An article by Jen Pinkowski in Mental Floss discusses the lack of genetic diversity in the most commonly cultivated V. vinifera grapes.
“The vast majority of wine produced across the world derives from a single grapevine species: Vitis venifera. The domesticated grape has thousands of varieties, and quite a lot of genetic diversity among them, according to a 2010 paper in PNAS that analyzed genome-wide genetic variation of more than 1000 samples of V. vinifera subsp. vinifera and its wild relative, V. vinifera subsp. sylvestris. But that’s not true for all grapes: Nearly 75 percent of cultivars had a first-degree relationship to at least one other. They were either parents or children.”