Blood Types
My mother had AB blood, I have AB Blood, and my daughter has AB blood. Her half brother by her mother has O. How is this possible?
-A curious adult from New York February 7, 2012 This is a perfect question for a geneticist. To figure out how this is possible (which it is by the way), we need to step back and do a bit of review on how genes work. Here are the basics of what we need to know:
1. We have two copies of each of our genes, one from mom one from dad
2. These genes can come in different versions
3. Sometimes one version dominates another
This is your only possible combination because there is only one way to get AB blood type. Now let's think about your daughter. Remember, she gets half her genes from you and half from her mother. So let's add her and her mother into the picture:
We know your daughter also has the AB blood type so I wrote that in. We also know that only one of those versions of the gene comes from you. Remember, you can't pass an AB down to your daughter by yourself...you can only give her one of your two blocks. Let's say you give her the A block. If you give her the A, that means the B must come from her mother since she gives your daughter the other half of her blood type. This is what we have so far then:
Do we know what her mother's other block is? Not yet. At this point, it could be A, B or O. Since she can only give her daughter one of her blocks, it means we don't know what her other block is. Until we look at your daughter's half brother that is... Now the real detective work can begin! You wanted to know how your daughter's half brother has O blood type. First of all, the "O" blood type actually means that you have two O's (just like two A's would be called an A blood type). So let's add the half brother and his father to this tree.
Ok, now let's figure out what the missing blocks are. Before, we didn't know what the mother's second block was, but now we know, right? It has to be an O since she gave an O to her son. And what about the half brother's father? We know what one of his blocks has to be...it's gotta be an O. But we don't know what the other block is based on this tree. It could be an A, B or another O.
And finally, for good measure, let's add your parents to the tree. At this point you can probably do it yourself, but just to remind you, here's the important information and rules to remember:
1. We have two copies of each of our genes, one from mom one from dad
2. These genes can come in different versions