Undark:
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.
Yeah, though “one specific disease” that hit cattle broadly enough could do that no matter what.
I’m not super worried about one targeting some notional Y-chromosome problem in Holsteins, myself.
(Note that humans have a single male common ancestor, but so long ago that significant mutational diversity has crept in.)
X-chromosome diversity in cows is probably more important for keeping dairy around – all actual dairy cows are FEMALE anyway, and don’t even have a Y; such a disease predicated on the Y chromosome would simply kill the Holstein bulls.
At worst a giant “kills all the male Holsteins in America” disease would lead to … the next generations of dairy cows having imported Holstein genetics, or being crossbred with one of the small percentage of other-breed dairy cows in America, etc.
I understand such a thing almost almost happened to bananas in the previous century.
see Cavendish.
As we approach 8 billion souls on this planet, avoiding homogeneity or avoiding a major crop blight should be a top-5-priority.
But then again, building the wall or sourcing cheaper MAGA hats from China really trumps that argument.