Blood types are determined by the presence or absence of certain antigens on the surface of red blood cells. The most well known blood group system is ABO, which consists of four major blood types: A, B, AB, and O. The wild type blood type refers to the ancestral or original blood type in the ABO blood group system.
What is the wild type ABO allele?
The ABO blood group is controlled by a single gene called ABO. This gene codes for glycosyltransferase enzymes that add sugar molecules to a precursor substance called the H antigen on red blood cells. The wild type ABO allele is considered to be allele O. The O allele does not produce a functional glycosyltransferase enzyme. This means thatindividuals with type O blood have red blood cells with just the H antigen and no A or B antigens.
The A and B alleles are considered to be variants or mutants of the ancestral O allele. The A allele codes for an enzyme that adds N-acetylgalactosamine to the H antigen to generate the A antigen. The B allele codes for an enzyme that adds galactose instead, creating the B antigen. The AB allele is a hybrid of A and B.
Evidence that O is the ancestral ABO allele
There are several lines of evidence that support blood type O being the original wild type in humans:
- Type O is the most common blood type globally. About 45% of people in the world population have type O blood. The high frequency suggests it is likely the ancestral phenotype.
- The distribution of ABO blood types varies by geography, but type O is the most prevalent across all populations. This indicates that it is the oldest blood type that predates modern human migration patterns.
- Most primates have red blood cells that appear to be group O. Their red blood cells react similarly to antibodies against human A and B antigens.
- Studies of ABO antigen expression patterns in embryos and fetal tissue are most consistent with O being the default blood type originally in adults.
- Genetic analysis shows the O allele has no mutations compared to the ancestral state of the ABO gene. By contrast, the A and B alleles have specific defining mutations.
What are the most common blood types?
The most abundant blood types in the human population are O positive and A positive. Here is a breakdown of the frequencies of the 8 major blood types:
| Blood Type | Frequency (%) |
|---|---|
| O positive | 37.4% |
| A positive | 35.7% |
| B positive | 8.5% |
| AB positive | 3.4% |
| O negative | 6.6% |
| A negative | 6.3% |
| B negative | 1.5% |
| AB negative | 0.6% |
Overall, about 85% of people are Rh positive while 15% are Rh negative. The most rare blood type is AB negative, present in less than 1% of the population. The high percentage of O positive and A positive types reflects the likely evolution of blood types from an original O ancestral group.
What determines your ABO blood type?
Your ABO blood type is inherited based on the ABO alleles you get from each parent. The gene is located on chromosome 9 and has three major allele forms – A, B, and O. Because humans are diploid organisms, we have two copies of each chromosome and therefore carry two ABO alleles.
The A and B alleles are co-dominant, so if you inherit one of each, you will have AB blood type. The O allele is recessive to A and B. So if you inherit an O allele along with an A or B allele, you will end up with A or B blood respectively. Only inheriting O alleles from both parents leads to O blood type.
The inheritance patterns based on parental blood types are summarized below:
| Parent 1 | Parent 2 | Possible Blood Types in Children |
|---|---|---|
| AA or AO | AA or AO | A |
| BB or BO | BB or BO | B |
| OO | OO | O |
| AA | BB | A, B, AB |
| AO | BO | A, B, AB, O |
What is the role of Rh factor in blood types?
In addition to the ABO system, blood types are also classified according to the presence or absence of the Rh antigen. This is designated with a positive or negative after the ABO type. People who have the Rh antigen on their red blood cells are Rh positive. Those without it are Rh negative.
The Rh blood group actually contains many different Rh antigens, but the D antigen is the most immunogenic. Inheriting the dominant RhD gene leads to Rh positive blood type. Being Rh negative means you have no functional copies of RhD.
Like ABO, Rh type is also inherited from parents. An individual who inherits two recessive d alleles will be Rh negative. A person with at least one RhD dominant allele will be Rh positive. Two Rh negative parents will always have Rh negative children. If one parent is Rh negative and the other is positive, there is a 50/50 chance for either Rh type in their offspring.
Rh Incompatibility
A special circumstance arises when an Rh negative mother carries an Rh positive fetus. The fetus can inherit Rh positive type from an Rh positive father. The mother may then develop antibodies against the Rh antigen, causing hemolytic disease of the newborn. This condition can be prevented by administering anti-D immunoglobulin to the mother during and after Rh incompatible pregnancies.
What are some key facts about the major blood types?
Here is an overview of some defining characteristics, compatibility, and functions of the ABO blood groups:
Type A
- Has A antigens on red blood cells
- Plasma contains anti-B antibodies
- Can receive A and O blood types
- Mostly found in Northern and Central Europe ancestries
Type B
- Has B antigens on red blood cells
- Plasma contains anti-A antibodies
- Can receive B and O blood types
- Mostly found in Asian populations
Type AB
- Has both A and B antigens on red blood cells
- Plasma does not contain anti-A or anti-B antibodies
- Universal recipient – can receive any blood type
- Rarest blood type
Type O
- No A or B antigens on red blood cells (only H antigen)
- Plasma contains anti-A and anti-B antibodies
- Universal donor – can donate to all blood types
- Most common blood type
Conclusion
The wild or original blood type in humans is group O. The high global frequency and ancestral inheritance patterns provide evidence that type O is the wild type from which A and B blood groups diverged later through mutations in the ABO gene. Both the ABO and Rh blood group systems contribute to the major blood types present across human populations today.