Dr. Steve Mount is an Associate Professor of Cell Biology and Molecular Genetics at the University of Maryland. In February 2011, he wrote an article about his experience submitting his DNA to 23andMe and his experiences matching his cousins. More specifically, he became interested in one particular segment of DNA trackable to a specific ancestor.
He shares these insights.
Distant relatives (4th cousins and beyond) often share no genetic material at all.
It is possible to share a segment with very distant relatives.
Sometimes, more distant relationships are more likely.
Most of your relatives may be descended from a small fraction of your ancestors.