Blood types are inherited from our parents through genetics. However, the inheritance patterns can sometimes be complex, and children do not always directly inherit either parent’s blood type. There are four main blood types (A, B, AB, and O) and two Rh factors (positive and negative). Which type you inherit depends on the specific blood types and Rh factors of each parent.
How Blood Types Are Inherited
There are two major blood group systems that determine your blood type – the ABO system and the Rh system.
ABO Blood Group System
The ABO blood group system consists of 4 blood types:
- Type A
- Type B
- Type AB
- Type O
Your ABO blood type is determined by the alleles you inherit for the ABO gene from each parent:
- The A allele produces the A antigen on red blood cells
- The B allele produces the B antigen
- Type O has no antigens
Here is how the alleles combine to determine blood type:
| Blood Type | Genotype |
|---|---|
| A | AA or AO |
| B | BB or BO |
| AB | AB |
| O | OO |
So you need one A or one B allele to be type A or B. You need both alleles to be type AB. And type O has no A or B alleles.
Rh Blood Group System
The Rh system consists of the following blood types:
- Rh positive – has Rh antigen on red blood cells
- Rh negative – does not have Rh antigen
Your Rh type is determined by the alleles you inherit for the Rh gene:
- Rh+ allele produces Rh antigen
- Rh- allele does not produce Rh antigen
So if you inherit at least one Rh+ allele, you will be Rh positive. You need two Rh- alleles to be Rh negative.
How Parents Pass Down Blood Type
Each parent passes down one of their two ABO alleles and one of their two Rh alleles. Here are the possibilities:
| Mother’s Blood Type | Mother’s Possible Alleles |
|---|---|
| A+ | AO Rh+ |
| A- | AO Rh- |
| B+ | BO Rh+ |
| B- | BO Rh- |
| AB+ | AB Rh+ |
| AB- | AB Rh- |
| O+ | OO Rh+ |
| O- | OO Rh- |
| Father’s Blood Type | Father’s Possible Alleles |
|---|---|
| A+ | AO Rh+ |
| A- | AO Rh- |
| B+ | BO Rh+ |
| B- | BO Rh- |
| AB+ | AB Rh+ |
| AB- | AB Rh- |
| O+ | OO Rh+ |
| O- | OO Rh- |
Each parent randomly passes down one of their two ABO alleles and one of their two Rh alleles to their child.
Blood Type Inheritance Patterns
Given all the possible allele combinations, here are some key facts about how blood types are inherited:
Rules for ABO Blood Types
- Two type A parents can only have a type A or O child.
- Two type B parents can only have a type B or O child.
- One parent AB and one parent O can only have A, B, or O children.
- Two type O parents can only have a type O child.
- Parents with AB+AB, A+O, or B+O have a 50/50 chance of passing either allele.
Rules for Rh Blood Types
- Two Rh+ parents will always have an Rh+ child.
- If one parent is Rh+ and the other is Rh-, there is a 50/50 chance of the child being Rh+ or Rh-.
- Two Rh- parents will always have an Rh- child.
Putting it all together, here are some examples:
| Mother’s Blood Type | Father’s Blood Type | Possible Child Blood Types |
|---|---|---|
| A+ | A+ | A+ or O+ |
| B- | AB+ | A+, B+, AB+, or O- |
| O+ | O+ | O+ |
| A- | B+ | A+, B+, O+, A-, B-, O- |
| AB+ | AB- | A+, B+, AB+, or A-, B-, AB- |
As you can see, the child’s blood type is not always a simple combination of the two parents’ types. There are many possibilities depending on the specific alleles passed down.
Special Cases
While the above patterns hold true in most cases, there are some rare exceptions:
ABO Bombay Phenotype
This very rare phenotype lacks the A, B, and H antigens. It can occur when a child inherits two recessive hh alleles, even if the parents have normal A, B, or O blood types.
Rh Blood Type Switch
In very rare cases, someone with an Rh- mother and Rh+ father may end up with Rh+ blood. This occurs through a process called recombination, and happens in less than 1% of Rh inheritance.
Chimerism
With chimerism, an individual has two genetically distinct cell populations derived from two different zygotes. This can happen when two fertilized eggs fuse early in development. The result is an individual with some tissues that may be one blood type, and other tissues that are a different blood type.
Conclusion
While children typically inherit an ABO allele and an Rh factor from each parent, the child’s actual blood type is not always a simple mixture of the parents’ blood types. There are specific inheritance patterns based on the interaction of the different blood group systems. In most cases, the child’s blood type will be consistent with one of the potential combinations from the parents’ blood types. But rare exceptions like the Bombay phenotype, Rh recombination, and chimerism can sometimes lead to unusual results. Overall, blood type inheritance follows a complex genetic pathway with many possible outcomes.