The "Nuremberg Laws" established a pseudo-scientific basis for racial discrimination. Only people with four German grandparents (four white circles in top row left) were of "German blood." Jewish is who descends from three or four Jewish grandparents (black circles in top row right).
In the middle stood people of "mixed blood" of the "first or second degree." These bizarre distinctions had deadly consequences. As there were no real racial differences, the Nazis had to use a trick: a Jewish grandparent was simply defined as a person who is or was a member of a Jewish religious community.

Chart published in 1935 to explain the "Nuremberg Laws"