How genetically close are third cousins?
First cousins share 12.5%, second cousins 3.125%, third cousins 0.78125%, and so on. But in real life, these are averages. Remember, first cousins should share 12.5% of their DNA while second cousins should share 3.125%. You can see these numbers in red in the table (1C is first cousin, 2C is second cousin).
What common ancestor do 3rd cousins share?
First cousins share a grandparent (2 generations) Second cousins share a great-grandparent (3 generations) Third cousins share a great-great-grandparent (4 generations)