Rhesus Negative Factor

[3sillydogs]

The either Rhesus Positive or Rhesus negative is the antigen.  The blood type of the person will either have it Rh+ or not Rh- .  Those that have it can receive blood from those that don't but not vice versa.

rhesus factor

    an antigen occurring on the red blood cells of many humans (around 85 per cent) and some other primates. It is particularly important as a cause of haemolytic disease of the newborn and of incompatibility in blood transfusions.

 
   
This explains it quite nicely
https://en.wikipedia.org/wiki/Rh_blood_group_system

[RobertCasey]

To answer the original question about using blood characteristics for tracing ancestry. Dr. Albert Casey tested thousands of Irish and other Europeans for 15 to 20 characteristics for blood (Rh included). He published a series of large source material books (16 volumes) on his Munster, Ireland ancestry but several volumes have his research on usage of blood characteristics to determine relationships between donors. Unfortunately, there is some Eugenics included as well (skull dimensions). If you want to know how blood characteristics can be used for tracing ancestry, this is a pretty interesting study of blood characteristics done in the 1950s which have a genetic genealogy component. These volumes are located in National Library of Ireland as well as the Hunstville, AL public library (and many other libraries).

These volumes have a lot of genealogical source material for County Cork and Kerry and these blood studies are extremely unique research for pre genetic genealogy research (they are pretty heavy reads and would help to have a good understanding of these blood characteristics):

http://www.worldcat.org/title/okief-coshe-mang-slieve-lougher-and-upper-blackwater-in-ireland/oclc/827660