A friend has learned that you are studying genetics, and she is into genealogy.

A friend has learned that you are studying genetics, and she is into genealogy. Her family has been in Maine for at least three generations, and she can trace her heritage back to a couple living on Mohegan Island.

She did notice something odd in her family, and you showed her how to draw a pedigree.

What is unusual about her family's pedigree?

Can you explain this pattern with Mendelian inheritance?  If yes, then what is a possible explanation?