Deductive reasoning

I was helping Tommy study for his science test the other night. And boy, I am always so amazed at everything I forgot the interesting things they are learning. This particular test was about genotypes and phenotypes. The study of genetics, basically. So after I grilled him with the questions, I decided to challenge him with a question not in his notes.

And so I asked him: If his dad and I have A+ blood types, and his three younger brothers also have this blood type, then why was his blood type 0+?

His answer: "I don't know. Because I'm the 0ldest?"

Oh. I see how you deducted that conclusion, Tommy. 0=oldest. Haha. That's funny. But, no. Being the oldest has nothing to do with it.

Now, who here can tell me how this can be possible? Do I see any hands?