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 shortwave@npr.org.

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 shortwave@npr.org.