What is the difference between ABO and Rh factor?

ABO blood group is determined by the presence or absence of A and B antigens on red blood cells. The Rh factor (Rhesus factor) is a separate protein antigen — if you have it, you're Rh+; if not, you're Rh-. These are inherited independently following Mendelian genetics.

Blood Type Calculator | Child Blood Type Predictor & Compatibility Checker

Select Parent Blood Types

Choose the ABO type and Rh factor for each parent to see possible child blood types.

Parent 1

Parent 2

Possible Child Blood Types

ABO Punnett Square (allele combinations)

How to Use This Blood Type Calculator

Three simple steps to understand blood type inheritance.

Select Parent Blood Types

Choose ABO type (A, B, AB, O) and Rh factor (+ or -) for both parents. The calculator considers all possible genotypes — for example, a type A parent could be AA or AO.

View the Punnett Square

The Punnett square shows how alleles from each parent combine. Each cell represents a possible genotype, and the probability bars show the overall likelihood of each blood type.

Check Compatibility

Switch to the Compatibility Matrix tab to see which blood types can donate to or receive from each other. O- is the universal donor, AB+ is the universal recipient.

Frequently Asked Questions

Common questions about blood type inheritance.

How is blood type inherited from parents?

Blood type is determined by the ABO gene, which has three alleles: A, B, and O. A and B are co-dominant, while O is recessive. Each parent contributes one allele to the child. For example, a parent with blood type A could have genotype AA or AO, and a parent with type B could be BB or BO. The combination of alleles determines the child's blood type. The Rh factor is inherited separately — Rh+ (D antigen present) is dominant over Rh- (D antigen absent). This means two Rh+ parents can have an Rh- child if both carry the recessive allele (heterozygous Dd).

Can two O-positive parents have an A-negative child?

No. Two O-type parents both have the OO genotype — they can only pass O alleles to their children. Therefore, their child must be type O. For the Rh factor, two Rh+ parents can indeed have an Rh- child — if both are heterozygous (Dd genotype), there is a 25% chance the child inherits dd and is Rh-negative. So O-positive parents can have an O-negative child, but never an A-type child.

What is the universal blood donor type?

O-negative (O-) is the universal donor — it can be given to patients of any blood type in emergencies. O-type red blood cells lack both A and B antigens, and Rh-negative blood lacks the Rh antigen, meaning the recipient's immune system will not attack the transfused cells. AB-positive (AB+) is the universal recipient — people with AB+ blood can receive red blood cells from any type because they already have A, B, and Rh antigens on their own cells. In plasma donation, the compatibility is reversed.

What is the difference between ABO blood group and Rh factor?

The ABO blood group system (discovered by Karl Landsteiner in 1901) classifies blood based on the presence or absence of A and B antigens on the surface of red blood cells. The Rh factor (discovered in 1937, named after the Rhesus monkey) is a separate system involving the D antigen. If you have the D antigen, you are Rh-positive; without it, you are Rh-negative. These two systems are inherited independently following Mendel's laws. Together they create the 8 common blood types: A+, A-, B+, B-, AB+, AB-, O+, O-.

How accurate is this blood type calculator?

This calculator uses standard Mendelian genetics to predict probabilities for ABO and Rh inheritance. The probabilities are mathematically accurate based on possible parental genotypes. However, the calculator cannot determine a child's exact blood type definitively because parents may carry hidden recessive alleles. For the most precise blood type determination, a laboratory blood test (serological typing) is always required. This tool is for educational and informational purposes — to help understand the genetic principles behind blood type inheritance.

Understanding Blood Type Genetics

Blood type is one of the most well-understood examples of Mendelian inheritance in humans. The ABO blood group system was discovered in 1901 by Austrian immunologist Karl Landsteiner, who later received the 1930 Nobel Prize in Physiology or Medicine for this work. The ABO gene, located on chromosome 9, codes for glycosyltransferase enzymes that add sugar molecules to the H antigen on red blood cells, creating either the A antigen (N-acetylgalactosamine), B antigen (D-galactose), or neither (O type, where the H antigen remains unmodified due to a frameshift mutation in the O allele).

The gene has three major alleles: A (produces A-transferase), B (produces B-transferase), and O (non-functional enzyme). A and B are co-dominant — when both are present (AB genotype), both antigens are expressed on the cell surface. O is recessive to both A and B. Each person inherits one allele from each parent, giving six possible ABO genotypes: AA, AO, BB, BO, AB, and OO.

Rh Factor Inheritance

The Rh (Rhesus) blood group system is the second most clinically important blood group after ABO. It is determined primarily by the D antigen, encoded by the RHD gene on chromosome 1. The presence of the D antigen makes a person Rh-positive (Rh+); its absence makes them Rh-negative (Rh-). The Rh+ allele is dominant — if you inherit at least one Rh+ allele (DD or Dd), you will be Rh-positive. Only those inheriting two Rh- alleles (dd) are Rh-negative.

Approximately 85% of Caucasians, 95% of Black Africans, and 99% of East Asians are Rh-positive. The Rh system is critical in pregnancy: if an Rh-negative mother carries an Rh-positive fetus, she may develop anti-D antibodies that can cause hemolytic disease of the newborn (HDN) in subsequent pregnancies. This is now routinely prevented with Rh immunoglobulin (RhoGAM) injections during pregnancy.

Blood Type Distribution Worldwide

Blood Type	Global Average	Caucasian (US)	African American	East Asian	Can Donate To	Can Receive From
O+	37%	37%	47%	29%	O+, A+, B+, AB+	O+, O-
O-	7%	8%	4%	<1%	All types (Universal)	O-
A+	34%	33%	24%	27%	A+, AB+	A+, A-, O+, O-
A-	6%	7%	2%	<1%	A+, A-, AB+, AB-	A-, O-
B+	10%	9%	18%	25%	B+, AB+	B+, B-, O+, O-
B-	2%	2%	1%	<1%	B+, B-, AB+, AB-	B-, O-
AB+	4%	3%	4%	10%	AB+ only	All types (Universal)
AB-	0.5%	1%	0.3%	<0.1%	AB+, AB-	AB-, A-, B-, O-

Source: Adapted from WHO and national blood service data. Distributions vary by ethnic group and geographic region.

Why Blood Type Matters

Blood Transfusions: Receiving incompatible blood can cause a life-threatening hemolytic transfusion reaction. ABO compatibility is the most critical factor — even a small amount of incompatible blood can cause acute kidney failure and shock.
Pregnancy: Rh incompatibility between an Rh- mother and Rh+ baby can cause hemolytic disease of the newborn. Modern RhoGAM prophylaxis has reduced this risk by over 99%.
Organ Transplantation: ABO compatibility is essential for solid organ transplants. An ABO-incompatible kidney transplant, for example, would cause hyperacute rejection unless the recipient undergoes desensitization therapy.
Disease Susceptibility: Studies suggest blood type may influence risk for certain conditions — type O individuals have a slightly lower risk of venous thromboembolism and pancreatic cancer, while type A individuals may have a slightly higher risk of stomach cancer (linked to H. pylori susceptibility) and severe COVID-19 outcomes.
Forensic Science: Blood type was one of the earliest forensic identification tools and remains useful as a rapid exclusion method at crime scenes.

Select Parent Blood Types

Possible Child Blood Types

Blood Type Compatibility Matrix

How to Use This Blood Type Calculator

Frequently Asked Questions

Understanding Blood Type Genetics

Rh Factor Inheritance

Blood Type Distribution Worldwide

Why Blood Type Matters