I hope that the following background information helps to answer your question. Blood type is an example of a trait determined by a single gene. Each of us has two copies of the gene for blood type. We inherit one copy of the gene from our mother, and the other copy from our father. There are three different forms (called alleles) of the gene for blood type called A, B, and O. Blood type is determined by the combination of alleles inherited from our parents.
A person’s blood type is called their phenotype, and the different genetic combination of the forms of the gene is called a genotype. An example of a genotype would be OB. In this example, the person has two forms of the gene, O and B. This person’s blood type or phenotype would be type B blood. As the previous poster mentioned, O is a recessive form of the gene. In order for an individual to have type O blood, they would have inherited one “O” allele from the mother and one from the father and have the genotype OO. A and B forms of the gene are codominant and can both be expressed together.
Type O blood is recessive so, to my limited understanding, two type O parents each have O-O genotype and will almost always have a type O child, barring a mutation of course.
So, assuming no initial error in blood typing was made with any of the parties, with a mutation it is possible, but the mutation is not likely.
There are genetic tests available for maternity/paternity that are more in depth than blood typing.
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