Blood Group Inheritance: Can Two A-Negative Parents Have an O-Negative Child?
Yes, two A-negative parents can have an O-negative child if both parents are heterozygous for the ABO blood group (genotype AO), each passing their O allele to the child.
Basic Genetics of ABO Blood Groups
The ABO blood group system follows Mendelian inheritance patterns where:
- Type A individuals can have genotype AA or AO (where A is dominant over O)
- Type O individuals must have genotype OO (homozygous recessive)
- Each parent contributes one allele to their offspring
When both parents have blood type A with genotype AO (heterozygous), the possible offspring combinations are:
- 25% chance of AA (Type A)
- 50% chance of AO (Type A)
- 25% chance of OO (Type O)
The Rh Factor (Negative Status)
The Rh-negative status is also inherited in a recessive pattern:
- Both A-negative parents must be homozygous recessive (dd) for the Rh factor
- All their children will therefore be Rh-negative (dd), as they can only receive d alleles from both parents
Clinical Confirmation
While extremely rare, documented cases exist of unusual ABO inheritance patterns. One case report described an O child from AB × O parents due to a structural mutation or deletion during oogenesis 1. However, the scenario you describe (O child from A × A parents) is straightforward Mendelian inheritance and requires no unusual genetic mechanisms—only that both parents are AO heterozygotes.
Practical Implications
If paternity or maternity is questioned based on blood typing alone, this is NOT grounds for doubt when two A-negative parents have an O-negative child, as this is a completely expected outcome occurring in approximately 25% of offspring when both parents are AO genotype.
Genetic marker testing or DNA paternity testing would be required to definitively establish parentage if there are concerns, as blood typing alone cannot exclude biological parentage in this scenario 1.