What’s the difference between yams and sweet potatoes?

Dear Cecil:

I always thought yam and sweet potato were two names for the same thing. However, when I asked to be passed the yams at a recent family gathering, I was informed by one of my snotty east-coast cousins that no yams were on the menu.  I was reduced to asking for the "freaking orange things, A-hole," which detracted from the air of refinement and class that I like to convey. What's the straight dope, Cecil?

Cecil replies:

I see you were able to communicate, which is the main thing. Technically, however, your cousin is right. Contrary to what even some grocery store produce guys think, yams and sweet potatoes are unrelated vegetables, though in both cases you’re eating the root of a tropical vine. Sweet potatoes, Ipomoea batatas (“batata” is the original Taino name, whence potato), are an American plant of the morning glory family, whereas yams are of the genus Dioscorea. Yams, which are rarely seen in the U.S. and Canada but are a staple in tropical regions, can grow up to seven feet in length. The name is thought to derive from the West African word nyami, “to eat,” which is heard in Jamaican patois expressions such as, Oonu wan fi nyam banana dem?, “Do you guys want to eat those bananas?”  Some blame (or credit) yams for the high rate of twin births in certain parts of Africa. Or so I’ve heard tell — I confess I haven’t been able to turn up any documentation.  Still, good material to dazzle the ‘tives with next time your cousins give you grief.

Send questions to Cecil via cecil@straightdope.com.

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