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Blood Type Calculator

Select both parents' blood types and Rh factors to instantly see which blood types their child could inherit, with probability percentages for every possible outcome.

How it worksReal-time

Mother

Blood type

Rh factor

A+

Father

Blood type

Rh factor

B+

Possible blood types for the child

8 possible · 0 impossible

ARh+17.6%

ARh−1.2%

BRh+17.6%

BRh−1.2%

ABRh+52.7%

ABRh−3.5%

ORh+5.9%

ORh−0.4%

Rh+ probability

94%

Rh− probability

Possible outcomes

Confirmed possible blood types

AB+
Blood Type AB+
Rh positive
52.7%
A+
Blood Type A+
Rh positive
17.6%
B+
Blood Type B+
Rh positive
17.6%
O+
Blood Type O+
Rh positive
5.9%
AB-
Blood Type AB-
Rh negative
3.5%
A-
Blood Type A-
Rh negative
1.2%
B-
Blood Type B-
Rh negative
1.2%
O-
Blood Type O-
Rh negative
0.4%

Interpretation

A child of a A+ mother and a B+ father has 8 possible blood types. The most likely outcome is AB+ (52.7%).

Rh factor: 94% chance of Rh positive, 6% chance of Rh negative.

Genetics guide

How blood type inheritance works

Your blood type is determined by genes inherited from both parents. The ABO blood group system is controlled by a single gene with three possible alleles: I^A, I^B, and i. The Rh factor is a separate system controlled by the RHD gene, which has two alleles: D (dominant, Rh positive) and d (recessive, Rh negative). Together, these two systems define the eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.

The ABO system and allele combinations

Every person carries two copies of the ABO gene, one inherited from each parent. The combination of those two alleles determines the blood type:

Type A: I^A I^A (homozygous) or I^A i (heterozygous)
Type B: I^B I^B (homozygous) or I^B i (heterozygous)
Type AB: I^A I^B (the only possible genotype)
Type O: i i (the only possible genotype)

I^A and I^B are codominant, meaning both are fully expressed when present together, producing type AB. The i allele is recessive, so a person needs two copies of it to have type O blood.

Why a type A parent could have a type O child

A person with type A blood can be either I^A I^A or I^A i. If they are I^A i (heterozygous), they carry one hidden O allele. If the other parent also carries an i allele, there is a 25% chance a child inherits i from both parents and becomes type O. This is why two type A parents can have a type O child, though they cannot have a type B or AB child.

Without a DNA test or family history, we cannot determine whether a type A or type B person is homozygous or heterozygous. This calculator assumes equal probability of each, which is the standard approach for educational and planning purposes.

The Rh factor

The Rh system adds a second layer of inheritance. People who are Rh positive carry at least one copy of the D allele (DD or Dd). People who are Rh negative carry two copies of the recessive d allele (dd).

Two Rh+ parents: The child will almost certainly be Rh+, but if both parents are heterozygous (Dd), there is a 25% chance of an Rh- child.
One Rh+ and one Rh- parent: If the Rh+ parent is heterozygous (Dd), the child has a 50% chance of being Rh-. If the Rh+ parent is homozygous (DD), all children will be Rh+.
Two Rh- parents: All children will be Rh-, because both parents can only contribute the d allele.

The Punnett square method

A Punnett square is a grid that shows all possible combinations of one allele from each parent. For ABO blood type, you list one parent's possible allele contributions across the top and the other parent's down the side, then fill in the grid with the resulting child genotypes. Each cell in the grid represents one equally likely outcome if the parents each contribute one allele at random.

For example, an A (I^A i) parent crossed with an O (ii) parent:

Parent A: I^A i Parent O: i i
Children: I^A i (type A, 50%) and i i (type O, 50%)

Why blood type compatibility matters

Blood type compatibility is critical for blood transfusions and organ transplants. Incompatible blood triggers an immune response because the immune system recognizes foreign antigens on red blood cells as threats. Type O- is the universal donor for red blood cells (it has neither A, B, nor Rh antigens), and type AB+ is the universal recipient (it can accept any type).

For pregnant women who are Rh-, there is also a concern about Rh incompatibility. If the baby is Rh+ and fetal blood enters the mother's circulation, the mother may develop antibodies against Rh+ blood. This is managed with Rh immunoglobulin (RhoGAM) injections during pregnancy.

Blood type inheritance: a complete reference table

Mother	Father	Possible child types
O	O	O only
A	O	A or O
B	O	B or O
A	A	A or O
B	B	B or O
A	B	A, B, AB, or O
AB	O	A or B
AB	A	A, B, or AB
AB	B	A, B, or AB
AB	AB	A, B, or AB

Note: Rh factor adds an additional layer. Each blood type in the table above can be Rh+ or Rh-, depending on the parents' Rh genes.

Disclaimer

This calculator provides estimates based on standard Mendelian genetics and assumes that Rh-positive parents are equally likely to be homozygous (DD) or heterozygous (Dd). Actual probabilities may differ based on known family history or genetic testing. This tool is for educational purposes only and is not a substitute for medical genetic counseling.

FAQ

Frequently asked

Can two O-type parents have an A or B child?+

No. Type O parents each carry only the recessive i allele and can only pass i to their children. Every child of two O-type parents will be type O. If a child has type A or B blood, it would suggest a more complex genetic situation or an error in reported blood types.

Can two A-type parents have a type O child?+

Yes, if both parents are the heterozygous I^A i genotype. In that case, each parent has a 50% chance of passing the i allele, giving a 25% chance of an ii (type O) child. Two A-type parents cannot have a type B or AB child.

What does it mean to be Rh positive vs negative?+

Rh positive means your red blood cells carry the Rh (D) antigen on their surface. Rh negative means they do not. This matters most for blood transfusions and pregnancy, where incompatible Rh factors can trigger an immune reaction.

Can two Rh+ parents have an Rh- child?+

Yes, if both Rh+ parents are heterozygous (carrying one D and one d allele). In that case, there is a 25% chance the child inherits d from both parents, making them Rh-. Two Rh- parents cannot have an Rh+ child.

Why are the probabilities sometimes unequal?+

Because a parent with blood type A or B might be homozygous (two A or B alleles) or heterozygous (one A or B allele plus one recessive O allele). Since we cannot tell without genetic testing, the calculator assumes each genotype is equally likely. This means some child blood types are weighted by multiple scenarios, producing unequal probabilities.

Is blood type inheritance always this straightforward?+

For the vast majority of people, yes. However, rare genetic phenomena like the Bombay phenotype (h/h genotype) can mask the ABO genes entirely, causing unexpected blood types that appear to violate standard inheritance rules. These cases are extremely rare and not covered by this calculator.

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