About

Blood type inheritance follows Mendelian genetics with the ABO system controlled by three alleles: I^A, I^B, and i. The alleles I^A and I^B are codominant over the recessive i allele. This means a person with genotype I^Ai expresses blood type A, while I^AI^B expresses type AB. The Rh factor operates independently as a simple dominant/recessive system where the D allele (Rh-positive) dominates over d (Rh-negative). Incorrect blood type predictions can have serious implications during pregnancy due to Rh incompatibility, where an Rh-negative mother carrying an Rh-positive fetus may develop antibodies causing hemolytic disease of the newborn (HDN).

This calculator performs genuine Punnett square analysis rather than simple lookup tables. It considers that phenotype A can arise from genotypes I^AI^A or I^Ai, and similarly for type B. The tool computes all possible parental genotype combinations weighted by population genetics, then crosses them to derive offspring probabilities. Note: without knowing parents' exact genotypes, the calculator assumes equal probability for heterozygous versus homozygous states. Actual probabilities may vary based on family history or ethnic background where allele frequencies differ.

Formulas

The ABO blood type system operates through codominant inheritance. Each parent contributes one allele to the offspring. The possible combinations follow standard Mendelian genetics with the following dominance relationships:

I^A = I^B > i

The Punnett square for ABO inheritance crosses maternal and paternal alleles. For parents with phenotypes that map to multiple possible genotypes, the calculator uses weighted probability:

P(offspring phenotype) = ∑i,j P(G_m = i) × P(G_p = j) × P(phenotype | i × j)

Where G_m represents maternal genotype and G_p represents paternal genotype. For Rh factor, the calculation is independent:

P(Rh+) = 1 − P(dd)

The combined blood type probability multiplies ABO and Rh probabilities since they segregate independently (located on different chromosomes: ABO on chromosome 9, Rh on chromosome 1):

P(A+) = P(type A) × P(Rh+)

Variable definitions: I^A = allele producing A antigen, I^B = allele producing B antigen, i = recessive allele producing no antigen, D = dominant Rh-positive allele, d = recessive Rh-negative allele.

Reference Data

Blood Type (Phenotype)	Possible Genotypes	Can Donate To	Can Receive From	Population Frequency (US)	Antigens Present	Antibodies in Plasma
A+	I^AI^A or I^Ai with Dd or DD	A+, AB+	A+, A−, O+, O−	35.7%	A, Rh	Anti-B
A−	I^AI^A or I^Ai with dd	A+, A−, AB+, AB−	A−, O−	6.3%	A	Anti-B
B+	I^BI^B or I^Bi with Dd or DD	B+, AB+	B+, B−, O+, O−	8.5%	B, Rh	Anti-A
B−	I^BI^B or I^Bi with dd	B+, B−, AB+, AB−	B−, O−	1.5%	B	Anti-A
AB+	I^AI^B with Dd or DD	AB+	All types	3.4%	A, B, Rh	None
AB−	I^AI^B with dd	AB+, AB−	A−, B−, AB−, O−	0.6%	A, B	None
O+	ii with Dd or DD	O+, A+, B+, AB+	O+, O−	37.4%	Rh	Anti-A, Anti-B
O−	ii with dd	All types	O−	6.6%	None	Anti-A, Anti-B
Allele Frequencies by Population
Caucasian	I^A: 0.28		I^B: 0.06		i: 0.66
African American	I^A: 0.19		I^B: 0.13		i: 0.68
Asian	I^A: 0.27		I^B: 0.17		i: 0.56
Hispanic	I^A: 0.22		I^B: 0.08		i: 0.70
Rh Factor Distribution
Rh-positive (D allele)	85% of population (Caucasian), 95% (African), 99% (Asian)
Rh-negative (dd)	15% of population (Caucasian), 5% (African), 1% (Asian)
Clinical Significance
HDN Risk	Rh− mother + Rh+ father = potential sensitization requiring RhoGAM prophylaxis at 28 weeks and postpartum
ABO Incompatibility	Mother type O with fetus type A or B: mild jaundice possible, rarely severe. Anti-A/B antibodies are IgM (don't cross placenta easily)
Universal Donor	O− for RBCs, AB+ for plasma (reversed logic for plasma products)
Bombay Phenotype	Rare hh genotype: cannot express A/B antigens regardless of I^A/I^B alleles. Appears as type O but incompatible with true O blood.

Frequently Asked Questions

No. Type O results exclusively from the genotype ii, meaning both parents can only pass the i allele. All offspring will be ii (type O). If a child tests as A or B with two O parents, possible explanations include lab error, undisclosed non-paternity, or the extremely rare cis-AB allele or Bombay phenotype variants.

Blood type phenotype does not uniquely identify genotype. A person with type A blood could be I^AI^A (homozygous) or I^Ai (heterozygous). Without genetic testing or family history, the calculator assumes equal probability for both genotypes. Population-specific allele frequencies would refine this estimate. For example, in populations with high i allele frequency, heterozygous I^Ai is more likely than homozygous I^AI^A.

Rh incompatibility can cause hemolytic disease of the newborn (HDN). During delivery or trauma, fetal Rh+ red blood cells may enter maternal circulation. The mother's immune system produces anti-D antibodies. In subsequent pregnancies with Rh+ fetuses, these antibodies cross the placenta and attack fetal red blood cells causing anemia, jaundice, or hydrops fetalis. Prevention requires RhoGAM (anti-D immunoglobulin) injection at 28 weeks gestation and within 72 hours postpartum.

Blood type can exclude paternity but cannot confirm it. If a child has blood type AB, neither parent can be type O. If both alleged parents are type O, the biological father must be someone else. However, matching blood types prove nothing since millions of people share the same type. DNA testing provides 99.99% accuracy for paternity determination versus blood typing's limited exclusionary power.

Rh-positive individuals can be genotype DD (homozygous) or Dd (heterozygous). If both parents are Dd, there is a 25% chance (1 in 4) the child inherits dd and is Rh-negative. The Punnett square: Dd × Dd = 1 DD : 2 Dd : 1 dd.

The Bombay phenotype (Oh) occurs in individuals homozygous for the h allele at the H locus (genotype hh). The H antigen is the precursor molecule onto which A and B antigens are added. Without H antigen, A and B cannot be expressed regardless of ABO genotype. These individuals type as O but carry anti-H antibodies, making them incompatible with all standard blood types including O. They can only receive blood from other Bombay phenotype donors. Prevalence: ~1 in 10,000 in India, rarer elsewhere.