Blood Type Inheritance: How ABO and Rh Factor Are Passed to Your Children

The ABO Blood Group System

The ABO blood group system, discovered by Karl Landsteiner in 1900, is determined by the ABO gene on chromosome 9q34. This gene has three functional alleles: I^A (produces the A antigen), I^B (produces the B antigen), and i (produces no antigen). Because you inherit one allele from each parent, your blood type depends on which two alleles you carry. I^A and I^B are codominant — both are fully expressed in AB individuals. The i allele is recessive, so blood type O only appears when a person inherits two i alleles (genotype OO or ii). Yamamoto and colleagues at Nature in 1990 sequenced the gene and showed that the A and B alleles differ by only seven nucleotides, four of which change the amino acid sequence of the glycosyltransferase enzyme that builds the antigens, while the O allele carries a single-base deletion that produces a non-functional truncated enzyme (PMID 2333095).

The molecular biology in one paragraph

The ABO gene encodes a glycosyltransferase that adds a specific sugar to the H antigen sitting on the surface of red blood cells. The A-transferase adds N-acetylgalactosamine; the B-transferase adds galactose. The O allele's truncated protein adds nothing, so red cells display only the unmodified H antigen. This is why blood type is fundamentally a question of which sugar coats your erythrocytes — and why antibodies against the 'wrong' sugar cause transfusion reactions within seconds.

How the Punnett Square Works

The Punnett square is a 2×2 grid that systematically crosses each parent's two alleles to predict offspring genotypes. Place parent 1's alleles across the top, parent 2's down the side, and fill each cell with the combination. Each cell represents a 25% probability. For example, an AO parent crossed with a BO parent produces AB (25%), AO (25%, phenotype A), BO (25%, phenotype B), and OO (25%, phenotype O) — children can be any of the four ABO types with equal probability.

Worked example: why two type-A parents can have a type-O child

If both parents are blood type A but each carries the AO genotype (heterozygous), the cross AO × AO produces AA (25%), AO (25%), OA (25%), and OO (25%). Three quarters of children will be phenotypically type A, but one in four will inherit the i allele from both parents and express type O. This is the single most common 'surprise' result in family genetics, and it does not imply non-paternity — it simply reflects hidden heterozygosity in the parents.

Worked example: A × B parents

If parent 1 is AA (homozygous type A) and parent 2 is BB (homozygous type B), every child will be AB. If both parents are heterozygous (AO × BO), children can be AB, A, B, or O — covering all four ABO types from two parents with only two visible blood types between them. This counter-intuitive result is one of the most cited examples in introductory genetics teaching.

The Rh Factor System

The Rh blood group system is controlled by the RHD gene on chromosome 1p36. The clinically critical antigen is D (RhD). Carrying at least one functional D allele makes you Rh positive (Rh+); two copies of the non-functional d allele make you Rh negative (Rh−). Rh inheritance follows classical dominant–recessive logic: D is dominant over d. About 85% of people of European ancestry are Rh positive; rates are higher (around 95%) in African and Asian populations. Because ABO and Rh sit on different chromosomes, they assort independently — your ABO type tells you nothing about your Rh status.

Rh Incompatibility in Pregnancy

Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus. During delivery, miscarriage, or trauma, fetal red blood cells can enter the maternal circulation and trigger anti-D antibody production. The first sensitized pregnancy is usually unaffected, but subsequent Rh-positive pregnancies can develop hemolytic disease of the newborn (HDN) as maternal IgG antibodies cross the placenta and destroy fetal red cells. Prevention with anti-D immunoglobulin (RhoGAM, 300 µg IM) at 28 weeks of gestation and within 72 hours postpartum is the standard of care and has reduced HDN by more than 90% since the 1970s.

Rare Exceptions That Break the Simple Rules

The Bombay phenotype (hh)

First described in Bombay (Mumbai) in 1952, the Bombay phenotype results from homozygous loss-of-function mutations in the FUT1 gene, which builds the H antigen that A and B sugars attach to. Without H, neither A nor B can be displayed — these individuals appear to be type O on standard testing but actually carry anti-H antibodies and will reject even type-O donor blood. Bombay individuals can only receive blood from other Bombay donors. Scharberg, Olsen and Bugert reviewed the H system comprehensively in Immunohematology in 2016 (PMID 27834485). The phenotype is extremely rare (roughly 1 in 10,000 in parts of India, far rarer elsewhere) but matters intensely for transfusion safety.

Cis-AB and weak D variants

Cis-AB is an exceptionally rare variant in which a single ABO allele encodes both A and B activity, so an AB-phenotype parent can pass both antigens to a child as a single inherited unit — producing apparent AB×O = AB children that classical rules forbid. Weak D variants (previously called Du) and partial D phenotypes are partial or quantitatively reduced RhD expressions caused by point mutations in RHD; they can be misclassified as Rh-negative by older typing methods and matter for safe transfusion and pregnancy management. Storry and Olsson reviewed the modern ABO landscape in Immunohematology in 2009 (PMID 19927620), and Daniels reviewed the molecular genetics of all blood group polymorphisms in Human Genetics the same year (PMID 19727826).

ABO Inheritance and Paternity Testing

Before DNA testing existed, ABO and Rh typing were used in paternity disputes. They have one fundamental limitation that is widely misunderstood: ABO/Rh can only EXCLUDE paternity in clear-cut cases — it can never CONFIRM it. For example, if a type-O mother and a type-A child have a putative father who is type AB, that man is excluded because an AB father can pass only A or B, never the i allele required for the mother's contribution. But if he is not excluded, it simply means he could be the father, along with millions of other men with compatible genotypes.

Why DNA testing replaced blood-group paternity tests

Modern paternity testing uses short tandem repeat (STR) DNA markers — typically 16 to 24 highly variable loci. Each locus has many possible alleles, and a correctly matched panel gives a probability of paternity above 99.99% (or definitive exclusion). Compared with ABO/Rh, which divides the population into only 8 broad groups (4 ABO × 2 Rh), STR testing effectively gives every individual a near-unique genetic fingerprint. Since the 1990s, courts in essentially every jurisdiction have accepted STR DNA testing as the gold standard and treat ABO/Rh as informational only.

Bottom line

For more than 99% of families, ABO and Rh blood types follow the simple Mendelian rules captured by the Punnett square: each parent passes one of their two alleles at random, codominance governs ABO, and dominance governs Rh. Use this calculator to estimate the probability of each blood type your children may have — and to understand why blood type alone is a screening tool, never a diagnostic one. For anything that matters clinically or legally, modern DNA-based testing is the correct standard.