Most U.S. Dairy Cows Come From 2 Bulls. That's Not Good.

NPR science correspondent Dan Charles explains why most of the dairy cows in America are descended from just two bulls, creating a lack of genetic diversity that can lead to health problems. He also visits a lab at Penn State University where scientists are trying to change that. Follow reporter/host Emily Kwong on Twitter @emilykwong1234. Email the show at [email protected].

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