-
- Father: Type A+
Possible genotypes:
Mother – Rh-Rh-
Father – Rh+Rh- or Rh+Rh+
The mother can only pass on an Rh- allele. If the father passes on an Rh+ allele, the child will be Rh+. If he passes on Rh-, the child will be Rh-. For the ABO blood group, there are 4 possibilities:
- A from mother, A from father: Genotype AA, Type A+
- A from mother, B from father: Genotype AB, Type AB+
- B from mother, A from father: Genotype AB, Type AB+
- B from mother, B from father: Genotype BB, Type B+
So in summary, the child could be A+, B+, or AB+ and either Rh+ or Rh-. The mother’s Rh- blood does not guarantee the child will also be Rh-.
Rh Incompatibility Between Mother and Fetus
While the Rh blood type is inherited from both parents, there is one special scenario involving Rh negative women.
If an Rh- woman is pregnant with an Rh+ baby, her immune system can develop antibodies against the Rh antigen. This is because some of the baby’s Rh+ blood cells can cross into the mother’s blood circulation. The mother’s body senses them as foreign and makes antibodies against them.
If it is her first pregnancy, this is usually not an issue. But in subsequent pregnancies with an Rh+ fetus, the Rh antibodies that she developed during the first pregnancy can cross the placenta and attack the fetal blood cells. This is called Rh sensitization and can lead to hemolytic disease of the newborn.
Therefore, Rh- mothers are usually given a medication called RhoGAM during pregnancy and immediately after delivery. This prevents her body from making antibodies against her baby’s Rh+ blood cells. This allows her to safely carry multiple Rh+ pregnancies.
Conclusion
In summary, both parents contribute to their child’s blood type through the inheritance of the ABO and Rh blood group genes. While the Rh- blood type of the mother can sometimes impact the fetus, the father’s blood type is equally important in determining the baby’s overall blood group. So while the mother’s blood type is a factor, it alone does not determine the blood type of the child.