Article — Blood Type Calculator
Blood Type Calculator: Predicting a Child’s ABO and Rh from the Parents
A child’s blood type is determined by two independent gene systems — the ABO group on chromosome 9 and the Rh factor on chromosome 1. Given both parents’ blood types, the calculator above runs a Punnett-square model to show the probability of each of the eight possible child blood types (A+, A−, B+, B−, AB+, AB−, O+, O−). A and B alleles are codominant; O is recessive; Rh+ is dominant over Rh−.
The calculator is an educational tool. Real human genetics includes rare variants — Bombay phenotype, cis-AB, weak D — that occasionally produce results outside the standard Mendelian prediction. This calculator is not a paternity test and should not be used to settle questions of parentage.
What the blood type calculator does
The calculator takes the ABO group (A, B, AB or O) and the Rh factor (+ or −) of each parent. It maps each phenotype to the possible underlying genotypes, runs a 2×2 Punnett square for each parent-genotype combination, and averages the results assuming each parent genotype is equally likely given the phenotype.
A type A parent could be AA or AO. Without family-history information, the calculator weights both possibilities equally. For Rh+ parents the same logic applies. Rh− parents are necessarily −/−, so no averaging is needed.
Karl Landsteiner discovered the ABO blood group system in 1901, earning the Nobel Prize for Physiology or Medicine in 1930. The Rh factor was discovered in 1937 by Landsteiner and Alexander Wiener while studying rhesus monkeys, which is where the “Rh” in Rh+ comes from.
The ABO blood type system
The ABO gene has three alleles. The A allele produces an enzyme that attaches an A antigen to red blood cells. The B allele attaches a B antigen. The O allele is non-functional and produces no antigen. A and B are codominant: a person carrying both alleles displays both antigens and has type AB blood. O is recessive: a person needs two O alleles to display type O.
Codominance is what makes the predictions interesting. AB parents can never have O children — they have no O allele to pass on — and two O parents can never have anything other than an O child.
AA, AO → A BB, BO → BAB → AB OO → OThe Rh factor and its inheritance
The Rh factor is named after the rhesus monkey species in which the antigen was first identified. The Rh blood group includes more than 50 antigens, but in clinical practice the D antigen dominates: Rh+ means the D antigen is present, Rh− means it is absent. The D antigen is the single most immunogenic blood group antigen besides ABO.
Rh follows simple dominance. A person needs only one + allele to display the D antigen. Two Rh+ parents can have an Rh− child only if both are heterozygous (+/−). About 15 percent of Americans of European descent are Rh−, but the frequency varies enormously by ancestry: it is below 1 percent in East Asian populations and as high as 30 percent among the Basque people of northern Spain.
- Rh+ frequency: 85 % in US, 99 % in East Asia, 70 % among Basques
- Rh+ genotypes: +/+ (homozygous) or +/− (heterozygous)
- Rh− genotype: −/− (always homozygous recessive)
- Two Rh+ parents: can have Rh− child only if both are heterozygous
- Two Rh− parents: must have Rh− child (no + allele to pass)
- D antigen: the dominant Rh antigen tested in clinical typing
The Punnett square for blood type
The Punnett square is a grid that arranges one parent’s alleles along the rows and the other parent’s alleles along the columns. Each cell holds the combined child genotype, with probability 1/4 if the parent genotypes are known. The calculator extends this to a 4×4 grid when parent genotypes are ambiguous, giving 16 equally likely outcomes.
Take an A+ parent crossed with a B+ parent. The A parent is either AA or AO; the B parent is either BB or BO. There are four combined parent-genotype scenarios, each given equal weight. Under each scenario, the child has 1/4 probability of each of four allele pairs. The result distribution over 16 equally likely outcomes is 9/16 type AB, 3/16 type A, 3/16 type B, 1/16 type O — assuming each parent has unknown genotype.
Blood type, paternity and surprises
Blood type can sometimes exclude a parent — an AB parent cannot have an O child, an O parent cannot have an AB child. But blood type cannot confirm parentage. Many men with the same blood type as the biological father would produce identical predictions, so the test has low resolution.
Real genetics also includes rare alleles that can break the textbook rules. The Bombay phenotype (genotype hh) blocks expression of A, B and O antigens regardless of the underlying ABO genotype, so a Bombay-phenotype person tests as type O even if they carry A or B alleles. Children of Bombay-phenotype parents can show blood types impossible under the standard Punnett square. Cis-AB, where both A and B antigens are encoded on a single chromosome, is another rarity that can surprise.
This calculator uses standard Mendelian inheritance and ignores rare variants that real human genetics displays. Do not use it for paternity questions. DNA testing of specific genetic markers is the proper tool for confirmed parentage decisions.
Rh− in pregnancy and RhoGAM
An Rh− mother carrying an Rh+ fetus can be exposed to fetal blood at birth, miscarriage or trauma during pregnancy. If exposed, her immune system may produce anti-D antibodies, a process called isoimmunisation. The first Rh+ pregnancy is usually unaffected because antibody production takes time, but subsequent Rh+ pregnancies can suffer hemolytic disease of the newborn — the maternal antibodies cross the placenta and destroy fetal red cells.
RhoGAM (anti-D immunoglobulin) prevents this. A single injection at 28 weeks of pregnancy and another within 72 hours of delivery blocks maternal antibody formation by clearing fetal Rh+ cells from the maternal circulation before the immune system can react. Modern obstetric care has reduced Rh-related hemolytic disease to a rare event in countries with universal prenatal testing.
If you are pregnant and Rh−, ask your obstetrician whether the father has been blood-typed. If he is Rh+ or unknown, RhoGAM is standard care. The injection is well tolerated and has decades of safety data.
Blood type and transfusion compatibility
Type O− is the universal red-cell donor because the cells carry no A, B or Rh-D antigens to trigger recipient antibodies. Type AB+ is the universal red-cell recipient because the immune system already recognises all the common antigens and does not produce anti-A, anti-B or anti-D antibodies.
Plasma compatibility runs the opposite way. AB plasma is the universal donor (it contains no anti-A or anti-B antibodies), while O plasma can only go to type O recipients. This is why blood banks separate whole-blood donations into packed red cells, plasma and platelets for targeted use. The American Red Cross and AABB maintain the protocols.