There are more than 9 million dairy cows in the United States, and the vast majority of them are Holsteins, large bovines with distinctive black-and-white (sometimes red-and-white) markings. The amount of milk they produce is astonishing. So is their lineage. When researchers at the Pennsylvania State University looked closely at the male lines a few years ago, they discovered more than 99 percent of them can be traced back to one of two bulls, both born in the 1960s. That means among all the male Holsteins in the country, there are just two Y chromosomes.
Any elementary science student knows that genetic homogeneity isn’t good in the long term. It increases the risk of inherited disorders while also reducing the ability of a population to evolve in the face of a changing environment.
This may seem like a silly story but it’s actually terrifying. All it would take is one specific disease and it could wipe out the entire dairy industry.