Unlike most dairy cows in America, which descended from just two bulls, this cow in Pennsylvania has a different ancestor: She is the daughter of a bull that lived in the 1960s called the University of Minnesota Cuthbert. The frozen bull semen was preserved by the U.S. Department of agriculture.

Dan Charles/NPR

Chad Dehow, a geneticist at Penn state University who studies dairy cows, explains how all of America’s cows turned out to be so similar to each other.

He picks up a website on his computer. “It’s a Select Sires company,” he says. It is one of the few companies in the United States that sells sperm from bulls for the purpose of artificial insemination of dairy cows.

Dehow chooses a lineup of Holstein bulls. It is a breed that dominates the dairy business. These are black and white animals that give a lot of milk.

Dairy farmers can go to this online catalog, choose a bull, and the company will supply doses of sperm to fertilize their cows. “There’s one bull, we believe he has more than a quarter of a million daughters,” says Dehow.

Companies rank their bulls based on how much milk their daughters have produced. Dechow picks one from the top of the list, a bull named Frazzled. “His daughters are projected to produce 2,150 pounds more milk than the daughters of the average bull,” he says, reading from the site.

Farmers like to buy sperm from the best bulls, and companies continue to breed even better bulls by pairing their best performers with the most productive cows. “They keep choosing the same families over and over again,” says Dehow.

A few years ago, Chad Dehow and some of his colleagues at Penn state made a discovery that shocked many people. All Holstein bulls used by farmers could trace their ancestry back to one of two male ancestors. “Everything goes back to two bulls born in the 1950s and 1960s,” he says. “Their names were Round Oak Rag Apple Rise and Pawnee farm Arlinda chief.”

This does not mean that the bulls in the catalog are genetically identical. They still had many different mothers and grandmothers. But it shows that this system of large-scale artificial insemination, with farmers repeatedly collecting top-rated bulls, has made cows more genetically similar. Meanwhile, genetic traits that existed in Holstein cows a generation ago have disappeared.

“We’ve lost genetic variation,” says Dehow. “Now, some of this variation was garbage that we don’t want to start with. But some of them were valuable things.”

To see what could be lost Dehcho decided to conduct an experiment. He found some old semen from other bulls that were alive 50 years ago, with names like University of Minnesota Cuthbert and Zimmerman All-Star Pilot. You can call them a relic of bulls. The USDA stores sperm samples in a deep freeze storage facility in Fort Collins, Colo.

Dehow used this sperm to fertilize some modern cows. They gave birth, and now you can see some lost pieces of the Holstein family tree come to life, in a barn in Pennsylvania in the form of three cows.

Dehow leads to the barn. He points to the cow, which looks at us suspiciously. “Here’s our old genetic line,’ cow ‘number 2869 –” he says.

To the untrained eye, this cow looks pretty much the same as everyone else. But Dehow sees what others can’t. “If you notice, if you look at her back to see how this cow to her left is a little more bony?”He says.

Once Dechows points it out, the difference is clear to see. “So it definitely carries more body condition. It’s a little thicker, ” he says.

Traditionally, dairy farmers did not like cows with excess fat. They thought that the ideal cow was skinny, because she made all of her feed into milk and not into fat. Therefore, farmers chose bulls, which, as a rule, produced such a daughter.

“We’re kind of chosen for tall, thin, cows,” says Dehow. “And it’s a very bad combination. They are barren, unhealthy. So we have to get away from it.”

Dehow believes frozen sperm from these long-forgotten bull relics could bring back valuable genes that went missing, maybe genes that would allow cows to thrive in warmer temperatures, for example.

For this to work, farmers actually have to use these bulls, and they will only do so if they are convinced that they will also produce a lot of milk.

So Chad Dehow is keeping a close eye on his experimental cows. So far, he said, everything is going well. Two out of three cows produce at least as much milk as the industry average.