Can an O positive mom and an O positive dad have a child with another blood-type?

A child can have O blood group only when two recessive O genes from both parents are inherited. A person is said to have blood type A or B even when a person possess one copy of “A” or “B” allele along with one copy of O allele, while only those who possess two copies of O are said to have a blood type of O.

Rhesus factor determines if the blood type is positive (+) or negative (-). Blood type is considered as A+ve or B+ve or O+ve if they have rhesus factor. People who don’t have rhesus factor are considered to have negative blood type such as O -ve. An O negative blood type is considered to be a universal donor since it contains nothing that would appear foreign to someone else’s blood. People with AB positive blood type is considered as universal recipients as they will not react to A, B, or O blood, and can receive a transfusion from a donor with or without the Rhesus antigen.

Now coming back to our question: Can an O positive mom and an O positive dad have a child with another blood type?

In ABO blood group system, If your mom is O positive and dad is also an O positive blood type then the child will have O blood type. Let’s try to understand this based on how alleles are passed on from parents to children.

Here, father and mother have O Blood group which means they have IoIo alleles. So In all combinations child will inherit Io from father and Io from the mother so which will become IoIo for the child. So in ABO blood group, a child will have O blood group. 

Read more on blood type inheritance: If Father has Blood Group A and Mother has Blood Group B , What will be the blood group of children ??

Let’s understand this again: Can an O positive mom and an O positive dad have a child with another blood-type?

Yes, It is possible in one of such case. One of the known couples have kids of blood type A and blood type B even though both parents are of type O blood. Though none cheated on each other still they had kids of different blood group than O. It’s strange but true. How did it happen? How could the child have different blood group even though parents belonged to O positive?

To understand this, we should understand how blood type detection test is done?

Blood group is detected by a simple laboratory test. In which blood sample is collected and made to react with different reagents. A person’s blood group is determined by different antigens. In ABO blood group people can have A blood type, B blood type, AB blood type and O blood type.

When sample blood group is exposed to reagents, the reaction may or may not occur. If blood has antibodies to particular blood group then reaction happens and it cannot belongs to that blood group. If blood group lacks A antigen then it belongs to B blood type. If blood group lacks B antigen then it belongs to A blood type. If blood group lacks antigen A and antigen B then it belongs to blood type O.

Blood type O also looks like Bombay blood group.  It is because Bombay Blood Group will not have antigen A and antigen B, so lab technicians will think that it belongs to blood group O unless blood sample is made to test against H antigen, Bombay blood group doesn’t contain H antigen too but people with blood type O contains H antigen. So it’s because of the way blood type is detected, it is more likely that Bombay blood group is misdiagnosed as blood group O. this Bombay blood group is also known as HH blood group.

Read More on to know Bombay Blood Group details:  Do you know Bombay Blood Group?

A person needs to have H gene in order to make A protein or B protein.  ABO gene codes for proteins that turn the H protein into A or B.  Since Bombay blood group type people lack protein H and their ABO gene cannot produce A or B . In such person even though the ABO gene may actually contain A  but since H protein is not there A might not be expressed. Or they may even have B gene but since again H is not there B might not have expressed too. So it’s more likely they look like O blood group type.

If either one of these couples belongs to Bombay phenotype and their child will express into other blood groups. Let’s find possible blood group of children when one belongs to Bombay blood group and other is actually O.

So in summary, one of the parents cannot make H protein which is required to produce A or B. but one of their ABO genes can code for either B or A protein which can be passed on to its child.

If the father has Bombay blood group and mother is having blood group O then the child can have following offsprings.

Assuming father can able to code for B but its not expressed because of broken H gene. So in that case we can see child having either blood group B or blood group O.

Assuming father is able to code for A but its not expressed because of broken HH gene. Then the child will have blood group of A or blood type of O.

Assuming father is able to code for A and B but its not expressed because of broken HH gene, then the child may have blood group A or blood group B or blood group O.

Hence, even if parents are detected with blood type O they may still have another blood type only when one of them belongs to Bombay blood type in real.