Blood Type Inheritance: How Parents Pass Blood Types to Children

The Basics: How Alleles Determine Blood Type

Every human inherits exactly two alleles for their ABO blood type — one from each biological parent. An allele is simply one possible version of a gene. For the ABO blood type gene, there are three possible alleles: A, B, and O.

Those two inherited alleles combine to produce your blood type. The six possible combinations and their resulting blood types are:

Genetic Combination	Blood Type
OO	O
AO	A
AA	A
BO	B
BB	B
AB	AB

Notice that both AO and AA produce the same outward blood type (A). This distinction matters when predicting children’s blood types — a parent who looks like “type A” might be carrying a hidden O allele that gets passed on.

Dominant, Recessive, and Codominant: How It Actually Works

The A and B alleles are codominant with each other — if you inherit one of each, both express themselves fully, giving you type AB. Neither one “beats” the other.

The O allele is recessive. It only produces type O blood when you inherit two copies of it (OO). A single O allele paired with A or B stays silent — the A or B takes over.

This is why:

• AB × AB parents can have A, B, or AB children — but never type O
• Two type-A parents can have a type-O child (if both are AO and the child gets the O from each)
• Two type-O parents will always have type-O children — no exceptions

Parent-Child Blood Type Combination Charts

Use this reference chart to see what blood types are possible (and impossible) given any two parents:

Parent 1	Parent 2	Possible Child Types	Impossible Child Types
A	A	A, O	B, AB
A	B	A, B, AB, O	None
A	AB	A, B, AB	O
A	O	A, O	B, AB
B	B	B, O	A, AB
B	AB	A, B, AB	O
B	O	B, O	A, AB
AB	AB	A, B, AB	O
AB	O	A, B	O, AB
O	O	O	A, B, AB

A few things stand out from this table:

• A × B is the most flexible combination — it can produce all four blood types
• O × O is the most restrictive — it can only ever produce type O
• AB is the only type that can never come from two O parents

Understanding It with Punnett Squares

A Punnett square is a simple grid tool used to visualize which allele combinations are possible between two parents. Here’s how it works for blood type:

Example: Two type-A parents who are each AO

Write one parent’s alleles across the top (A and O) and the other’s down the side (A and O), then fill in the four boxes:

	A	O
A	AA	AO
O	AO	OO

Result: 25% chance of AA (type A), 50% chance of AO (type A), 25% chance of OO (type O). So despite both parents being type A, each child has a 1-in-4 chance of being type O.

Example: One AB parent × one O parent

	A	B
O	AO	BO
O	AO	BO

Result: 50% type A, 50% type B. No type O or AB children are possible from this pairing.

How Rh Factor Is Inherited

The Rh factor (positive or negative) follows the same inheritance logic — you get one Rh allele from each parent. The key difference is that Rh positive is dominant over Rh negative.

• A person who is Rh positive may carry either two positive genes (+/+) or one positive and one negative (+/-)
• A person who is Rh negative carries two negative genes (-/-)

What this means practically:

Parent 1 Rh	Parent 2 Rh	Possible Child Rh
Positive (+/+)	Positive (+/+)	Positive only
Positive (+/-)	Positive (+/-)	Positive or Negative
Positive (+/-)	Negative (-/-)	Positive or Negative
Negative (-/-)	Negative (-/-)	Negative only

Two Rh-negative parents will always produce Rh-negative children — the only scenario with a guaranteed Rh outcome.

Can Blood Type Prove or Disprove Paternity?

Blood typing can sometimes rule out a potential father. If a child is type AB, for instance, a potential father with type O can be excluded — there’s no way to produce an AB child from an O parent.

But blood typing cannot confirm paternity. Most human populations carry primarily A and O alleles. Because so many people share the same blood type, there’s usually a large pool of potential fathers who would produce the same child blood type. The system just isn’t precise enough.

For definitive paternity determination, DNA testing is required. DNA profiling compares specific genetic markers across the entire genome — far more discriminating than three alleles in a single gene.

Why This Matters Beyond Paternity

Understanding blood type inheritance has real medical implications beyond curiosity or paternity questions.

For expectant parents where one is Rh-negative, knowing whether the other parent carries Rh-positive genes helps predict the baby’s likely Rh status — important for managing the risk of hemolytic disease of the newborn (where maternal antibodies attack fetal red blood cells). See more on this in our blood type compatibility article.

For patients with rare blood types, understanding inheritance helps identify which family members might be compatible donors — a critical advantage when rare-type compatible blood is nearly impossible to find in standard blood banks.

And for anyone managing hereditary blood disorders like sickle cell disease or thalassemia, knowing both parents’ genetics helps assess the risk of passing those conditions to children.