Blood Type Calculator (child’s possible ABO & Rh types)

Understanding the blood type calculator — concept and principle

This calculator works out which ABO and Rh blood types a child of two parents could have, and how likely each is. Because a parent's blood type does not reveal their hidden genes, it weights those genes by how common each type is in the chosen population. Parents, students learning genetics and curious families use it.

A (likely AO) × O (OO) — with the default inputs: O 39.8% · A 60.2%.

How it is calculated

Each parent passes on one of two ABO genes (A, B or O) and one of two RhD genes (D or d)
A and B are codominant; both are dominant over O. D is dominant over d.
A type-A parent may be AA or AO; an Rh+ parent may be DD or Dd
Hidden pairs are weighted by gene frequencies derived from population blood-type shares (Hardy–Weinberg)
Every possible gene from each parent is combined to give the child's chances

Population shares: Korea about A 35%, B 27%, O 27%, AB 11%; Rh-negative about 0.1–0.3%.

The principle

Blood type follows Mendel's laws. A child receives one ABO gene and one RhD gene from each parent. A type-A parent who carries a hidden O gene can have a type-O child with a type-O partner. How likely that hidden O is depends on how common O genes are in the population, which the Hardy–Weinberg principle estimates from blood-type frequencies. In Korea, where Rh-negative is very rare, an Rh+ parent is very unlikely to carry d.

Worked example

Defaults: parent 1 A, Rh+; parent 2 O, Rh+; Korea; Rh-negative share 0.2%.

Possible types: Type O, Type A · Rh+ or Rh−.
Probability parent 1 is AO is about 0.80, so a type-O child = 0.80 ÷ 2 = 39.8%; type A = 60.2%; B and AB 0%.
Each Rh+ parent carries d with probability about 0.086; Rh-negative child ≈ 0.086² ÷ 4 = 0.2%; Rh-positive 99.8%.

How to read the result

The percentages are population-based estimates for parents whose genes are unknown. If parents know their genotypes, for example from their own parents' types, the real chances can be quite different. Rare exceptions, such as cis-AB, the Bombay phenotype or weak and partial D, are not covered.

Common mistakes

Using blood type to prove or disprove parentage; it cannot.
Assuming two Rh+ parents cannot have an Rh− child.
Using Korean frequencies for a family from another population.
Ignoring rare variants when results seem impossible.

Keep in mind: For education only. Blood type cannot establish parentage; medical transfusion and pregnancy decisions rely on laboratory testing.

Frequently asked questions

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

Yes, if both carry a hidden O gene.

Can an AB parent have a type-O child?

Normally no, because an AB parent passes on A or B; rare exceptions exist.

Why does the population matter?

It determines how likely a parent is to carry a hidden O or d gene.

Sources: Mendelian inheritance of the ABO and RhD blood groups with Hardy–Weinberg allele frequencies · American Red Cross, blood type facts; Military Manpower Administration conscription-exam data reported by YTN (October 2023)

Checked 9 Nov 2026. Mendelian inheritance of the ABO and RhD blood groups (A and B codominant, O recessive; D dominant); hidden genotypes weighted by Hardy–Weinberg allele frequencies. Population shares: Korea — Military Manpower Administration conscription-exam data reported by YTN (Oct 2023): A 35%, B 27%, O 27%, AB 11%; United States — American Red Cross blood type facts (about O 45%, A 40%, B 11%, AB 4%).

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