In order to really understand varietals we need to start at the beginning. Way back at the beginning, with seeds and parents, clones and pollination. So let’s get geeky about wine.
Grapes have seeds. So, we could grow a new plant from each of these seeds; however, the seed would not necessarily grow a plant identical to its parents. This is so despite the fact that the grape is hermaphroditic and can self-pollinate. Grapevines are diploid organisms, which means they have two copies of each chromosome. When pollination occurs, each gene has a 50% chance of being different from the combination its parent had.
Grapes have thousands of genes, so the different combinations would be endless. Each set of new combinations from the parent would result in a different expression of vine characteristics in the new plant. You could search through thousands of new plants and likely not find one identical to the parent. So, in order to produce a vineyard of identical plants, we must grow each new vine from a cutting from another plant. This is called vegetative propagation, and it ensures that each new plant is genetically similar to the original vine.
Over time, all organisms accumulate mutations in their genetic code. This could be due to UV radiation or just simple DNA degradation or mistakes. This type of mutation can yield a new clone of an existing variety. The new plant is almost genetically identical to the original, and is still considered the same variety; however there may be a slightly different expression that the grape grower will find beneficial. Normally these mutations will spring up spontaneously in the field, a grower will notice that the plant is different or maybe better than its neighbors and will then choose that plant’s wood to propagate new plants.
In Cabernet Sauvignon, for example, we have many clones that we work with to create variability and complexity, but also to match with the site where we are growing grapes. These all carry numbers that they are identified by the nurseries that provide plant material for new vineyards.