How Is Blood Type Inherited?
Blood type is determined by two genetic systems: the ABO system and the Rh factor. Each person inherits two alleles — one from each parent — which combine to produce their blood type.
In the ABO system, there are three alleles: A, B, and O. The A and B alleles are codominant (both express when present together), while O is recessive. This means:
- Blood type A: genotype AA or AO - Blood type B: genotype BB or BO - Blood type AB: genotype AB (one A and one B allele) - Blood type O: genotype OO (two recessive O alleles)
The Rh system is simpler: Rh-positive (+) is dominant over Rh-negative (-). A person is Rh-positive if they have at least one + allele.
What Are the Possible Blood Types?
There are 8 common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−. The distribution varies by ethnicity, but globally the most common blood type is O+, followed by A+.
AB-negative is the rarest of the main 8 blood types, occurring in less than 1% of the global population. O-negative is the universal donor for red blood cells, as it lacks A, B, and Rh antigens.
How Does the Punnett Square Work?
A Punnett square maps out all possible allele combinations from two parents. For blood type:
1. Determine the possible alleles each parent can pass down based on their blood type 2. Combine each possible allele from Parent 1 with each from Parent 2 3. Translate each allele combination into a blood type phenotype
Because we typically do not know a parent's exact genotype (for example, whether a type A parent is AA or AO), this calculator uses population-level probability assumptions to estimate likely outcomes.
Why Can't Certain Blood Types Appear?
Some parent combinations make certain child blood types impossible. For example, two O-type parents (OO x OO) can only produce O-type children, since there are no A or B alleles to pass down. Similarly, two Rh-negative parents cannot have an Rh-positive child.
This is why blood type is sometimes cited in paternity questions — though DNA testing is far more definitive and reliable for confirming biological relationships.
Rh Factor and Pregnancy
Rh incompatibility can be clinically significant during pregnancy. If a Rh-negative mother carries an Rh-positive fetus, her immune system may produce antibodies against fetal red blood cells — a condition called Rh sensitization. This typically does not affect the first pregnancy, but can cause hemolytic disease of the fetus and newborn (HDFN) in subsequent pregnancies.
To prevent this, Rh-negative pregnant women are routinely given Rh immunoglobulin (brand name RhoGAM), which prevents sensitization. If you are Rh-negative, always inform your healthcare provider when pregnant.
How to Use This Tool
1. Select Parent 1's blood type from the dropdown (for example, A+, O-, AB+) 2. Select Parent 2's blood type from the second dropdown 3. Click "Calculate Child Blood Types" 4. Review the table of possible child blood types with their estimated probabilities 5. Note any Rh incompatibility warnings if applicable
FAQs
Q: Can two A-type parents have an O-type child? A: Yes. If both parents are genotype AO (they both carry the recessive O allele), there is a 25% chance each child will be type O. Two A-type parents cannot have a B or AB child.
Q: Can blood type identify the father? A: Blood type can rule out certain people as the biological father. If the child's blood type is impossible given the alleged father's type, he can be excluded. However, blood type cannot positively confirm paternity. DNA testing is required for that.
Q: Do identical twins always have the same blood type? A: Yes. Identical twins share the same DNA and therefore always have the same blood type.
Q: What is the rarest blood type? A: AB-negative is the rarest major blood type, occurring in less than 1% of the global population. There are also very rare blood types outside the standard ABO/Rh system (like the Bombay phenotype) that occur in extremely small populations.
Q: Why is O-negative called the universal donor? A: O-negative red blood cells lack A, B, and Rh antigens, meaning they will not trigger an immune response in recipients of any blood type. This makes O- blood critical for emergency transfusions when the recipient's type is unknown.
Q: Are these predictions accurate? A: The predictions are based on simplified population genetics models using assumed allele frequency distributions. Actual blood type inheritance depends on exact parental genotypes, which require genetic testing to determine precisely. This tool is for educational purposes only.
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