My mom has O+ blood, my dad is B+, and I'm A-. Is it true I'm the only one with Rh blood? 

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I am a bit confused by the wording of your question, but I can help you with most of it.  The + symbol in blood types refers to having the Rh factor.  The Rh factor is a type of protein on the surface of red blood cells.  If a person has the Rh factor, they have positive (+) blood.  If the Rh factor is not present, the person has negative (-) blood.  Since you are A-, you do not have the Rh protein on your red blood cells.  

I am at a loss as to how you have type A blood.  Your mom is type O, which means that she can only donate the O type gene.  Genotypically, her blood type is OO.  She can only ever donate the O.  I can't know your dad's genotype for sure, but because he is type B, he can either be a pure type B or a type B that carries the gene for O type blood.  A pure B would genotypically be BB.  If he carries the O gene, then his blood genotype is BO.  

In either case, your dad can only donate an O or a B.  Your mom can only ever donate an O.  Neither parent can donate the gene for type A blood, so your blood type should not be type A.  

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