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Blood Type Inheritance Calculator

See the possible ABO and Rh blood types of a child, and the chance of each, from both parents' blood types.

Level: Grades 7–8

  • Common Core standard 7.SP.C.8.bRepresent sample spaces for compound events using methods such as organized lists, tables and tree diagrams. For an event described in everyday language (e.g., "rolling double sixes"), identify the outcomes in the sample space which compose the event.
  • Common Core standard 7.SP.C.8.aUnderstand that, just as with simple events, the probability of a compound event is the fraction of outcomes in the sample space for which the compound event occurs.
Printable worksheet

Blood Type Inheritance Calculator

Inputs
Parent 1 ABO type
Parent 1 Rh factor

A parent with type O blood among their own parents or children must carry O.

Parent 2 ABO type
Parent 2 Rh factor

A parent with type O blood among their own parents or children must carry O.

Result

Possible child blood types

A+, A−, B+, B−, AB+, AB−, O+, O−

A child of these parents could have any of A+, A−, B+, B−, AB+, AB−, O+, O−.

Chance of A+
17.58%
Chance of A−
1.17%
Chance of B+
17.58%
Chance of B−
1.17%
Chance of AB+
52.73%
Chance of AB−
3.52%
Chance of O+
5.86%
Chance of O−
0.39%
Not expected from these parents
None: every type is possible

See the step-by-step solution ↓

Chance of each blood type for one child
Chance of each blood type for one child
Blood typeChance
A+17.58%
A−1.17%
B+17.58%
B−1.17%
AB+52.73%
AB−3.52%
O+5.86%
O−0.39%

Step-by-step solution

  1. List the ABO gene pairs (genotypes) each parent could have. A and B are dominant, so they can hide an O

    Parent 1: AA 50% or AO 50%; Parent 2: BB 50% or BO 50%

  2. Each parent passes on one of their two ABO alleles at random. A Punnett square of every pairing gives the child's ABO chances

    A 18.75%, B 18.75%, AB 56.25%, O 6.25%

  3. Do the same for Rh. One D allele is enough for positive, so Rh+ is DD or Dd and Rh− is dd

    Parent 1: DD 50% or Dd 50%; Parent 2: DD 50% or Dd 50% → Rh+ 93.75%, Rh− 6.25%

  4. The two genes are passed on separately, so multiply the ABO chance by the Rh chance for each type

    A+ 17.58%, A− 1.17%, B+ 17.58%, B− 1.17%, AB+ 52.73%, AB− 3.52%, O+ 5.86%, O− 0.39%

  5. Possible blood types for a child

    A+, A−, B+, B−, AB+, AB−, O+, O−

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out which blood types a child could have, and how likely each one is, from the parents' blood types. Blood type comes from two separate genes. The ABO gene has three common versions, called alleles: A, B and O. A and B both win out over O, which means they are dominant. A child who gets one A and one B shows both, as type AB. The Rh (RhD) gene decides the plus or minus. Positive is dominant, so a person is Rh-negative only when they carry two negative alleles.

The calculator builds a Punnett square for each gene. That's a small grid showing every way the parents' alleles can pair up. It then combines the two grids and lists all eight types, from A+ to O−, with the chance of each. Do you know whether a parent carries a hidden O or Rh-negative allele? Add it for a sharper answer.

Where it's used

Why this math matters outside the classroom:

  • Biology class. Students draw a 2 × 2 Punnett square for AO × BO and find each of A, B, AB and O has a 1 in 4 chance.
  • Family trees. A student sees that two Rh+ parents who are both Dd have a 25% chance of an Rh− child.
  • Compound events. A class multiplies 25% for type A by 75% for Rh+ to find an 18.75% chance of A+.
  • Codominance. Students learn why an A parent and a B parent can have an AB child who shows both markers.
  • Science fairs. A student surveys classmates' blood types and compares them with the types their parents' types allow.

Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.SP.C.8.b, 7.SP.C.8.a. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose each parent's ABO type and Rh factor (positive or negative).
  2. If you know a parent's genes, add them. For example, a type A parent who has a type O parent of their own must carry O. Their genotype, or gene pair, is AO.
  3. Read the list of possible child types, and the chance of each in the table.

What the inputs mean

Parent 1 ABO genes, if known
A type A person may have the gene pair AA or AO, and a type B person BB or BO. When you choose 'Not known', the calculator gives the two options equal weight. That's a simplification, not a figure measured in real populations.
Parent 1 Rh genes, if known
An Rh-positive person may have the gene pair DD or Dd. D is the positive allele and d is the negative one. A parent who has an Rh-negative parent or child must be Dd.
Parent 2 ABO genes, if known
A parent with type O blood among their own parents or children must carry O.

Formula and method

each parent passes one of their two alleles, each with probability ½
A + A or A + O → A;  B + B or B + O → B;  A + B → AB;  O + O → O
D + anything → Rh+;  d + d → Rh−
P(type and Rh) = P(ABO type) × P(Rh)

In these lines, A, B and O are the three ABO alleles. D is the Rh-positive allele and d is the Rh-negative one. P( ) means "the chance of". Each parent has two alleles for each gene and passes on one of them at random.

For one pair of parent genotypes, there are four ways to pick one allele from each parent, and all four are equally likely. The calculator counts which of the four give each blood type. When a parent's genotype isn't known, it weights each possible genotype by how likely it is, then adds them up. That gives the ABO chances, and the Rh gene is handled the same way.

The ABO and Rh genes sit on different chromosomes, so they're passed on independently. That means the chance of a combined type such as A− is the chance of A times the chance of Rh−.

Worked example

One parent is A+ with the gene pairs AO and Dd. The other is B+ with the gene pairs BO and Dd. What blood types could their child have?

  1. ABO Punnett square, AO × BO: the four pairings are AB, AO, BO and OO, each 25%. So A, B, AB and O are each 25%.
  2. Rh Punnett square, Dd × Dd: DD 25%, Dd 50%, dd 25%. So Rh+ is 75% and Rh− is 25%.
  3. Combine the two: each ABO type is 25% × 75% = 18.75% positive and 25% × 25% = 6.25% negative.
  4. Possible blood types for a child: A+, A−, B+, B−, AB+, AB−, O+, O−.

Result: All eight types are possible. Each positive type has an 18.75% chance, and each negative type a 6.25% chance.

Understanding the result

The chances apply to each child separately. A 25% chance of type O doesn't mean one child in four will be type O. Every child is a fresh draw, like a new coin toss.

A type listed as not expected can still happen in rare cases. Examples are the Bombay phenotype and the cis-AB allele, which are rare gene variants, and new mutations. That's why this tool can't settle questions of parentage.

Assumptions and limitations

  • Inheritance follows the simple textbook (Mendelian) pattern, with the common A, B and O alleles and the RhD gene.
  • When a genotype isn't known, both possibilities get equal weight. Real chances depend on family history and ancestry.
  • Rare blood group variants aren't modelled, and neither are other blood group systems (such as Kell or Duffy).
  • This is an educational genetics tool, not medical or legal advice. It isn't a paternity test.
  • Questions about Rh status in pregnancy should go to a doctor or midwife.

Frequently asked questions

Can two O parents have an A child?

Under standard inheritance, no. Both O parents are OO, so they can only pass on O. Very rare variants can produce unexpected results. That's one reason blood types aren't used to decide parentage.

Can two Rh-positive parents have an Rh-negative child?

Yes, if both carry the negative allele (they're both Dd). Then each child has a 1 in 4 chance of getting d from both parents. That makes the child dd, which is Rh-negative.

Why does the result change when I say a parent carries O?

A type A parent who is AA always passes on A, while an AO parent passes on O half the time. Knowing which one makes the chances exact, instead of an average of the two.

What blood types can an AB parent and an O parent have?

Only A and B, each with a 50% chance. The AB parent passes A or B and the O parent always passes O, giving AO (type A) or BO (type B).

How this calculator is checked

Method: Mendelian segregation for the ABO gene (A and B codominant, both dominant over O) and the RhD gene (D dominant), assumed to assort independently. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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