How can a baby have O+ (O positive) blood group if the father has B+ (B positive) blood group and the mother has A+ (A positive) blood group?

How a Baby Can Have O+ Blood Group from B+ Father and A+ Mother

This scenario is genetically possible when both parents are heterozygous carriers of the O allele (father is BO and mother is AO), allowing the baby to inherit an O allele from each parent, resulting in OO genotype and O+ blood type.

Genetic Explanation

ABO Blood Group Inheritance Basics

• The ABO blood group system is controlled by allelic genes at a single locus, with A and B being codominant and O being recessive 1
• Each parent contributes one ABO allele to their child 1
• A person with blood type B can have genotype BB (homozygous) or BO (heterozygous) 1
• A person with blood type A can have genotype AA (homozygous) or AO (heterozygous) 1

The Specific Scenario

• Father with B+ blood group: If his genotype is BO (heterozygous), he can pass either the B allele or the O allele to his child 1
• Mother with A+ blood group: If her genotype is AO (heterozygous), she can pass either the A allele or the O allele to her child 1
• Baby with O+ blood group: This occurs when the baby inherits the O allele from the father AND the O allele from the mother, resulting in OO genotype 1

Probability and Genetic Possibilities

When both parents are heterozygous (father BO, mother AO), the possible offspring blood types are:

• 25% chance of type A (AO genotype)
• 25% chance of type B (BO genotype)
• 25% chance of type AB (AB genotype)
• 25% chance of type O (OO genotype) 1

Rh Factor Inheritance

• The Rh positive (+) trait is dominant over Rh negative (-) 2
• Since both parents are Rh positive, the baby can easily inherit Rh positive status from either or both parents 2
• Even if both parents were heterozygous for Rh (carrying one negative allele), there would still be a 75% chance of an Rh positive baby 2

Rare Alternative Explanations

De Novo Genetic Recombination

• Extremely rare cases of de novo recombination in the ABO gene during meiosis have been documented, where hybrid alleles can form 3
• One documented case showed a child with A blood type from a B mother and O father due to recombination between B and O alleles creating a hybrid allele with A specificity 3
• However, this mechanism is extraordinarily rare and should not be the first consideration 3

Structural Mutations

• Very rare structural mutations or deletions in the A or B gene can result in O phenotype expression despite having A or B genotype 4
• One case documented an O phenotype child from AB × O parents due to a structural mutation in the maternal A or B gene 4
• These cases are exceedingly uncommon and require specialized genetic testing to confirm 4

Clinical Implications

ABO Incompatibility Monitoring

• When a mother has blood type O and the baby has a different blood type (A, B, or AB), there is risk for ABO hemolytic disease of the newborn 5, 6
• However, in this scenario where the baby is O+ and mother is A+, there is no risk of ABO incompatibility since the baby's blood type O does not express A or B antigens that could be targeted by maternal antibodies 5
• The American Academy of Pediatrics recommends that when maternal blood is group O, testing cord blood for infant blood type and direct Coombs test is optional but surveillance should be ensured 5

Paternity Considerations

• This blood group combination (B+ father, A+ mother, O+ baby) is completely consistent with biological parentage and does not suggest non-paternity 1
• ABO blood typing alone should never be used to exclude paternity, as demonstrated by documented cases where apparent exclusions were later proven to be due to rare genetic mechanisms 3
• Modern DNA fingerprinting and microsatellite markers are far more reliable for paternity testing than blood group analysis 3

Common Pitfalls to Avoid

• Do not assume non-paternity based solely on this blood group combination, as it is genetically plausible through standard Mendelian inheritance 1
• Do not confuse ABO incompatibility risk: In this case, there is no maternal-fetal ABO incompatibility since the mother is A+ and baby is O+ 5
• Remember that blood type alone reveals genotype incompletely: A person with type A or B blood could be either homozygous or heterozygous for their allele 1