I am not an adult with PWS, my 18 month old daughter has a PWS. I read this thread to learn all I can. That being said, she is the 1st on either side of our families to have a PWS. She is the youngest of three girls and my oldest had a small strawberry mark that went away when she was 3. My middle daughter has a small brown birthmark on her back.
I also wanted to add that a good friend of mine growing up has a PWS covering her right cheek. She has two sons and neither has a PWS. Hope this helps.