What Blood Type Will a Child Have When One Parent Is AB and the Other Is O?
Understanding how blood types are inherited is essential when exploring the possible blood types of children based on their parents' genetics. Human blood types are determined by specific antigens—proteins or sugars—present on the surface of red blood cells. These antigens are controlled by genes passed down from both parents, following well-established genetic principles.
Blood Type Basics: What Do AB and O Mean?
The ABO blood group system classifies blood into four main types: A, B, AB, and O. These types depend on which antigens are present on red blood cells. Individuals with type AB blood have both A and B antigens on their red blood cells, making them universal plasma donors but recipients of only AB-type blood. On the other hand, people with type O blood lack both A and B antigens entirely—their red blood cells are often described as "blank" or antigen-free.
This distinction is crucial when predicting the blood type of offspring. Each person inherits one allele (gene variant) for blood type from each parent, resulting in a combination that determines their phenotype—the observable blood type.
Genetic Inheritance: How Blood Types Are Passed Down
Blood type inheritance follows Mendelian genetics. The A and B alleles are co-dominant, meaning if both are present, they are both expressed (resulting in AB blood). The O allele is recessive, so it only appears when no A or B allele is present.
In this scenario, one parent has AB blood, meaning their genotype is AB—they carry one A allele and one B allele. The other parent has O blood, which means their genotype is OO—they carry two recessive O alleles.
Possible Combinations for the Child
During reproduction, each parent contributes one allele to their child. The AB parent can pass on either the A or the B allele—but not both, and certainly not an O (since they don't carry it). The O parent, however, can only pass on an O allele, as that's the only type they possess.
Therefore, the child will inherit either:
- A from the AB parent + O from the O parent → Blood type A (genotype AO)
- B from the AB parent + O from the O parent → Blood type B (genotype BO)
Why Can't the Child Be AB or O?
AB blood type requires inheriting an A allele from one parent and a B allele from the other. Since the O parent has no A or B allele to give, this combination is impossible.
Likewise, for a child to have type O blood, they must inherit an O allele from both parents. While the O parent can contribute an O allele, the AB parent does not carry an O allele at all—so they cannot pass it on. Thus, an OO genotype cannot occur.
Final Outcome: Only Two Possibilities
As a result, children born to one AB parent and one O parent will always have either type A or type B blood—each with a roughly 50% probability. There are no exceptions under standard genetic inheritance patterns.
This predictable outcome is useful not only in biology classrooms but also in real-world applications such as paternity considerations, medical planning, and understanding family health history. While blood type alone cannot confirm parentage, unexpected results may prompt further genetic testing.
In summary, genetics plays a powerful role in determining who we are—including something as fundamental as our blood type. When AB meets O, the result is never AB or O in the child, but rather a balanced chance between A and B—nature's way of mixing traits while following strict biological rules.