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Are They His? Kids’ Blood Group is Different From Dad’s.

January 11, 2020

How does blood group inheritance occur between close family members like siblings or even – parents and children?

It seems reasonable to expect that people within the same family should share the same, if not similar blood groups. But is this always the case?

Consider this recent enquiry about this situation:

Hi Doc, my two children are blood group O+ while I am A+; should I be worried about the paternity of the children? Although my wife is O+ …

Dad cuddling two small children - do they share the same blood group?

Understanding Blood Groups

Generally humans fall into one out of a number of blood group classifications.

the most common example is the ABO classification.

Another is the Rhesus type classification where a person can be Rhesus positive or Rhesus negative. It is typically expressed along side your ABO group by the addition of a + or -.

How individuals develop blood group

You are born with your blood group. Thus however it makes up, you can only have received the component genes – one each from either parent.

If someone has group ‘A’ they actually have 2 genes that could make up the group – AA or AO.

Despite the ‘O’ gene’s presence, the person’s group is ‘A’ , because when they occur together, ‘A’ gene is dominant while the ‘O’ gene is not.

So you may have 2 individuals – one with AO genes; another with AA genes who both have the ‘A’ group.
(Likewise, someone with B group can either have BB genes or BO genes but is known as ‘B’ blood group).

The Blood Group difference and Paternity

So back to the specific issue – when you have A group, can you have offspring with O group?

When you have A group, you are either AO or AA genetically.

Now when a person with group A (with AA genes) partners a group O individual (i.e. OO genes) they have kids with AO genes.

This means the children will be blood group A (as the ‘O’ is the recessive gene in the AO gene combination).

If the person with group A (genes AO) partners an O, their kids can be AO and OO (A blood group and O blood group)

Terminology clarifications

So yes, your kids can have OO (O group) even if you are A group. In that case, your genes are AO, not AA.

Once you realise that a person with group A, may carry O genes, you will see that if they partner with another person with O genes, having kids with blood group O is very possible!


This is the simplest way to explain a rather complex genetic association and sometimes an individual may not even fit so nicely into one of the two patterns we describe here.

So if you genuinely feel you have good reason to doubt the paternity of the kids, the best way to address this is with a paternity test rather than blood groups alone.

You can delve deeper into blood groups here and here.

Read about other blood type challenges such as with Sickle Cell disease here.

Have you any questions about blood groups or blood type? Ask or Comment below

More Reading

Editing by AskAwayHealth Team


All AskAwayHealth articles are written by practicing Medical Practitioners.

The advice in our material is not meant to replace the management of your specific condition by a qualified health care practitioner.
To discuss your condition, please contact a health practitioner or reach us directly

Image Credits: Canva

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