The squares behind it
Type A can be AA or AO, and type B can be BB or BO, so more than one square may apply. Each is shown with the odds it produces.
Why a blood type is not a genotype
ABO blood type runs on three alleles, not the usual two: A, B and O. A and B are codominant, so someone carrying both shows both antigens and is type AB. O is recessive to both.
That means type A blood is either AA or AO, and a blood test cannot tell you which. The same goes for type B. Only AB and O are unambiguous: AB must be AB, and O must be OO. This is exactly why two type A parents can produce a type O child, and why that result is not evidence of anything except recessive alleles doing what recessive alleles do.
The one cross worth memorising. An AB parent and an O parent can never have an AB or an O child. AB always passes A or B, O always passes O, so every child is AO or BO, which means type A or type B. It shows up on exams constantly.
Rh factor is a separate gene
Rh inheritance is ordinary dominance and has nothing to do with the ABO gene. Rh positive is DD or Dd, Rh negative is dd. Two Rh positive parents who both carry a hidden d have a 1 in 4 chance of an Rh negative child. Two Rh negative parents can only have Rh negative children, since neither has a D to pass on.
Because the two genes are independent, the calculator above multiplies the ABO odds by the Rh odds when you set both.
This cannot establish paternity. A Punnett square can sometimes exclude a parent, if a child's blood type is impossible from that cross. It can never confirm one, because millions of people share any given blood type. Real testing uses DNA markers.
Common questions
Can two parents with type A blood have a type O child?
Yes. Type A blood can be genotype AA or AO. If both parents are AO, each can pass the O allele, so there is a 1 in 4 chance of an OO child, who is type O. This surprises people because neither parent is type O, but the O allele is recessive and can be carried silently for generations.
Can an AB parent and an O parent have an AB or O child?
No. An AB parent always passes either A or B, never O. An O parent is OO and always passes O. So every child is AO or BO, meaning type A or type B. This is the one blood type cross that rules out both parental types entirely, and it is a classic exam question.
What blood types can a type A and a type B parent have?
All four. If both parents are heterozygous (AO and BO), the children can be AB, A, B or O, each with a 1 in 4 chance. It is the only phenotype pairing that can produce every blood type.
Why can a blood type not tell you the genotype?
Because A and B are dominant over O. A person with type A blood is either AA or AO, and there is no way to tell from a blood test alone, only from family history or genetic testing. Type AB and type O are the exceptions: AB must be AB, and O must be OO.
How is Rh factor inherited?
Separately from ABO, and as a simple dominant trait. Rh positive is DD or Dd, Rh negative is dd. Two Rh positive parents who are both Dd have a 1 in 4 chance of an Rh negative child. Two Rh negative parents can only have Rh negative children.
Can a blood type Punnett square prove paternity?
It can sometimes exclude, but it can never confirm. If a cross makes a child's blood type impossible, that rules the parent out. But millions of people share any given blood type, so a compatible result proves nothing. Actual paternity testing uses DNA markers, not blood type.
Why is ABO inheritance called codominance?
Because when someone inherits both A and B, neither masks the other and both antigens appear on the red blood cells, giving type AB. That is different from incomplete dominance, where you would get a blend. ABO also involves three alleles rather than the usual two, which is why it is a standard example of multiple allelism.
Related tools
General Punnett square calculator, for monohybrid and dihybrid crosses with any letters.
Hardy-Weinberg calculator, for allele frequencies across a whole population.
Sources
ABO inheritance follows the standard three-allele codominant model described in Griffiths et al., Introduction to Genetic Analysis. Rh is modelled as a single locus with D dominant to d, which is the usual teaching simplification. Real Rh genetics involves several genes and more than two haplotypes, so this page is right for coursework and not for clinical use.