My four-year-old daughter started the day by asking me a question about blood types. I was delighted to have the opportunity to share some of my knowledge on the subject with her. It made me want to go back to all those genetics classes I used to have and try to explain everything in a way that made sense. Since she didn’t seem interested in the topic, I’ll quickly run through a few things about blood types and Rh factors here (it’s a public holiday today, so I have some time).
possible blood types
Like many of our traits, blood types are genetically inherited. In simplified terms, it’s important to remember three things:
- We have two copies of almost all of our genes.
- Genes act as recipes for making proteins.
- We have something called alleles, which are different versions of our genes.
Our blood type is determined by three different versions of a gene, which we can call ABO.
Since we inherit two versions of each allele, one from our mother and one from our father, each of our two copies can have a different version. The following table is a straightforward way to comprehend this:
| Genes inherited | Blood type resultant |
| AA | A |
| AO | A |
| BB | B |
| BO | B |
| AB | O |
| OO | O |
There are tiny details outside our blood cells called antigens. It’s pretty simple.
| Blood type | Antigen |
| A | A |
| B | B |
| AB | AB |
| O | NO PROTEIN |
However, there is always an interesting new factor to consider: our DNA can change. It’s expected to change during our lifetime since there are many factors that can cause mutations in our DNA, such as our diet, sun exposure, age and chemicals in the air.
These include our diet, sun exposure, age and chemicals in the air. It is thought that everyone had type A blood in the distant past, and that types B and O are mutations.
DNA is made up of four different elements, which we call bases. Each gene is a formula for making a protein.
The difference between type A and type O is one base (type O is missing this base), while the difference between type A and type B is seven bases.
Our genes are read by cells in groups of three bases. Therefore, if just one base is different, it can alter the entire code. O is simply a modified version of A, which occurred at some point in the distant past and was able to survive and spread successfully.
This justifies a mutation leading from an A type to an O type.
We can also observe that an O type can generate an A type, but this is much rarer. The odds of this specific change occurring are around 1 in 50 million. Therefore, it is possible for two type O parents to have a type A child, but it is not very probable.
Similarly, it is possible for O-type parents to have a B-type child, but this is highly unlikely, as it is improbable that seven bases would change simultaneously to produce a recipe for B-type blood from O-type blood.
There are different types of mutation that can result in two B-type parents having an A-type child. This happens because of recombination.
Since each person has two different forms of a gene (called alleles), one from each parent, we can see how recombinations can occur.
An example:
A woman have type B and type O alleles, which results in type B blood. When her body produces eggs, it chooses which allele it will carry. It can be type O, coming from one parent, or type B, coming from the other.
Type O alleles are almost type A alleles, they’re just missing one little base. If, for a reason, this one base is replaced, it can works just like a type A.
This way, if this woman have children with a O type father, the child could have A type blood, even though neither parent have an A type allele. If she have kids with an B type man, the child could have AB blood type.
We have something called amino acids, that are like Lego parts to build a protein. There are 20 different types of this building blocks.
The ABO gene have the following sequence of DNA Bases:
GTC CTC GTG GTG ACC CCT TGG
This sequence is read by the cell as instructions to make a protein using the following amino acids (Lego pieces):
Val Leu Val Val Thr Pro Trp
What if one of the “G” base, right on the middle, is taken out? All the following bases would get shifted to the left, resulting in something like
GTC CTC GTG GTA CCC CTT GGC
Which would result in a protein builded with the pieces
Val Leu Val Val Pro Leu Gly
This is a mutation. As you can see, it’s a completely different set of amino acids (what makes a totally different protein!).
How the cell know when it reached the end of the gene, so it can stop reading? As you can see on the table above, there’s certain combinations that acts like a stop sign. In this case, when the “G” base got missing, the cell finds a stop signal earlier than it should. This is exactly why two type A can have type O children!
rare exceptions on blood type inheritance
There are rare exceptions on the blood rules. We can basically summarise it as
- Chimerism
- Cis-AB type
- Uniparental dissomy
- Bombay group
- New Mutations
chimerism
We call chimerism when one person has two different sets of DNA. It can happen because of:
- Bone marrow transplant, when the recipient will have the blood type of the donor, but their sperm or eggs would still have the original DNA and blood type
- Fusion chimerism, when a twin pregnancy combines generating a single baby with both twin’s sets of DNA
- Blood chimerism, when fraternal twins share blood during pregnancy, causing mixing cells
cis-AB TYPE
There’s a rare version of ABO gene, which can result in AB type by itself. On this case, two AB parents could have an O type child and one AB type parent + one O type parent could have an AB type child.
uniparental dissomy
It happens when a child inherits both sets of chromosomes from a single parent. It’s very rare.
bombay group
It’s a recessive blood type caused by a variant at different blood type genes. This type look like an O type, no matter which ABO type they have.
new mutations
This is pretty rare, but new and less documented mutations can occur.
When we consider this exceptions, the have a table of combinations that look like that:
possible blood types chart
| Parent 1 | Parent 2 | Expected blood type resultant | Rare exceptions |
| A | A | A, O | B, AB – If either parent is a chimera |
| A | B | A, B, AB, O | Non existent |
| A | AB | A, B, AB | O – If the AB parent is a chimera or has cis-AB type. Also possible that both parents carry Bombay group |
| A | O | A, O | B – If either parent is a chimera or if O parent carries Bombay AB – If the O parent is a chimera or carries Bombay |
| B | B | B, O | A, AB – If either parent is a chimera |
| B | O | B, O | A – If either parent is a chimera or if the O parent carries Bombay AB – If the O parent is a chimera or carries Bombay |
| AB | AB | A, B, AB | O – If both parents are carriers of Bombay or have cis-AB type |
| AB | O | O | O – If the AB parent is a chimera or has cis-AB type or if both parents carriers Bombay AB – If the AB parent has cis-AB type or if the O parent carriers Bombay |
| O | O | O | A – If either parent is a chimera or carriers Bombay B – If either parent is a chimera or carriers Bombay AB – Both parents are chimera or carriers of Bombay group |
Besides blood types, there’s also a lot of things to consider when we think about Rh factor, but this will have to be discussed on a further post, since Fritz is already boring meowing for me.