ABO Blood Group Inheritance: Father B × Mother A
The children can have blood types A, B, AB, or O, depending on whether each parent is homozygous or heterozygous for their ABO alleles.
Genetic Inheritance Patterns
The possible blood types of offspring depend on the specific genotypes of both parents:
Most Common Scenario: Both Parents Heterozygous
- Father genotype: BO (blood type B)
- Mother genotype: AO (blood type A)
- Possible offspring blood types:
- Type A (25% probability - genotype AO)
- Type B (25% probability - genotype BO)
- Type AB (25% probability - genotype AB)
- Type O (25% probability - genotype OO)
This represents the classic Mendelian inheritance pattern where each parent contributes one allele 1.
Alternative Scenarios Based on Parental Genotypes
If father is homozygous BB:
- Offspring can only be type AB (genotype AB) or type B (genotype BO)
- Type A and O are impossible 1
If mother is homozygous AA:
- Offspring can only be type AB (genotype AB) or type A (genotype AO)
- Type B and O are impossible 1
If both parents are homozygous (BB × AA):
- All offspring will be type AB (genotype AB) 1
Clinical Relevance for Newborn Management
Blood Type Testing Recommendations
- For newborns, cord blood testing for ABO and Rh(D) blood types helps identify potential hemolytic disease of the newborn (HDN), which can cause anemia, hyperbilirubinemia, and in severe cases, kernicterus or death 2
- When maternal blood is group O, testing is particularly important due to higher risk of ABO incompatibility 3
- In this A × B pairing, the risk of clinically significant ABO hemolytic disease is substantially lower than with type O mothers 3
Hemolytic Disease Risk
- Type A or B mothers are 5.5 times less likely to have erythrocyte sensitization compared to type O mothers 3
- A-B and B-A mother-infant pairs with positive antiglobulin tests have similar rates of significant hyperbilirubinemia compared to those with negative tests 3
- Sensitization is much rarer when the mother has blood type A or B than when she has blood type O 3
Rare Genetic Exceptions
Cis-AB Phenomenon
- Extremely rare cases exist where a single allele produces both A and B antigens, which can result in apparently paradoxical inheritance patterns (such as an O child from an AB parent) 4
- This occurs due to mutations creating hybrid enzymes capable of producing both antigens 4
De Novo Mutations
- Rare recombination events during meiosis can create hybrid alleles, potentially resulting in unexpected blood types 5
- One documented case showed a child with type A from a type B mother and type O father due to recombination between maternal B and O alleles 6, 5
Practical Clinical Application
For routine clinical purposes, assume standard Mendelian inheritance with the four possible blood types (A, B, AB, O) when both parents are heterozygous, which is the most common scenario 1. The exact probabilities depend on whether each parent carries one or two copies of their expressed ABO allele, but without additional family history or genetic testing, all four blood types should be considered possible outcomes.