Anderson, who has bred the birds for 26 years, said the key technical advance was artificial insemination, which came into widespread use in the 1960s, right around the time that turkey size starts to skyrocket. The reason is that turkeys over 30 pounds are "inefficient" breeders: It's difficult for them to actually perform the natural mating act. With artificial insemination, the largest birds can still be used as sires, even if they have a hard time walking, let alone engaging in sexual reproduction.
"You can spread the one tom around better. It adds a whole new level of efficiency. You can spread him over more hens," Anderson said. "It takes the lid off how big the bird can be. If the size of the bird keeps them from mating, then you're stuck."
This process, compounded over dozens of generations, has yielded turkeys with genes that make them very big. In one study in the journal Poultry Science, turkeys genetically representative of old birds from 1966 and modern turkeys were each fed the exact same old-school diet. The 2003 birds grew to 39 pounds while the legacy birds only made it to 21 pounds. Other researchers have estimated that 90 percent of the changes in turkey size are genetic. Perhaps the most obvious change in turkey genetics is that, unlike the colorful pictures we all drew in elementary school, modern, factory-farmed birds are all white. The Broad Breasted White turkey became the dominant commercial breed in the middle of the 20th century.
These fast-growing, big birds are more energy efficient than their forebears. They can convert 2.5 pounds of feed into a pound of body weight. Legacy breeds take a longer time to add weight and can need over 4 pounds of feed to add a pound of weight.
But all that bulk comes with consequences. Commercial turkeys can't fly and researchers have even invented a way of quantifying how impaired the birds' walking has become. The 1-to-5 scale ranges from "birds whose legs did not have any defect" to bowlegged birds who have "great difficulty walking." After 30 years of breeding, Ohio State's big birds average a 3.
The birds also have a hard time regulating their own food intake. In essence, they eat too much and get fat. "Commercial broiler breeder strains, selected for rapid growth and high meat yields, do not adequately regulate voluntary feed intake commensurate with their energy needs," wrote two USDA scientists last year. "Consequently, these birds must be given a limited amount of feed to avoid overconsumption that can lead to excessive accumulation of energy stores [fat tissue]."
And some food lovers argue that fast growth and genetic change have robbed turkey meat of its distinctive taste. Some are turning to heritage-breed turkeys like the Blue Slate variety that pack pre-industrial genomes.
"One thing I would say about a modern turkey is that they have a lot less flavor," said food historian Curtin. "If you've ever had a chance to taste a heritage breed, there's subtleties in turkey."
Turkey isn't the only element of the iconic Thanksgiving dinner that science has given an overhaul. Corn breeding has made corn six times sweeter than the variations that the Pilgrims probably encountered back in 1620.
We eat a type of corn called, appropriately enough, sweet corn. The maize that American Indians grew in the 17th century would have been more like the type we feed to animals now, said Bill Tracy, a corn agronomist at the University of Wisconsin-Madison.