DEAR DR. GOTT: Is it possible for three sisters with O positive blood to be related to one with A positive blood? She has always seemed different in looks and personality and we just found out about her different blood type. Both of our parents are dead and we do not know what their blood type was. We will not tell her the results, but it would explain a lot of things to us if it were true. Can you help us?
DEAR READER: The simple answer is yes, it is possible. However, without knowing your parents blood type, I cannot give you a more definitive answer. Only DNA testing can prove relation. Blood typing can merely disprove relation in cases where parental and child blood types are known. That being said, I will give you a more in-depth look at how this situation may be possible.
Both A and B blood types can be created in two different ways; types AB and O in one. A can appear as AA or AO, B as BB or BO, O as OO, and AB as AB. This means that you and two of your three sisters are represented by OO. Your third sister can either be represented by AA or AO. If she were AA, she could not be related; however, if she were AO, there is a possibility.
If either of your parents had AB or B type blood, the situation would be different. With B type blood, your A positive sister couldn’t be related to you and one or both of your parents; with AB type, the three of you with O positive couldn’t be related to one or both of your parents.
As you can see the situation is complex. Personality, looks and blood type cannot be used to judge relation. Your oddball sister may simply have more recessive genetic traits, etc. Perhaps she isn’t related or is a half-sister. It has always been my opinion that genetic similarity doesn’t make a family; love does. If you love your sister and she loves you in return, be happy. Don’t nitpick about the details.
If you are truly concerned or feel you need a definitive answer, seek out genetic testing. Your physician, local hospital, or an attorney may be able to provide with a reliable laboratory that can do DNA testing to determine if the four of your are, in fact, related.
