Blood type is the textbook example of a trait that really follows a Punnett square. This blood type calculator crosses the ABO and Rh genotypes of both parents in one 4 × 4 square and lists every blood type a child can have with its probability. Because ABO has three alleles and Rh is inherited separately, it is also a compact demonstration of codominance and a dihybrid cross at the same time.
A blood type hides information. Someone who is type A may be AA or AO, and someone who is Rh positive may be ++ or +−. The hidden O or − allele is what lets two positive-looking parents have a negative child, so the pickers ask for the genotype: A (AO), A (AA), B (BO), B (BB), AB or O (OO) for ABO, and Rh+ (+−), Rh+ (++) or Rh− (−−) for Rh. If you only know the phenotype, the AO and +− options are the safest guess, because they include every possibility.
The A and B alleles are codominant: an AB person shows both antigens. O is recessive, so it only shows in an OO child. Rh positive is dominant over Rh negative. Each parent’s gametes carry one allele of each gene, giving four gamete types per parent. The square puts one parent’s four gametes across the top and the other’s down the side, and each of the 16 cells is a child genotype. Cells are tinted by ABO group, the red-blood-cell icons show the antigens as markers on the cell surface, and the Rh factor is written after the letter. Switch the view to Text if you prefer plain labels, and toggle the odds between percentages and sixteenths.
Take an A parent carrying O who is Rh+ carrying − (AO +−) and a B parent with the same hidden alleles (BO +−). The ABO half gives AB, AO, BO and OO in equal measure, so A, B, AB and O each have a 25% chance. The Rh half gives ++, +−, −+ and −−, so 75% of children are Rh positive and 25% negative. Multiplying the two, the calculator lists eight blood types: A+, B+, AB+ and O+ at 3/16 (18.75%) each, and A−, B−, AB− and O− at 1/16 (6.25%) each. Set both parents to O (OO) and Rh− (−−) and the whole square collapses to a single outcome, O negative.
Tap any cell to highlight all cells with the same blood type and read its fraction. The odds list below the square sorts the possible types by probability and separates ABO from Rh, and the download row saves the square as PNG, JPG or SVG, or the odds as a CSV table. The share button copies a link that reopens the same parents, which is handy for homework or a class handout. For a single-gene version of the same reasoning, see the monohybrid cross page.
No. Type O is the genotype OO, so each parent can only give an O allele and every child is OO, type O. If both parents are O, the square shows 100% O.
It depends on the hidden alleles. AO × BO gives A, B, AB and O at 25% each; AA × BB gives only AB. Pick the parents' genotypes in the calculator to see the exact odds for your case.
Yes. Rh negative is recessive. If both parents are +−, the Rh part of the square gives −− in 1 cell out of 4, so each child has a 25% chance of being Rh negative.
Because it follows two genes at once. Each parent makes four kinds of gametes, one ABO allele paired with one Rh allele, so 4 × 4 combinations fill the square, just like a dihybrid cross.
Yes. An A parent and a B parent can have an O child if both carry a hidden O allele, and two A parents can have an O child the same way. AB is also new when it comes from an A parent and a B parent.
No. It can only rule out combinations that are impossible under Mendelian rules, such as an AB child from two O parents. Matching blood types never prove a relationship; only a DNA test can do that.