Children

Blood type, how is it inherited? All the various combinations

How do you inherit the blood type (A, B, AB or 0) from mom and dad? Here are all the combinations and whether it can be used as a paternity test.

Children inherit the blood type (A, B, AB , or 0) from both their father and mother. But what are the possible and not possible combinations ? Here are the tables.

Table of Contents

Blood group table: mother-father-child combinations

This table allows you to calculate all possible blood groups of children starting from the group of the mother and father (without taking into account the Rh factor).

As can be seen from the table, some combinations of parental blood types can only lead to specific blood types in children.

For example, two parents of group 0 will have children of group 0 , while two parents 0-A or AA will have children of group 0 or group A.

A similar discourse concerns the  color of the eyes , which is always inherited from the parents.

Blood group as an indicator of paternity

Since a child’s blood type is a combination of that of the father and mother, it can be an  indicator of paternity  (and even motherhood if not certain) of the child.

However, it is important to underline that there are  rare genetic mutations  for which children of group A or B can be born from parents of group 0 (Bombay phenotype), which would not be possible according to standard tables such as the one we have illustrated.

When in doubt, the only way to get certain answers is DNA analysis  which gives 100% certain results.

What are the blood groups?

In a simplified way, a person’s blood group is indicated with the AB0 (AB-ZERO) system. This system identifies 4 blood groups: A, B, AB or 0 .

Together with the group, another value called “Rh factor” is also added, which is indicated with a + (plus) or a – (minus).

By making all the combinations, we obtain 8 categories and each person belongs to only one of these (see figure below).

Remember: it is absolutely recommended to know your group and keep a card that shows it in your wallet.

How blood groups work (in detail)

Warning: we do not use strictly scientific terminology to be clearer and more understandable.

Determining a person’s blood group depends on the  combination of two parts , called alleles, which can be “strong” (dominant) or “weak” (recessive).

For example, in the case of  group A , the two components (alleles) that determine the group can both be of type A or one of type A and one of type 0.

This happens because type 0 is “weak” (recessive), while type A is “strong” (dominant). The part of type 0 is then “silenced” by the part of type A (below the part 0 is specially written in small print).

A+A → A or A+  → A

Similarly, group B can be determined by two parts type B or by one part type A and one type 0.

B+B → B or B+  → B

Another example is the  AB group : in this case the person has a component of type A and one of type B and, since they are both “strong” (dominant), both “express” creating the AB group.

A+B → AB

In the case of group 0, both sides are “weak” (recessive) and therefore are not “hidden”.

0 +  → 0

In biology these two “parts” are the alleles and are indicated with the letter “i”, using a capital letter when the part is dominant and the lower case when the part is recessive.

The writing therefore becomes a little more complicated: the allele type A is dominant and is written as I A , the allele type B is also dominant and is written I B , the allele of type zero is the only one to be recessive and is indicated with a simple i.

As seen, each blood group is determined by two alleles and therefore by combining them we obtain:

  • Group 0 (zero) is determined by two recessive alleles which we will indicate with ii.
  • Group A is determined either by two dominant alleles A (I A I A ) or by one dominant allele A and one recessive i (I A i).
  • Group B is determined either by two dominant alleles B (I BI ) or by a dominant allele B and a recessive allele i (I Bi ).
  • The AB group is determined by the presence of both the dominant alleles A and B (I A I B ).

Summarized in a table:

Blood group First allele Second allele
0 the the
TO A the
TO A A
b b the
b b b
AB A b

How blood types are inherited from parents

Each child inherits only one allele of the blood group from the father and from the mother and it is this that allows the combinations of groups that we saw at the beginning to be created.

For example, those with blood type 0 (zero) must have inherited a recessive “i” allele from their mother and a recessive “i” allele from their father. Perhaps the mother also had the dominant allele A and therefore group A, but she “passed” the zero type allele to her son.

This is why the mother and father can both be A or B and the child be type 0: both also had the recessive allele (which did not manifest itself in them) and both donated it to their child.

A second child could instead inherit only the dominant alleles (A and B), one from the mother and one from the father, and be of group AB.

Let’s see the various cases with tables showing the various combinations and also the probabilities for a child to have a certain group.

Both parents of group 0

Only and exclusively a child with group 0 can be born: 100% ii.

the the
the ii (group 0) ii (group 0)
the ii (group 0) ii (group 0)

Both parents of group A

Case 1: both parents with dominant alleles

A A
A A I A (group A) A I A (group A)
A A I A (group A) A I A (group A)

100% group A children are born.

Case 2: one parent with dominant allele and one with recessive allele

A the
A A I A (group A) A i (group A)
A A I A (group A) A i (group A)

100% group A children are born.

Case 3: both parents with recessive alleles

A the
A A I A (group A) A i (group A)
the A i (group A) ii (group 0)

They are born with a 75% probability of children of group A and with a 25% probability of children of group O.

Both parents of group B

Case 1: both parents with dominant alleles

b b
b B I B (group B) B I B (group B)
b B I B (group B) B I B (group B)

100% group B children are born.

 Case 2: parent with dominant allele and one with recessive allele

b the
b B I B ( group B) B i (group B)
b B I B ( group B) B i (group B)

100% group B children are born.

Case 3: both parents with recessive alleles

b the
b B I B ( group B) B i (group B
the B i (group B) ii (group 0)

They are born with a 75% probability of children of group B and with a 25% probability of children of group 0.

One parent of group A and the other of group 0

As we saw at the beginning, an A blood group could be determined by two different cases and therefore we create two distinct tables.

Case 1: Group A parent has both dominant alleles

A A
the A i (group A) A i (group A)
the A i (group A) A i (group A)

In this case the child can only be of group A (100% I A i).

Case 2: The group A parent has one dominant and one recessive allele

A the
the A i (group A) ii (group 0)
the A i (group A) ii (group 0)

In this case the child has a 50% chance of being born in group 0 (ii) and a 50% chance of being born in group A (I A i ) .

One parent of group B and the other of group 0

Blood group B could also be determined by two different cases, so let’s create two separate tables.

Case 1: Parent of group B has both dominant alleles

b IB extension
the B i (group B) B i (group B)
the B i (group B) B i (group B)

In this case the child can only be of group B (100% I Bi ).

Case 2: The group B parent has one dominant and one recessive allele

b the
the B  i (group B) ii (group 0)
the B i (group B) ii (group 0)

In this case the child has a 50% chance of being born in group 0 (ii) and a 50% chance of being born in group B (I B  i).

One parent of group A and one of group B

This time there are several sub-cases, let’s see them together.

Case 1 : both parents carry a recessive allele

A the
b B I  (group AB) B i (group B)
the A i (group A) ii (group 0)

The child can be born with the following probabilities: 25% of group AB, 25% of group 0, 25% of group A and 25% of group B.

Case 2: A group A parent with dominant alleles and a group B parent with one dominant and one recessive allele

A A
b B I  (group AB) B I  (group AB)
the A i (group A) A i (group A)

The child can be born with the following probabilities: 50% of group A and 50% of group AB.

Case 3: A group B parent with dominant alleles and a group A parent with one dominant and one recessive allele

A the
b B I  (group AB) B i (group B)
b A I B (group AB) B i (group B)

The child can be born with the following probabilities: 50% of group B and 50% of group AB.

Case 4: both parents with dominant alleles

A A
b B I  (group AB) A I B (group AB)
b A I B (group AB) A I B (group AB)

In this case the offspring can only be of group AB (100%).

One parent of group AB and one of group 0

the the
A A i (group A) A i (group A)
b B i (group B) B i (groupB)

In this case, children of group A (50%) or group B (50%) can be born.

One parent of group AB and one of group A

Case 1: The group A parent has a recessive allele

A the
A A I  (group A) A i (group A)
b A I B (group AB) B i (group B)

They can be born with a 50% probability of children of group A, with 25% children of group AB and with 25% children of group B.

Case 2: The group A parent has the dominant alleles

A A
A A I  (group A) A I A  (group A)
b A I B (group AB) A I B (group AB)

The offspring have a 50% chance of being group A and a 50% chance of being group AB.

One parent of group AB and one of group B

Case 1: The parent of group B has a recessive allele

b the
A A I  (group AB) A i (group A)
b B I B (group B) B i (group B)

Children of group AB (25%), A (25%), B (50%) can be born.

Case 2: The parent of group B has both dominant alleles

b b
A A I  (group AB) A I B  (group AB)
b B I B (group B) B I B (group B)

Children of group AB (50%) and B (50%) can be born.

Both parents of group AB

A b
A A I  (group A) A I B  (group AB)
b A I B (group AB) B I B (group B)

Children of group A (25% probability), group B (25% probability) and group AB (50% probability) can be born.

Dr Kathryn Barlow

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

Leave a Reply

Your email address will not be published. Required fields are marked *