(NPR) Chad Dechow, a geneticist at Pennsylvania State University who studies dairy cows, is explaining how all of America's cows ended up so similar to each other.
He brings up a website on his computer. "This is the company Select Sires," he says. It's one of just a few companies in the United States that sells semen from bulls for the purpose of artificially inseminating dairy cows.
Dechow chooses the lineup of Holstein bulls. This is the breed that dominates the dairy business. They're the black-and-white animals that give a lot of milk.
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