There are over nine million dairy cows in the US, 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. Media reports says when researchers at the Pennsylvania State University looked closely at the male lines a few years ago, they discovered more than 99% 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.
