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Hi, Diane ~ thank you for the compliment! :-)Yes, it does make a difference to Os, and a number of other blood types as well. Here's a question to which, happily, there is a definitive answer!
It started as a regional thing in the southern U.S. to call sweet potatoes "yams," with variations such as "garnet yams," etc. The dual usage has since spread countrywide ~~ and nobody cared, until BTD came along. ;-> In my organic grocer, they complicate the matter by using both terms. It all began with an imported word which canny veg distributors originally used to distinguish the heritage pale-yellow sweet potatoes from the darker-pigmented new-and-improved strain. The word, "nyami," was conveniently heisted from a food item which is botanically and physically quite distinct from the sweet potato. However, rest easy: all those little tubers, be they pale yellow, dark orange, dark red, etc. are sweet potatoes.
In your supermarket, I doubt they have true yams. They have them in mine, because I live in a predominantly Dominican immigrant neighborhood. If you frequent Asian grocers, you'll see species of yam there, too. They all have a dark brown, almost woody-looking, somewhat shiny skin and chalk-white innards. Their size varies from long-sweet-potato to huge -- the bodega a few blocks from me regularly displays torpedo-sized specimens.
The yams of food list and database fame have a dry, hard, starchy flesh, and belong to the Dioscorea family of plants ~ while our HB sweet potatoes (from meek yellow to outright magenta) hail from the Ipomoea clan, with the softer, sweeter qualities we're all familiar with.
Here's a great reference page from the North Carolina Cooperative Extension Service. Have a read, and spread the word! :-D