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It is not possible for a person with O- and O+ to have a child with A-.
Remember that each parent gives half of their alleles to their offspring.
A parent with a blood type of O has OO for their alleles. In order for a child to inherit the A one of the parents needs to have an A, and if you have at least one A then your blood type will be A.
Also, the negative is the recessive trait meaning that the positive parent needs to be O-O+ in order to have a chance at passing on the negative trait.
In this scenario, the parents would need to be A+ or A- and O- or O+ in order to have a child with a blood type of A-.
The short answer is , no.
Human blood types are referred to as being "ABO" - based upon their containing the "A'" protein (or "antigen"), the "B" protein, both proteins ("AB") or neither A or B proteins (called"O" ) and the additional "+" or "-" are describing a second factor called the "Rh" factor. The + means that the Rh factor is present, while the - means it is absent.
Blood type is determined by the presence of two alleles, one donated from each parent, which together form the blood type of the offspring.
Type "AB" blood would have alleles of one "A" and one "B"
Type "A" blood could be one "A" allele and another "A" allele or it could be one "A" allele and one "O" (since "O" is recessive, the "A" would be expressed)
Type "B" blood, similarly, could be "BB" or could be "BO"
Type "O" blood could ONLY be two "O" alleles.
Therefore type "O" parents (both "OO", could only make an "OO" child)
The Rh factor could occur as in your question, since the negative factor is recessive and a heterozygous positive (one "+" along with one "-" ) and a recessive (two "-" alleles) could have a combination in which a "-" from each parent unites to form the ofspring's Rh factor.
Back to your question. Parents with AO and OO..........AA and OO...........AO and AO...........AA and AO..........AA and AA.........AA and AB...........AO and AB.........and ..........AB and AB..........can ALL have a type "A" offspring...........but no two parents of type "O" can have an "A" offspring.
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