I Share DNA with my Adopted Son

This chart shows my ancestry to William Moulton (b. about 1615) and Margaret Page (b. about 1623). They are one set of my 9th great grandparents.  

A few days ago I was researching the Spalding ancestors of my son, whom I adopted as an infant. This is what I discovered.  

This chart starts with my son's birth mother, so if we add one generation to the bottom of the chart for my son, we find that William Moulton and Margaret Page are his 10th great grandparents. Even though I adopted my son, it's wonderful to see that we share some DNA through our common lineage to this couple.