Blood Type Inheritance Calculator

The blood type inheritance calculator predicts the possible ABO blood groups and Rh factors of a child from the blood types of both parents. Human blood type is one of the cleanest real-world examples of Mendelian genetics, which is exactly why it appears in nearly every biology classroom and why this blood type calculator is so widely used by students, parents, and genetics enthusiasts. Each person inherits one ABO allele and one Rh allele from each parent, and the combination of those alleles determines the blood type that is expressed.

This calculator works on two independent genetic systems at once. The first is the ABO blood group system, controlled by a single gene with three alleles: A, B, and O. The second is the Rh factor (Rhesus) system, controlled by the D gene that produces either Rh-positive (Rh+) or Rh-negative (Rh-) blood. Because these two genes sit on different chromosomes, they are inherited independently, so the calculator multiplies the ABO probabilities by the Rh probabilities to give the full blood-type prediction such as A+, O-, or AB+.

To produce accurate results, the tool asks for each parent's phenotype (A, B, AB, or O and Rh+ or Rh-) plus the underlying genotype when it cannot be determined from phenotype alone. A parent with type A blood, for example, could be genetically AA (homozygous) or AO (heterozygous), and that hidden allele dramatically changes what a child can inherit. By combining a Punnett square for ABO with a Punnett square for Rh, the blood type inheritance calculator reports every possible offspring blood type along with its probability.

The ABO gene has three common alleles. The A and B alleles are codominant with each other, meaning a person who inherits one of each (genotype AB) expresses both A and B antigens and has type AB blood. The O allele is recessive to both A and B, so it is only expressed when a person inherits two copies (genotype OO), producing type O blood. This is why type O is sometimes called the "silent" or "universal donor" type for red cells.

Because O is recessive, a person with type A blood hides a second allele that can be either A or O. The table below shows how each phenotype maps to its possible genotypes, which is exactly the logic the calculator uses when you select a parent's genotype.

Notice that types AB and O have only one possible genotype, so the calculator does not ask you to choose a genotype for them. Types A and B each have two possible genotypes, so you must specify whether the parent is homozygous (AA or BB) or heterozygous (AO or BO). This choice changes which alleles the parent can pass on, and therefore the entire offspring prediction.

The Rh (Rhesus) factor is governed mainly by the RHD gene, modeled here with two alleles: D (Rh-positive, dominant) and d (Rh-negative, recessive). Anyone carrying at least one D allele is Rh-positive, so genotypes DD and Dd both produce Rh+ blood. Only the homozygous recessive genotype dd produces Rh-negative blood.

This dominance pattern means an Rh-positive parent, like a type A parent, can be hiding a recessive allele. The calculator therefore asks Rh-positive parents whether they are homozygous (DD) or heterozygous (Dd). An Rh-negative parent is always dd, so no genotype choice is needed. The Rh prediction is calculated with its own 2x2 Punnett square and then combined with the ABO result.

The Rh factor matters far beyond classroom genetics. During pregnancy, an Rh-negative mother carrying an Rh-positive baby can develop antibodies in a condition called hemolytic disease of the newborn. Modern medicine prevents most cases with Rh immunoglobulin (RhoGAM), but understanding Rh inheritance is why the blood type inheritance calculator treats Rh as a separate, equally important prediction alongside ABO.

The calculator builds a 2x2 Punnett square for each genetic system. It takes the two alleles from parent 1 and crosses them against the two alleles from parent 2, generating four equally likely offspring genotypes. Each genotype is then mapped to its phenotype, and matching phenotypes are summed. Because ABO and Rh are inherited independently, the probability of a full blood type is the product of the ABO phenotype probability and the Rh phenotype probability.

For each system the parental alleles are crossed exhaustively: every allele from parent 1 pairs with every allele from parent 2, yielding four combinations. The fraction of those four combinations that produce a given phenotype is its probability. The combined probability for a full type such as A+ is then the ABO count divided by 4, multiplied by the Rh count divided by 4.

Using the blood type calculator takes only a few clicks. Follow these steps to predict your child's possible blood types:

1. Select Parent 1's ABO type. Choose A, B, AB, or O for the first parent.
2. Select Parent 1's Rh factor. Choose Rh+ or Rh-.
3. Set the genotype if asked. If you picked A or B, choose homozygous (AA/BB) or heterozygous (AO/BO). If you picked Rh+, choose DD or Dd. Types AB, O, and Rh- have a fixed genotype and need no choice.
4. Repeat for Parent 2. Enter the second parent's ABO type, Rh factor, and genotypes the same way.
5. Read the results. The calculator instantly shows every possible offspring blood type with its percentage, along with separate breakdowns for ABO type and Rh factor probabilities.

If you do not know a parent's exact genotype, the heterozygous option is the most informative default because it shows the widest range of possible children. Remember that the percentages describe the probability for each child independently, not a guaranteed split across multiple children. Two type A heterozygous parents can, by chance, have several type O children even though type O has only a 25 percent chance per pregnancy.

The results panel groups information three ways. The Possible Blood Types grid lists each full type (for example A+, B+, AB+, O+, A-, and so on) with its combined probability. The ABO Type Probabilities section shows the chance of A, B, AB, or O ignoring Rh, and the Rh Factor Probabilities section shows the chance of Rh+ versus Rh-. The Parental Genotypes box confirms the exact allele pairs the calculator used.

A useful sanity check is that the ABO probabilities always sum to 100 percent and the Rh probabilities always sum to 100 percent. The full blood-type percentages also sum to 100 percent because they are simply the product of those two independent distributions. If two heterozygous parents (AO x BO and Dd x Dd) are entered, you will see four ABO types at 25 percent each combined with Rh+ at 75 percent and Rh- at 25 percent, giving full types at 18.75 percent (A+) and 6.25 percent (A-), and so on.

Keep in mind that this genetic calculator uses the standard textbook ABO and Rh model. Rare phenomena such as the Bombay phenotype, weak D variants, cis-AB alleles, and chimerism can produce results outside these predictions. The calculator is excellent for learning Punnett-square genetics and estimating likely outcomes, but it is not a substitute for clinical blood typing or paternity testing.