What blood types can children have when the father is blood type AB and the mother is blood type O?

Blood Type Inheritance: AB Father × O Mother

Children from an AB father and O mother can only have blood type A or B—never AB or O.

Genetic Basis of ABO Inheritance

The ABO blood group system follows Mendelian inheritance patterns where:

• The AB father has genotype AB (one A allele, one B allele) and can only pass either an A or B allele to offspring 1, 2
• The O mother has genotype OO (two O alleles) and can only pass an O allele to offspring 1, 2
• Each child receives one allele from each parent, resulting in either AO (blood type A) or BO (blood type B) genotypes 1, 2

Expected Offspring Blood Types

The only possible blood types for children are:

• Blood type A (genotype AO) - 50% probability
• Blood type B (genotype BO) - 50% probability

Impossible blood types for children:

• Blood type AB - cannot occur because the mother cannot provide an A or B allele 1, 2
• Blood type O - cannot occur because the father cannot provide an O allele 1, 2

Critical Exception: Cis-AB Phenomenon

There exists an extremely rare exception where an O child could theoretically be born from an AB × O mating:

• The cis-AB phenotype involves a single chromosome carrying both A and B genes, which can produce an O child through rare structural mutation or deletion during oogenesis 1, 3
• This represents a genetic anomaly, not standard inheritance, and occurs in fewer than 0.001% of cases 1, 3
• The cis-AB enzyme creates both A and B antigens with a single enzyme through amino acid interchange between normal A and B enzyme sequences 3, 2

In practical clinical terms, if paternity is questioned because an O or AB child was born from an apparent AB × O couple, consider:

• Cis-AB inheritance in the AB parent (extremely rare) 1, 3
• Non-paternity (far more common explanation) 1
• Laboratory error in blood typing 3

Clinical Counseling Approach

When counseling couples about blood type inheritance:

• Provide clear, nondirective information about expected blood types based on parental genotypes 4
• Explain that standard AB × O matings produce only A or B children with 50% probability each 1, 2
• If unexpected blood types occur, genetic testing can definitively establish parentage with probability >99.99% 1
• Document blood type information in the medical record for future reference 5