When you sit down to Thanksgiving dinner today you can rest assured that everything on the table has been the subject of meticulous, long-term research by the U.S. Department of Agriculture. If you can eat it, someone at the department studies it, in numbing detail.

Take Dr. Alfred Jones, for instance, resident sweet potato breeder at the U.S. Vegetable Laboratory in Charleston, S.C. Next June he's hanging up his gene mappers after 29 years of pushing, prodding, poking and pollinating one of America's most venerable crops.

At 58, Jones is the department's authority on sweet potatoes, which automatically makes him a world expert (if not the expert). This is his last Thanksgiving on the job.

"Every year we get some calls about this time," Jones said. "I like sweet potatoes, and I enjoy working with sweet potatoes. It's been a nice way to make a living."

But with some irritating moments. Like when Jimmy Carter was president and wanted to develop alternative energy sources. Jones and the Vegetable Lab came up with hidry (pronounced "high dry"), a high-starch, low-water variety of sweet potato designed to produce large amounts of ethyl alcohol, a gasoline additive.

"But by the time we got hidry ready, we had a new administration, and they could have cared less what Jimmy Carter wanted," Jones said.

Overall, however, things have gone well. Virtually hired out of graduate school, Jones has moved once in his professional life, done the same kind of research since 1962, worked with the same team since 1975 and put his mark on a half-dozen new breeds of sweet potato. A soft-spoken North Carolinian who actually says pshaw!, he knows everything about sweet potatoes. Like:

They are not yams, "an entirely different species altogether," Jones says. Yams are a root crop grown exclusively in the tropics, mostly as a high-starch staple food.

Sweet potatoes (which are often mistakenly called yams) are morning glories, just like the vine with the pretty blue flowers. Don't be digging up morning glories by the roadside, however, cautions Jones. Sweet potatoes are the only morning glories with roots you can eat.

Sweet potatoes have up to 25 different genetic traits, so "it takes a fairly comprehensive program" to breed them, Jones added. This is why a geneticist can work with sweet potatoes for 29 years without repeating himself.

Farmers don't sweat breeding, though. Instead, they just plant sweet potatoes until they sprout, then plant the sprouts. They can also plant pieces of vine if they want. As a result, virtually all sweet potatoes in the United States are clones.

The sweet potato is native to the Caribbean islands and probably was brought to what is now the mainland United States shortly after Juan Ponce de Leo'n set foot in Florida in 1513.

By 1648 sweet potatoes were being grown commercially, although it is doubtful that the Pilgrim fathers ate them with any regularity until much later. Sweet potatoes need sandy soil and warmth, and do not grow in any quantity north of the Mason-Dixon line.

In the South, however, they have been prevalent and popular for years. North Carolina, which harvested 560 million pounds in 1988, and Louisiana, with 252 million pounds, are the two biggest producers. In all, the United States in 1988 produced 1.15 billion pounds of sweet potatoes worth $154.67 million.

Jones notes that this is only about 5 percent of world production, most of which is centered in tropical countries. Jones also notes that these countries use sweet potatoes and yams as staples. Cooked for the table, foreign sweet potatoes are yellow, or even white, starchy and much harder.

Our favorites are the jewel variety: copper skin, blocky shape, orange pulp, soft and sweet when cooked. The industry standard is the jewel "U.S. No. 1," 2 to 3 1/2 inches across, 3 to 9 inches long. Anything smaller is a "canner," bigger a "jumbo."

The jewel is as American as cranberries or pumpkins. We eat all we grow and sell them nowhere else. And because we like them so much, jewels have a certain tyranny over the market.

"When you develop a new variety, it's very difficult to get people to drop the old one," Jones said. "They know they can market the old one, and it's awful hard to get them to try something new."

Farmer inertia can drive a geneticist crazy. For any sweet potato to have a chance in the U.S. market, Jones says, "the root has to be shaped attractively," and "the flesh has to be orange."

Even then, it's not a sure thing. Beginning in the early 1980s, Jones's team developed six varieties of sweet potato, all of which have the shared characteristic of being highly pest resistant.

These include resisto (very pest resistant); regal (regal red skin); southern delight (rose skin, dark orange flesh, excellent eating); excel (lighter skinned, with a yield per acre that is 12 to 15 percent higher than jewel); the ill-fated hidry, and sumor (a yellow variety designed as a summer potato to be grown in a hot climate).

"On the face of it, the new varieties would just have to be grown," Jones said. "The problem is in the marketing process: if the buyer won't buy it, then the farmer can't sell it. The market wants the U.S. No. 1."