For humans, blood type is determined by three possible alleles which are A,B, and O. The child's blood type is dependent on which two alleles are inherited by the offspring from the parents' gametes. The Rh factor is another inherited trait associated with blood type and there are two alleles for this which are Rh positive (+) and Rh negative (-).
The A and B allele are codominant and the O allele is recessive to both A and B. Also, Rh positive allele is dominant to Rh negative. If the two parents are both type O positive, they are homozygous for the O allele which is recessive. In terms of the Rh factor, their genotype can be two Rh + genes, or one Rh + and one Rh - gene. Either of these combinations produces an Rh + individual.
Their gametes will contain an O and an Rh+ or an Rh - gene and once fertilization occurs, the offspring can only be homozygous for blood type O and may be Rh positive and homozygous, Rh positive and heterozygous, or even Rh- if each parent was a heterozygous carrier.
It is not possible for two parents of the genotype described in the question to have a type A+ offspring.
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