Blood Group Inheritance

Which blood groups can a child of these parents have? The ABO group comes from three alleles (A and B are codominant, O is recessive) and the Rh factor from one more gene.

GeneticsBlood groupsCodominance

Inputs

Input examples

Blood group A can mean genotype AA or AO (and B: BB or BO). If you do not know, try both: an O child is only possible when both parents carry O.

Mother

ABO blood group

Rh factor

Father

ABO blood group

Rh factor

How Blood Groups Are Inherited

The ABO gene has three alleles. A and B are codominant: someone with one of each has blood group AB. O is recessive, so blood group O means two O alleles, while group A can hide an O (genotype AO). The Rh factor is a separate gene: one + allele makes you Rh+.

That is why two parents with blood groups A and B can have a child with any of the four groups, and two Rh+ parents can have an Rh− child. Two O parents, though, can only have O children.

A + Bare codominant

Ois recessive

AO × BOall four groups

O × Oonly O

Punnett Square (ABO × Rh)

mother × father

Result

Enter the values above

Key Relations

ABO Phenotypes

A and B both show when present together; O only shows without either of them.

Two Independent Genes

ABO and Rh lie on different chromosomes (9 and 1), so the chances for the two genes multiply.

Example:

Impossible Groups

A group is impossible when no combination of the parents’ alleles produces it — a classic way to exclude paternity before DNA tests.

Sources & Verification

References

K. Landsteiner, “Über Agglutinationserscheinungen normalen menschlichen Blutes”, Wiener klinische Wochenschrift 14, 1132–1134 (1901).
G. Mendel, “Versuche über Pflanzen-Hybriden”, Verhandlungen des naturforschenden Vereines in Brünn 4, 3–47 (1866).