From the article This doesn't mean that the bulls in the catalog are genetically identical. They still had lots of different mothers, as well as grandmothers.

From the article This doesn't mean that the bulls in the catalog are genetically identical. They still had lots of different mothers, as well as grandmothers. But it does show that this system of large-scale artificial insemination, with farmers repeatedly picking top-rated bulls, has made cows more genetically similar. Meanwhile, genetic traits that existed in Holstein cows a generation ago have disappeared