You are absolutely right. It seems that the pedigree is collapsing pretty hard in recent generations because it seems I hear of health problems if one sort or another in almost every Amish or Mennonite family (if not THAT family, a cousin's family).
What I always tell my kids is, say hi to to the Amish because they are your cousins. Now, my it was my great-great grandmother who was the last Amish, but there have been Mennonites and Dunkard Brethren going back to the 18th century and I have traced our family tree back to the 1500's in Germany through one line. I tell my kids that we are cousins because if we go back that many generations we ARE related, we all have some common ancestor who came to Pennsylvania from Germany and Switzerland and intermarried for many generations. My mother's family eventually settled in Goshen, Indiana, another large Amish enclave but they all come from the original stock that settled in Lancaster and Berks counties in the 18th century.
Genetics is indeed a fascinating subject...
With the exception of an Ott that I have been unable to trace back to Germany from early 19th c. South Carolina, the rest of our family is exclusively British - mostly Scottish and Irish with a heavy admixture of English and a sprinkling of Welsh.
The frightening birth defects out there for those of us of almost exclusively British ancestry are the neural tube defects (spina bifida etc.) We watched the kids in utero like hawks for that one . . . and I took folic acid until it was coming out my ears. Thank heavens both born free of that trouble.